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Dew point

• All air contains water vapour of varying quantities. The dew point indicates the amount of moisture in the air. The higher the dew point, the higher the moisture content of the air at a given temperature. Conversely, the dew point of humid air will be higher than the dew point of dry air.
•  Dew point temperature is defined as the temperature to which the air would have to cool (at constant pressure and constant water vapour content) in order to reach saturation. A state of saturation exists when the air is holding the maximum amount of water vapour possible at the existing temperature and pressure.
• Condensation of water vapour begins when the temperature of air is lowered to its dew point and beyond. The dew point, like other measures of humidity, can be calculated from readings taken by a hygrometer
EXPLAINING DEWPOINT AND RELATIVE HUMIDITY
• The public has a good grasp on how the weather makes them feel. One approach to explaining dewpoint would be to say, dewpoints above 65 F make it feel sticky and humid outside while dewpoints less than 65 F are comfortable with respect to the stickiness of the air. The higher the dewpoint is, the more moisture that is in the air. The higher the dewpoint is above 65 F, the stickier it will feel outside (feels like you have to breathe in a bunch of moisture with each breath). 75 F or above dewpoint, the air really feels sticky and humid.
• RH can be more difficult to explain. The public pretty much understands that a RH of 100% means it is either foggy, very wet, or saturated outside. One misconception people have is that the RH is 100% only when it is raining. EX 1: The RH is often 100% in the early morning hours when temperature has dropped to dewpoint. EX 2: When rain first begins, it takes time for the air to saturate. RH is often much less than 100% when it is raining (it takes time and lots of evaporation to saturate air that previously has a RH of 50% for example). If the rain is not heavy enough or does not last long enough, the rain will not saturate a previously drier PBL.
•  RH can be explained to the public as the "closeness the air is the saturation". When the RH is less than 40%, it feels dry outside, and when the RH is greater than 80% it feels moist outside (dewpoint will determine if it is uncomfortably moist or just regularly moist). Between 40 and 80% RH is comfortable if the temperature is also comfortable.
• The worst combination for human comfort is a high dewpoint (65 F or above) combined with a high RH. If the dewpoint is above 65, it will generally always feel uncomfortably humid outside. Obviously, the temperature could climb to over 100 and result in a low RH, but the quantity of moisture in the air is still high and will be noticed.
• The optimum combination for human comfort is a dewpoint of about 60 F and a RH of between 50 and 70% (this would put the temperature at about 75 F). The air feels dry outside when BOTH the dewpoint is below 60 F AND the RH is less than 40%.
• Now the dilemma, how does the public differentiate the "meaning" between a high dewpoint and a high RH when they both indicate the air is humid??? Dewpoint is related to the quantity of moisture in the air while relative humidity is related to how close the air is to saturation. How the public is to understand this difference in meaning can be a challenge. The challenge can be overcome by describing how the weather feels and relate that information to the current dewpoint and relative humidity.
Observing humidity or moisture in the air
• The amount of water vapour in the air varies. The percentage of water vapour in the air compared to what the air can hold at that certain temperature is called humidity. People often confuse the terms humid and heat when talking about what they are feeling. Heat, as discussed in the section on temperature, relates to the amount of solar energy contained in the air. Humidity relates to the amount of water vapour within the air and is recognised by the discomfort through stickiness.
•   Humidity is measured using instruments called hygrometers. The standard hygrometer measures the difference in temperature of the air and that of a thermometer attached to a wet wick known as a wet and dry bulb thermometer. As evaporation occurs, it represents a cooling process and hence the temperature on the thermometer containing the wick will give a lower reading of temperature. A conversion scale is used to convert the difference in temperature of both thermometers to the relative humidity reading at that dry air temperature. Other hygrometers have scales that indicate humidity directly. The humidity scale is measured on a scale 0 to 100 since it is a percentage. When humidity reaches 100%, condensation occurs. At 0%, the air would be extremely dry.
• To compare the amount water vapour in the air, it is important to consider both the humidity and temperature. For instance, a day of about 27 degrees Celsius and 85% humidity has more water vapour than a day with a temperature of 27 degrees Celsius and 43% humidity. Air on a day that is warmer can hold more water vapour. Therefore, often the trend on a normal fine sunny day is to observe the relative humidity to drop during the day as the temperature rises and increase as the temperature drops during the evening. On a wet day, the relative humidity is quite high and remains almost unchanged if the rain persists and there are no significant changes in the wind speed.
•  Examples of observations of humidity other than discomfort are fog, mist, cloud and rain that indicate environments of very high humidity. Over the long term, if the rate of evaporation decreases dramatically such as when drying clothes, the humidity is high. If the evaporation rate increases dramatically at a particular temperature, humidity has dropped significantly. Remember that humidity is defined according to the temperature and does not directly indicate how much water vapour is in the air.

Observing weather at Night Observing weather events during the night Most people tend to associate observing the weather as a daytime activity. The darkness of the night tends to draw most people indoors. The weather, however, is still as active during the night as it is during the day. Some significant weather situations such as rain, thunderstorms, and wind can and do occur during the night. In particular, the conditions associated with cold weather such as snow, frosts, and fogs are more likely to occur during the night.  Night weather conditions therefore influence everyone in one way or another. Cold conditions require heating indoors. Thunderstorms occurring the night can cause blackouts. On the other hand, there are those people that work during the night in shifts. They are more directly influenced by the external night weather conditions. For instance, travelling during the night introduces reduced visibility. Flooded creeks or large puddles can be difficult to see. This section covers briefly many observations that can be made during the night using various techniques. The various types of night observation techniques Night observations require a variety of special techniques. Although it is dark, it is still possible to observe sound as well as light illuminated images. And, of course, the night does not hamper observations of the various weather conditions that we can feel during the night such as temperature and humidity. The most effective weather observations at night still require some source of light to illuminate the various features in the sky. Such light sources include any of the following: lightning, city or town lighting (on a large scale), street lighting (on a local scale), back ground light from the moon and stars, and automobile headlights (on a local scale). Such weather observation techniques require a lot of daytime weather experience and a previous knowledge of weather features such as cloud structure and cloud types. Those that have hearing impairments rely on sight and feeling when observing the weather and obviously must view weather differently to the rest of the population. Sound observations are also effective observation techniques. In fact, it is normal for sounds associated with weather to be more audible during the night due to the lack of daytime noise such as those associated with traffic. Again, experience is needed to observe certain sounds during the night especially since we tend to observe noise simultaneously with other observations. Those who are visually impaired rely on sound observations and feeling both during the day and night. They will obviously have a different view of the weather and may never be able to appreciate weather from the view of the rest of the population.  They way we perceive (like or dislike) certain weather conditions is definitely influenced by the impact the weather has on our bodies. For instance, most people seem to wake up in a good mood if they can view the sunrise. In other circumstances, high humidity makes people feel uncomfortable. Nevertheless, despite how weather makes people feel, feeling the weather is a form of observation and it is as important during the night as it is during the day. Such observations include feeling temperature, humidity, rain, wind, dust, and vibrations associated with thunder. It can even include feeling static electrical discharges especially during dry weather! Observing thunderstorms during the night

Of course, the most common form of night time observations are those linked to thunderstorms: namely lightning and thunder. However, it is not as obvious as it may seem. Lightning and thunder activity reveal an abundance of information about thunderstorms including location, severity, precipitation cascades, types of precipitation, speed and direction of the storm's movement, evidence of wind, cloud structure and the extent thunderstorm activity
• Structure is the most important observation to note from lightning effects of thunderstorms. Internal lightning activity such as sheet lightning can reveal the shape of the thunderstorm cloud structure. Lightning can reveal precipitation intensities, base structures such as shelf clouds, the extent of the front, side and rear anvils, and the downdraughts from the storm as revealed by wind blown precipitation.
•  For those who believe in light green tinges, I have noted night time hailstorms to reveal greenish effect due to the lightning. In normal cases, lightning will appear pinkish. The difference in structure of hailstorms must have an impact on various light frequencies such that green dominates. I don't really have an explanation but definitely, I have observed several times green during hailstorms.
• Another type of observation is thunder. The distance to thunderstorm can be determined and calculated (approximately) by measuring the time from the occurrence of lightning and until when the associated thunder is heard. It must be the associated thunder and not just any thunder heard. Once the time is measured, multiply this by 340 and the answer will be in metres. For instance, a 10 sec lag between lightning and thunder is about 3400 metres or about 3.4 kilometres. The fact that thunder is heard means it is nearby up to a distance of up to 20 kilometres or so. This depends on the severity of lightning. Just being aware of lightning means you may expect thunder and therefore you are not startled.

Other types of night observations

The following observations are only discussed briefly. The observer is more or less made aware of the surroundings.

Because of the light created by bushfires, they are easily observed during the night. But in rugged terrain, it may be difficult to observe until you get close. Be alert during times of dry conditions and high bushfire danger levels especially when travelling through forested regions.

Precipitance

Rain or Snow.
Dependent upon temperature

Precipitation typically forms high in the atmosphere where the temperature is below freezing. As ice crystals form aloft and fall toward the surface, they collect each other to form large snowflakes. If ground temperature is above 32 F, the freezing level must be located somewhere above the ground. As the falling snow passes through the freezing level into the warmer air, the flakes melt and collapse into raindrops. During the summer months, it is not uncommon for the freezing level to be found at a level above cloud base.
When the air temperature at the ground is less than 32 F, the snowflakes do not melt on the way down and therefore reach the ground as snow
Occasionally, we observe snow reaching the ground even though the outside temperature is above freezing. This occurs when a very thin layer of warm air is found near the surface.
Since the layer of warm air is so shallow, the precipitation reaches the ground as snow before it has a chance to melt and become rain. For more about precipitation, visit the precipitation section of this module.
Rain and Hail
Liquid and ice precipitation

Rain develops when growing cloud droplets become too heavy to remain in the cloud and as a result, fall toward the surface as rain. Rain can also begin as ice crystals that collect each other to form large snowflakes. As the falling snow passes through the freezing level into warmer air, the flakes melt and collapse into rain drops. The picture below shows heavy rain falling from a Texas thunderstorm.

Hail is a large frozen raindrop produced by intense thunderstorms where snow and rain can coexist in the central updraft. As the snowflakes fall, liquid water freezes onto them forming ice pellets that will continue to grow as more and more droplets are accumulated. Upon reaching the bottom of the cloud, some of the ice pellets are carried by the updraft back up to the top of the storm.
As the ice pellets once again fall through the cloud, another layer of ice is added and the hail stone grows even larger. Typically the stronger the updraft, the more times a hail stone repeats this cycle and consequently, the larger it grows. Once the hail stone becomes too heavy to be supported by the updraft, it falls out of the cloud toward the surface. The hail stone reaches the ground as ice since it is not in the warm air below the thunderstorm long enough to melt before reaching the ground

Freezing Rain

super cooled droplets freezing on impact
•   Ice storms can be the most devastating of winter weather phenomena and are often the cause of automobile accidents, power outages and personal injury. Ice storms result from the accumulation of freezing rain, which is rain that becomes super cooled and freezes upon impact with cold surfaces. Freezing rain is most commonly found in a narrow band on the cold side of a warm front, where surface temperatures are at or just below freezing.
•  The diagram below shows a typical temperature profile for freezing rain with the red line indicating the atmosphere's temperature at any given altitude. The vertical line in the center of the diagram is the freezing line. Temperatures to the left of this line are below freezing, while temperatures to the right are above freezing.
Freezing rain develops as falling snow encounters a layer of warm air deep enough for the snow to completely melt and become rain. As the rain continues to fall, it passes through a thin layer of cold air just above the surface and cools to a temperature below freezing. However, the drops themselves do not freeze, a phenomena called supercooling (or forming "supercooled drops"). When the supercooled drops strike the frozen ground (power lines, or tree branches), they instantly freeze, forming a thin film of ice, hence freezing rain.

Ice-Crystal Mechanisms

the formation of freezing rain

Freezing rain can develop either through ice crystal processes or super cooled warm-rain processes. Ice crystals high in the atmosphere grow by collecting water vapor molecules, which are sometimes supplied by microscopic evaporating cloud droplets. In the figure below, the blue line represents the temperature of the atmosphere and the black line represents the 0C (32F) isotherm (a line of equal temperature). When the blue line is to the right of the black line, the atmosphere is warmer than 0C and when the blue line is to the left, the atmosphere is colder than 0C.
As the snow falls, it encounters a layer of warm air where snow and ice particles completely melt and collapse into raindrops. As the raindrops approach the ground, they encounter a layer of cold air and cool to temperatures below 0C. However, since the cold layer is so shallow, the drops themselves do not freeze, a phenomena called supercooling (or forming "supercooled raindrops"). The supercooled raindrops are raindrops that are colder than 0C and freeze on contact when they strike the ground.

Supercooled Warm-Rain Processes’
the formation of freezing rain
•  A less common way that freezing rain forms is through supercooled warm-rain process (SWRP), where cloud top temperatures are warmer than about -10C. Supercooled raindrops develop as microscopic cloud droplets collect one another as they fall. Ice processes are not involved in the formation of these raindrops.
• The precipitation falls to the surface as supercooled rain or drizzle and freezes instantly on contact. The raindrops do not freeze within the cold layer because there are very few ice nuclei in the presence of warmer temperatures.
•  Below is a sounding that typically results in freezing rain through Supercooled Warm Rain Processes (SWRP). Throughout the sounding profile, the temperature never exceeds 0C (32F). The process begins as supercooled raindrops grow by collision and coalescence, and since the temperature throughout the cloud is warmer than about -10C, the cloud is generally free of ice crystals. This is important because if ice crystals were present, the cloud drops would instead grow by ice crystal processes, producing snow and not freezing rain. Winds are typically out of the west or northwest at the surface, veering to the southwest in middle and upper levels.
• In the second sounding, the temperature exceeds 0C (32F) above the cloud. This type of sounding is commonly observed in the Southern Plains where very warm mid-level air from the Mexican plateau overrides colder air in the surface layers. Winds at the surface are usually from the east or southeast and veer around to the southwest or west in middle and upper levels.
•  The best way to forecast freezing rain is to examine forecast model soundings for profiles similar to the ones described in this module. One web site providing these products is the Northern Illinois University's Storm Machine. One can choose the city and model and receive the appropriate forecast soundings, from which a forecast can be made about when and where freezing rain will occur.
Sleet
frozen raindrops that bounce on impact with the ground

Progressing further ahead of the warm front, surface temperatures continue to decrease and the freezing rain eventually changes over to sleet. Areas of sleet are located on the colder side (typically north) of the freezing rain band
.
• Sleet is less prevalent than freezing rain and is defined as frozen raindrops that bounce on impact with the ground or other objects. The diagram below shows a typical temperature profile for sleet with the red line indicating the atmosphere's temperature at any given altitude. The vertical line in the center of the diagram is the freezing line. Temperatures to the left of this line are below freezing, while temperatures to the right are above freezing.
• Sleet is more difficult to forecast than freezing rain because it develops under more specialized atmospheric conditions. It is very similar to freezing rain in that it causes surfaces to become very slick, but is different because its easily visible.

Snow
an aggregate of ice crystals

• Progressing even further away from the warm front, surface temperatures continue to decrease and the sleet changes over to snow.
• Snowflakes are simply aggregates of ice crystals that collect to each other as they fall toward the surface. The diagram below shows a typical temperature profile for snow with the red line indicating the atmosphere's temperature at any given altitude. The vertical line in the center of the diagram is the freezing line. Temperatures to the left of this line are below freezing, while temperatures to the right are above freezing.

The development of freezing rain

In most cases, freezing rain results from the process of warm moist air "overrunning" colder air. Perhaps the most common overrunning scenario occurs as warm moist air flows up and over a warm front associated with a midlatitude cyclone. The rising air cools, the water vapour condenses, producing a narrow band of freezing rain ahead of the front. This band is typically less than 50 kilometres (30 miles) wide and is represented by region #1 (shaded in orange) in the diagram below. This band is often wrapped around and behind the low pressure center by counter clockwise winds flowing around the cyclone. Some of the most devastating ice storms occur in association with this narrow band of freezing rain.

• A second area of freezing rain is typically found behind the cold front, (region #2 shaded in orange in the diagram above). Freezing rain develops as southerly winds at upper levels push warm moist air up and over the cold front, producing precipitation that falls into the colder air. Freezing rain associated with the cold front is usually very light and scattered, and in rare cases, even observed ahead of the front
• Stationary fronts can be associated with the production of freezing rain. A stationary front separates cold air to the north from warm moist air to the south. Freezing rain develops as upper-level winds (typically light and south-westerly) push warm moist air over the colder air north of the stationary front, producing a narrow band of freezing rain on the cold side of the frontal boundary.
•  Arctic air masses are typically very shallow and have been known to produce devastating ice storms. Behind a cold front, the air mass is maritime polar (mP), but behind an Arctic front, the air mass is continental polar (cP). As Arctic air advances, it lifts the warm moist air (ahead of the front), producing precipitation that falls into the Arctic air. Sometimes, a band of freezing rain wider than 50 kilometres develops in association with an Arctic front.
• When precipitation approaches, the curtain appears to be more of a smooth, white sheet. The boundary where the precipitation meets the cloud base is by this stage almost overhead. On the ground, it is possible to see the rain approaching. Any trees, buildings or hills will become obscured by a shade of white as the rain moves through those areas. On the ocean or on a lake, the precipitation cascade can be observed obscuring the background with a distinct change in colour where the precipitation meets the ocean. By following the movement of this boundary where the cascade meets the ground, lake or ocean, you will be able to determine if and when the rain will strike your area. You will also be able to, with practice, determine where the rain is falling such as on nearby mountains. It is an advantage to be on an isolated hill with little or no obstructions to be able to observe precipitation in surrounding regions 50 or more kilometres away.
• Before the rain begins to fall, often drops of rain are observed. The drops of rain that fall often are an indication of the type of rainfall that is approaching. For instance, if there are medium to large drops of rain, rain to heavy rain may follow. If tiny drops of rain fall initially, then drizzle or showers are most probable. The size of the drops of rain can be observed on impact with surfaces such as the amount of spread on cement, the length of the drops when striking vertical surfaces especially windows, or how far they rebound when they hit surfaces such as metal or water. Drops of rain also appear larger and more frequent than normal on windscreens whilst driving against the rain.

• To observe if it is raining, signs to look out for are reduced visibility in the distance, rain falling when using a dark background to create contrast, or hearing rain falling on roves especially metal roves (corrugated iron). Rainbows are another method that can be used to observe if rain is falling. Occasionally, rain mixed with dust has its own distinct scent especially after a long hot day. If heavy rain occurs suddenly, as is the case with many thunderstorms, it can be heard approaching from a few hundred metres away striking other house or shed roves.
• Rainfall (and the other forms of precipitation) is measured using what we call the rain gauge. Rain gauges are easy to erect and maintain. They are usually not very expensive to purchase. The graduations (or units of measurement) are in millimetres in the metric system or points of inches in the imperial system, both representing height of water collected. They work on a principal that a flat area of land will fill with the amount of water measured in the rain gauge.
•   If you experience a thunderstorm that produces deep hail or live in an area where it snows, it is possible to estimate approximately the amount of precipitation it represents using a special technique. However, it is best to consult meteorological organisations in your country on measurements of the various forms of precipitation and the appropriate times to take measurements

Observing clouds with precipitation Observing precipitation from cumulus Precipitation can occur from some cumulus and can vary from light to heavy. As the height and base area of cumulus decreases, the intensity (and duration) of precipitation decreases if it develops. Precipitation develops due to the fact that the rising (convective) air currents cannot support the excess moisture. The heavier precipitations are associated with darker precipitation cascades and darker bases. The higher cumulus will also contain larger sized droplets. The smaller cumulus normally produces light precipitation in a region with high humidity such as over oceans.

In order to determine which direction the cumulus with precipitation is moving, the observer must follow the direction of movement of the cumulus or make predictions as to the region that may be developing. Having studied an area for many years, often the development and movement of cumulus can be predicted. The observer must take into account that clouds below the cumulus cloud may be moving in various directions and that they are not necessarily producing the precipitation. This often confuses even some experienced amateur meteorologists. If you are not sure, you can back up your skills by watching the radar images that are shown on weather reports.

The duration of the precipitation varies and depends on the following factors:

the size of the base area, the speed of the cloud, and the rate of further development of the cloud at the rear, if any. Such development is true for all cloud types that produce precipitation. Horizontal development towards the rear decreases the relative motion of the cloud and associated precipitation.
This therefore increases the duration of precipitation

• In other words, if the cumulus cloud is large, slow moving and still expanding from development, then long duration, heavy precipitation is most likely. In fact, cumulus that rapidly develops and produces heavy rainfall often may develop into a thunderstorm. Hail can also develop as a result of rapid cumulus vertical development.
• In cases of heavy precipitation, the cascade may appear to spread out at the base due to the downdraught flow.

Observing precipitation from stratocumulus

Stratocumulus can produce precipitation with intensity varying from light to heavy showers. Stratocumulus is the main type of cloud to produce drizzle. In its patchy form, stratocumulus produces intermittent showers. The larger the cloud tops, the greater the potential for heavy showers. Since stratocumulus is normally faster moving than cumulus, showers are shorter in duration. In fact, like cumulus the duration depends on the amount of development, the size of the base area and the height of the clouds. Stratocumulus in sheets tend to produce more of the longer duration precipitation

Although stratocumulus can produce precipitation
, it is not necessarily responsible for the precipitation on a day with precipitation. Think of the situation where stratocumulus develops with more than one type of cloud such as cumulus or nimbostratus. The stratocumulus may be below the rain bearing cloud and not producing precipitation. In this case the stratocumulus will thicken and become darker in the precipitation cascade region . The observer must determine which the rain bearing cloud is in order to make predictions such as the direction of movement. The direction of movement of the rain bearing cloud is often different to that of the stratocumulus. This is often the case with thunderstorms

Observing precipitation from stratus

Stratus, being so thin, only produces drizzle or light showers. As in the case of stratocumulus, stratus often lies below the precipitation bearing cloud and becomes darker and thicker within the region of the precipitation cascade.

Observing precipitation from altocumulus

As discussed in the section on clouds, altocumulus occurs in various forms. Therefore there are various situations where precipitation exists. Precipitation from altocumulus varies from light to moderate. Heavy precipitation from altocumulus normally relates to the existence of larger cumulus or cumulonimbus clouds above the altocumulus
In normal circumstances, precipitation is light to moderate rain rather than showers. Because altocumulus is higher than lower level clouds, there are more ice crystals and hence, larger droplets. The thicker the vertical structure of the altocumulus, the more precipitation that can be produced

•  Because altocumulus has a patchy appearance and structure, rainfall from altocumulus is often intermittent and variable. If precipitation from altocumulus is associated with rapid development such as during unstable conditions, it will also develop rapidly. Often under these circumstances, larger cumulus develops with heavier precipitation. If altocumulus development occurs at the rear, precipitation associated with this development will persist for a lot longer but again intermittent.
• The precipitation cascades are usually well defined since light can pass through the breaks. However, the more widespread and thicker the altocumulus cover, the lower the contrast of the precipitation cascade and hence the harder it is to see.

Observing precipitation from altostratus and nimbostratus

Altostratus produces light to moderate precipitation, usually rain. Like altocumulus, it consists of ice crystals which results in larger droplets. Precipitation can vary from intermittent, patchy rain to persistent rain or showers. However, we normally associate persistent rainfall with nimbostratus. Altostratus will increase and thicken before precipitation develops. It then becomes known as nimbostratus.

•   Precipitation from nimbostratus is usually of long duration ranging from a few hours to a few days. In the latter case, a large cloud mass is normally responsible.
•  The more sunlight that can pass through the altostratus or nimbostratus cloud, the lower the contrast of the precipitation cascade. Such days are usually dull with the precipitation cascades poorly defined.
• Like altocumulus, precipitation can and does develop continuously from nimbostratus. However, development from the rear is not as pronounced since altostratus already exists and just thickens to form an overcast nimbostratus cover.
Observing precipitation from cirrus, cirrostratus and cirrocumulus

Cirrus, cirrostratus and cirrocumulus do not produce any rain.
However, on days with thunderstorm activity, dissipating thunderstorms continue to produce precipitation even when the top (or thick cirrus) section of the thunderstorm remains. Precipitation here usually consists of medium to large drops and gradually decreases. The rain area may occur from the rear of the cloud and hence rainbows are common with a lowering of the sun towards the horizon.

Observing past rainfall

There are several observations that indicate rain has occurred. Water runoff along the sides of the roads indicate moderate to heavy rain within the past hour or so. This depends of course whether rain is still draining out from drainage pipes linked to households. Large puddles along the side of the road are an indication of previous moderate to heavy rain. Long periods of moderate showers can also create puddles.

Observing precipitation falling from clouds

·         The techniques used to observe clouds are just as appropriate in observing precipitation. In most cases, it is very easy to observe precipitation falling out of a cloud. Precipitation falls mostly from the base of clouds when saturation occurs within the cloud and the currents of air cannot support the rain drops. Therefore, precipitation must be observed from around the base of the cloud and reaching the ground in a continuous vertical curtain. It will normally appear in the form of a uniform, greyish shade or tinge. At this stage, we will refer to rainfall as the major form of precipitation.

•  The motion of the rain cascade will obviously be the same as the cloud producing it. However, do not get confused when several clouds are present. Usually, only one of the cloud types is responsible for the rain. This situation will be discussed later with the more advanced observational techniques.
•  Depending on the conditions within the cloud, the intensity and duration of the rainfall varies. The heavier rainfall consists of larger droplets. This means that the cascade of rainfall allows less light to pass through making it look a darker shade of grey as compared to light rain or drizzle
• Another important point to consider is the depth of this cascade. Does it extend to a thickness of many kilometres or is it just a thin band?  The more the depth of the bands of rain (precipitation), the darker it should look because it will allow less background light to pass through. Contrast between the adjacent surroundings will be pronounced. Where there are no areas of precipitation, more clouds will be visible in the background.
•  Let us assume that an approaching rain band consists of uniform rainfall (equal rate of rainfall). If you observe such a rain band at different angles, in other words, you observe several areas almost simultaneously, you will find areas to the left and right of your perpendicular line of sight may appear darker. This occurs because diagonally there is more depth of rainfall, and hence less light penetrates
• Rain bands do not necessarily approach perpendicular to your line of sight. Quite often they approach at an angle, with nearby areas receiving the first rain. Therefore, it is essential to observe the parent cloud producing the rain to determine if the rain is approaching you and which part.
•  With practice in the techniques discussed in this section, you will be able to predict the intensity and extent of the approaching rainfall

Virga

Virga (aka fallstreak, fall-stripe) are generally streaks of rain or snow appearing to hang under a cloud or tapering down from the cloud base, descending and evaporating before reaching the ground. The name of this supplementary cloud feature derives from the Latin virga meaning 'rod' or 'stripe'. Virga (vir) are especially good to observe during sunrise or sunset when the back-lighting from the sun illuminates the reddish stringy trails.

Virga is accessory clouds associated with cumulus, cumulonimbus, altocumulus, stratocumulus, cirrocumulus, nimbostratus, and altostratus. However, they may not be confused with the supplementary cloud feature precipitation -
visible rain, drizzle, snow, and hail or ice pellets actually reaching the ground.

O ar, ao elevar-se, expande-se e, como conseqüência, se resfria, continuando a expandir- se e a resfriar-se enquanto se eleva. Ao atingir determinado nível, o ar seco pára de subir, por estar suficientemente frio. Só continuaria a se elevar se houvesse uma fonte de energia a aquecê-lo nesse novo nível de altitude. No ar seco não há fonte de energia, mas no ar úmido há uma forte fonte de energia, que é o calor latente armazenado no vapord’água contido no ar. Então, uma das condições favoráveis para intensificar o processo  convectivo é o ar estar bastante úmido. Com a intensificação da convecção, formar-se-ão nuvens de grande desenvolvimento vertical, às quais estão associadas tempestades e trovoadas.

As tempestades isoladas ocorrem associadas às nuvens cumulonimbus. As trovoadas, manifestações características de nuvens cumulonimbus, ocorrem durante todas as estações do ano nos trópicos e do final do inverno até o outono nas zonas temperadas.
As trovoadas formam-se, em geral, sobre grandes áreas líquidas, com maior probabilidade de ocorrerem à noite (devido à pouca estabilidade do ar sobre a água neste período), ou ao longo das frentes frias. Nas linhas de instabilidade (associadas com a
ruptura da dianteira de frentes frias, que se manifestam como uma súbita rajada de vento e uma violenta instabilidade, provavelmente com granizo e trovão) formam-se cumulonimbus e trovoadas devido a efeitos locais.
Para que haja formação de cumulonimbus e trovoadas, é indispensável uma intensa convecção, com fortes correntes verticais. Estas correntes verticais originam-se por um ou mais dos seguintes processos:

• influência topográfica;
•  convergência; e
•  aquecimento do ar próximo ao solo.
Tais processos permitem classificar as trovoadas, de acordo com a forma pela qual
a ascensão inicial do ar é realizada, em:

•  De frente quente;
•  de frente fria;
•  pré-frontal (linha de instabilidade); e
• de frente oclusa.
• Convectivas;
•  orográficas; e
Ocorrem como resultado de correntes verticais ascendentes criadas por uma frente ou por movimentos a ela associados.
As trovoadas de frente quente normalmente ocorrem quando o ar quente e úmido se superpõe à massa de ar frio e instável.
As trovoadas de frente fria ocorrem próximas à superfície frontal. Uma linha contínua e paralela à superfície frontal é a característica distintiva. Devido ao fato da maioria das trovoadas ser visível, elas são fáceis de serem reconhecidas enquanto a frente está seaproximando de qualquer direção. As bases das trovoadas de frente fria são normalmente mais baixas que as do tipo de frente quente. Elas são mais ativas durante a tarde e, geralmente, mais violentas que as do tipo de frente quente.
São trovoadas que ocorrem com oclusões do tipo frente fria e do tipo frente quente e são semelhantes às das outras frentes, com menor extensão e tempo menos severo. As trovoadas de frentes oclusas são associadas mais freqüentemente com a oclusão do tipo frente quente.
Como no caso da trovoada de frente quente, as trovoadas de frente oclusa são quase sempre envolvidas por nuvens estratiformes e dão pequeno, ou nenhum, aviso de sua presença.

Apresentam duas características básicas:
1.  Formam-se, normalmente, no interior de uma massa de ar quente e úmida; e
2.  geralmente, são isoladas ou esparsas sobre uma grande área.
As trovoadas convectivas ocorrem com maior freqüência que qualquer outro tipo de trovoada de massa de ar. Elas ocorrem sobre terra ou água, na maior parte das áreas do globo, sendo muito comuns nas zonas temperadas durante os meses de verão. Quando o processo convectivo ocorre na área marítima, observa-se a importância da contribuição da umidade para intensificação da convecção. Em vista disso, constata-se o extraordinário desenvolvimento convectivo que ocorre na Zona de Convergência Intertropical (ITCZ) e na região tropical marítima. A fabulosa energia que sustenta os furacões provém da umidade do ar marinho, razão pela qual eles enfraquecem e se dissipam ao penetrarem no continente. Sobre os oceanos, as trovoadas convectivas ocorrem mais no inverno e nas horas avançadas da noite, quando o aquecimento da superfície do mar durante o dia é pequeno e insuficiente para formar correntes convectivas fortes. Durante a noite, a superfície do mar e o ar inferior úmido se resfriam aos poucos, enquanto que o ar superior se resfria rapidamente pela radiação. A diferença de temperatura se torna maior à noite e, conseqüentemente, as razões de variação de temperatura necessárias à convecção se tornam mais freqüentes neste período, originando correntes convectivas fortes e trovoadas.
As trovoadas orográficas se formam quando o ar úmido e instável é forçado a ascender por terrenos montanhosos. A saraiva é comum nestas trovoadas, quando elas se desenvolvem ao longo de encostas de montanhas elevadas.
As trovoadas advectivas ocorrem quando há advecção (movimento horizontal do ar) de ar frio sobre áreas quentes (quase sempre correntes marítimas quentes), estando o ar instável ou condicionalmente instável. O ar frio sobre as águas aquecidas tenderá a ter sua camada inferior, mais próxima da superfície, igualmente aquecida, o que dá início à formação da trovoada. Sob certas condições, também pode ocorrer advecção de ar quente e úmido sob uma atmosfera instável ou condicionalmente instável, e isto acarretará o início da formação. Essas trovoadas acontecem à noite e, por isso, são também chamadas de noturnas.

Vento: os cumulonimbus provocam ventos em rajadas, variando em direção, e sua intensidade pode atingir 40 a 80 nós de velocidade. Quando a chuva pára, os ventos tornam-se fracos e com a direção acompanhando o sistema de pressão predominante na área.
Precipitação: à medida que as gotas d’água ou cristais de gelo que compõem as nuvens vão aumentando de tamanho, elas começam a cair rapidamente e atingem o solo em forma de precipitação, salvo quando retidas por correntes ascendentes ou evaporadas durante a queda. A precipitação adquire diferentes formas (granizo, saraiva, precipitação em forma de pancadas fortes, moderadas e fracas), dependendo da temperatura na qual ocorra a condensação e das condições encontradas durante a queda das partículas em direção ao solo.
Granizo: grãos de água congelada, semitransparentes, redondos ou cônicos. Cai apenas durante a trovoada, e constitui um dos perigos dos cumulonimbus, porque a intensidade de seu impacto sobre as embarcações e aeronaves é capaz de causar danos às mesmas.
O radar pode mostrar áreas de granizo, que devem ser evitadas. As áreas que contêm granizo normalmente apresentam coloração esverdeada.
Saraiva: precipitação em forma de pedras de gelo mais ou menos ovais, variando em diâmetro de 5 a 50 mm, ou mais. É composta de gelo vidrado ou de camadas opacas e claras alternadamente. É encontrada, ocasionalmente, no ar claro próximo à trovoada. Nas trovoadas tropicais e subtropicais, a saraiva raramente alcança o solo. Ocorre nas Latitudes médias e altas.
Precipitação em forma de pancadas (aguaceiros): precipitação em que a intensidade aumenta ou diminui com interrupções regulares, cujos períodos são sempre maiores que os períodos de precipitações. O início de qualquer precipitação é usualmente
• abaixamento da base da nuvem; e
• abaixamento do nível de congelamento.
Visibilidade: os fenômenos associados às nuvens cumulonimbus afetam a visibilidade, reduzindo-a, em geral, para 1 a 2 km.
Estado do Mar: as nuvens cumulonimbus produzem rajadas de vento e intensa precipitação de duração entre 15 e 30 minutos. O estado do mar durante a precipitação,devido aos ventos em rajadas que atingem de 34 a 40 nós de intensidade, poderá apresentar ondas de 3 a 4 metros de altura. Quando ocorrem vagalhões moderados, as cristas que se formam quebram em borrifos e a espuma é espalhada em faixas bem definidas, na mesma direção do vento.
Trombas-d’água e tornados: fenômenos já mencionados no Capítulo 42; a nuvem afunilada de uma tromba-d’água se forma associada com uma trovoada e, quando atinge a superfície líquida do mar, capta a água violentamente.
Turbulência: por definição, é a agitação vertical das moléculas de ar. Esta agitação provocará um vôo desconfortável, pois a aeronave tem sua altitude alterada seguidamente, o que provoca variações em sua sustentação. A turbulência pode, também, tornar o controle da aeronave muito difícil e, em casos extremos, resultar em avaria estrutural.
A turbulência na atmosfera é classificada como leve, moderada, forte e severa.
A turbulência no ar atmosférico é causada por vários fatores:
• Térmico;
•  frontal;
•  mecânico;
• cortante do vento em grande escala; e
• produzida pelo homem.
A turbulência térmica é causada pelo maior aquecimento da superfície da Terra, enquanto a turbulência frontal é provocada pela chegada de uma frente. Uma região turbulenta deve ser evitada, sempre que possível, pois é no interior ou nas proximidades de
nuvens cumuliformes que surgem as maiores dificuldades aos aeronavegantes.

Thunderstorms

Thunderstorms are local storms accompanied by lightning and thunder and a variety of weather phenomena, such as heavy rain, hail or - in winter - snow, high winds and sudden temperature changes. Thunderstorms originate when intense heating causes a parcel of moist air to rise from the earth's surface into upper levels of the atmosphere, a process called convection. Thunderstorms are therefore also known as convective storms.

At any given moment, it is estimated there are 2000 thunderstorms in progress around the world. They occur most frequently in the tropics but are also common in the mid-latitudes.

Thunderstorm ingredients:
• Thunderstorms need an ample supply of moisture, preferably in the lower and mid-levels of the atmosphere, as they are mainly powered by latent heat released as water vapour condenses.
• Thunderstorms need unstable air, a temperature profile with warm air near the ground and cold air aloft. When an air parcel is given an initial push upwards it will continue rising without additional force. Thus thunderstorms are more likely in the spring and summer than in the fall and winter. The sun warms the ground, which warms the air near the ground. In spring the air aloft retains its winter cold and thus will be more unstable than in the fall when the air aloft retains its summer warmth.
• Thunderstorms need a source of lift. This can be (1) differential heating when air near the ground is warmer than in upper levels, (2) orographical effects when air has to rise to pass a mountain ridge, (3) frontal boundaries when air masses of a different temperature clash, (4) drylines when air masses with differing humidity but similar temperatures clash and (5) Land/Sea breezes. Thunderstorms can get started by even faint air boundaries and thus sometimes seem to pop up out of the blue sky.
• Thunderstorms are often accompanied by severe weather and lightning is among the biggest weather killers. However, less then one percent of all thunderstorms produce hail bigger than the size of a golf ball and/or strong downburst winds. Only a small fraction of severe storms actually produce tornadoes or waterspouts.
• No place in Europe is completely immune from the threats of thunderstorms. Severe weather can strike at any place, and at any time. Thunder and lightning occur simultaneously but thunder is heard later than lightning is seen, as light travels faster than sound. A good measure of distance from a storm is 1 mi (1.6 km) for every 5 seconds between flash and thunder.
Thunderstorm Probability

The Lifted Index is a measure of atmosphere's stability (or instability) and Meteorologists use it to determine the thunderstorm potential. It doesn't accurately predict the intensity of every single storm, but it is a useful tool to estimate the atmosphere's potential to produce severe thunderstorms

Parcels" (or bubbles) of air start to rise on their own if they are warmer than the surrounding air.
• This process is called convection. Consider an air parcel as it begins to rise through the atmosphere after being heated by the sun and the warming ground.
• The Lifted Index is defined as a rising parcel's temperature when it reaches the 500 millibars level (at about 5,500m or 18,000 feet asl), subtracted from the actual temperature of the environmental air at 500 millibars.
• If the Lifted I index is a large negative number, then the parcel will be much warmer than its surroundings, and will continue to rise.
• Thunderstorms are fuelled by strong rising air, thus the Lifted Index is a good measurement of the atmosphere's potential to produce severe thunderstorms.
Conditions necessary for thunderstorm development

There are many synoptic situations that exist where thunderstorms will develop. Thunderstorms can develop ahead of, with, or behind the passage of cold fronts.(There are no specific patterns that exist to suggest thunderstorms of certain intensities develop with certain types of synoptic conditions).Thunderstorms can also develop with lows or troughs of low pressure usually in the eastern section. Other situations that favour development of thunderstorms are upper ai disturbances, cold pools of air in the upper atmosphere and trough lines which can exist ahead of cold fronts or in a trough of low pressure. These synoptic situations provide the necessary atmospheric conditions where thunderstorms can develop. Uplift is the flow of air from the lower layers towards the upper layers. Types of uplift include >unstable air, cold fronts acting as wedge for warm air, cold air in the upper atmosphere or orographic forcing as air meets a mountain range. Sometimes more than one form of uplift can occur simultaneously. It requires a minimum level of uplift for cumulus to develop further into thunderstorms.

Types of Thunderstorms
• single cell,
• multi cell clusters,
• multi cell lines
• and super cells
Single cell,

Single cell storms typically do not produce severe weather and usually last for 20-30 minutes. Also known as pulse storms, single cell storms seem quite random (perhaps because of our lack of understanding) in the production of brief severe events such as downbursts, hail, some heavy rainfall, and occasional weak tornadoes.

This is a single cell storm, looking east from about 15 miles. The storm was moving east (into the photo). Some of the anvil cloud has been left behind the storm, but the greater portion of the anvil is blowing off in advance of the storm and is not observable from this perspective.
(May storm in the Texas Panhandle near Amarillo.)

Evolution of a Single Cell Storm
Typical lifespan less than one hour

The upper sequence depicts the life cycle of a non-severe single cell storm in weak wind shear, with white cloud shapes and gray shades of progressively heavier radar reflectivity. Note the quick collapse of the rainy downdraft through the updraft. The bottom sequence depicts the radar history of the severe pulse storm. Note that the initial radar echo in the pulse storm develops at higher levels than in the non-severe single cell storms. Stronger radar reflectivities aloft with the pulse storm cascade down, resulting in a quick burst of severe weather, possibly hail but more likely downbursts, just before storm dissipation.

• This is another single cell storm with tops near 40,000 feet, but on a day with virtually no vertical wind shear. Contrast the height of the cloud base with that in the previous photo. This storm had a much higher base, about 1/5 of the way to the storm top, or approximately 8,000 feet above the ground. The temperature and dew point temperature on this August day were 102 and 61, respectively. The low-based storm in the previous photo occurred with values of 94 and 74.
• Storms that occur with 30 to 50 degree surface temperature/dew point spreads have relatively high microburst potential. (This is not to say that environments without such huge spreads will not produce microbursts!) This combination of surface observations and high-based Cbs should serve as "red flags" to pilots and aviation weather forecasters. Several short-lived storms that occurred in the Fort Worth area on this day produced microbursts.
Multicell clusters

Multicell severe weather can be of any variety, and generally these storms are more potent than single cell storms, but considerably less so than supercells. Organized multicell storms have the higher severe weather potential, although unorganized multicells, which are simply conglomerates of single cells, can produce pulse storm-like bursts of severe events.

Components of Multicell Clusters
Moderate dangers with some severe risk

The close proximity of updrafts within the multicell cluster storm results in updraft competition for the warm, moist low-level air. Thus, updrafts never attain extremely strong vertical velocities and each has a short life span when compared to a supercells updraft. Naturally, multicell severe weather usually is less intense than that from supercells, but still can be quite potent, with marble to golf ball size hail and 60 to 80 MPH winds not uncommon.

Development of Multicell Cluster Storms
The flanking line and varying sheared environments

A multicell cluster storm, the most common of the four basic storm types, evolves as an organized sequence of cells in various stages of development and decay at any given time. When multicell storms form in environments with winds which veer from southerly to westerly and increase with height, new updraft development usually occurs in the upwind (usually southwest) quadrant of the complex, with older cells decaying in the downwind quadrant.

Multicell Clusters from Different Perspectives
Viewing from the northeast and southeast

We are looking northeast from about 15 miles, along the axis of the flanking line into this multicell storm. Note the several "humps" of multicellular Cb top embedded in the anvil.

Evolving Storm
An unusually severe multicell cluster storm

This is how some multicell cluster storms will appear as they approach, again assuming good visibility. The ominous shelf cloud, appearing like a moustache with this storm, is the leading edge of the storm outflow. Observe the rain-free updraft bases ahead of and above the shelf cloud.

The storm was unusually severe, packing hail from 1 to 3 inches in diameter and 70 MPH winds. Most of the hail was from 1/2 to 1 inch in diameter. Looking east, note the steam fog arising from the fresh hail fall, as the storm ends. From the backside we watch as the same storm cluster moves away to the east. Observe the south-eastward-tilt of the clouds in the short flanking line and the precipitation area to the east. The flow aloft was from northwest to southeast (rather than southwest to northeast), influencing the tilt of the storm tops.
• It is curious that this storm showed updrafts on the leading (east) edge as it approached, and on the back (northwest) side as it moved off. The storm was definitely multicultural, although not as "clear-cut" about preferred updraft locations as other multicell storms we have viewed. Again, nature does not always allow us to label and catalogue everything neatly!
• Concerning storms in northwest flow aloft, it has been observed that the updraft area frequently shifts to the southeast flank, when rain-cooled air keeps warm, southerly winds from providing a continual feed to the northwest flank updrafts. Thus, with this storm it is possible that the leading (southeast flank) updraft area became predominant once heavy precipitation began, with the northwest updraft area no longer benefiting from the "prime" air.

Multicell Lines
Also known as squall line
•  Multicell line storms consist of a line of storms with a continuous, well developed gust front at the leading edge of the line. An approaching multicell line often appears as a dark bank of clouds covering the western horizon. The great number of closely-spaced updraft/downdraft couplets qualifies this complex as multicellular, although storm structure is quite different from that of the multicell cluster storm.
• Also known as squall lines Multicell line storms consist of a line of storms with a continuous, well developed gust front at the leading edge of the line. An approaching multicell line often appears as a dark bank of clouds covering the western horizon. The great number of closely-spaced updraft/downdraft couplets qualifies this complex as multicellular, although storm structure is quite different from that of the multicell cluster storm.

•  Multicell line storms are better known as squall lines, which is the term that we will use from here on. The former name is for positioning squall lines in the thunderstorm spectrum.
• Squall lines most frequently produce severe weather near the updraft/downdraft interface at the storm's leading edge. Downburst winds are the main threat, although hail as large as golf balls and gustnadoes can occur. Flash floods occasionally occur when the squall line decelerates or even becomes stationary, with thunderstorms moving parallel to the line and repeatedly across the same area.

• Squall lines with a confirmed severe weather history allow for the issuance of reliable warnings. Pilots should be extremely cautious, as they should for all thunderstorms, particularly near the squall line's leading updraft/downdraft interface
Squall Lines

• The precipitation downdraught associated with an individual cell tends to be concentrated towards the leading edge of the storm where the cold heavy outflow spreads out at ground level forming a small high pressure cell, a meso-high, 10 – 15 nm across. The dense air lifts the warmer, moist air in its path and may initiate a self amplifying convective complex, in which neighbouring storm cells consolidate into a towering squall line of large thunderstorm cells ranged across the prevailing wind direction. At locations in the path of the squall line the resultant line squall occurs as a sharp backing in wind direction, severe gusts, temperature drop, hail or heavy rain and possibly tornadoes. If the squall line is formed in an environment of strong mid-level winds the surface gusts may exceed 50 knots
• Squall lines vary in length, some of the longest being those which develop in a pre-frontal trough 50 -100 nm ahead of a cold front. These squall lines may be several hundred nautical miles in length and 10 – 25 nm wide moving at typically 25 knots. The pre-frontal lines form ahead of the front as upper air flow develops waves ahead of the front; downward wave flow inhibiting and upward wave flow favouring, uplift.
• During daylight hours the squall line may appear as a wall of advancing cloud with spreading cirrus plume but the most severe effects will be close to each of the numerous Cb cells. The convective complex releases a tremendous amount of latent heat and moisture which may be sufficient to generate a warm core mesoscale cyclone lasting several days
On Satellite Images
Squall lines and mesoscale convective systems

This infrared satellite view of an eastward moving squall line, extending from the Ohio River Valley south-westward into Louisiana, shows the extreme lengths that thunderstorm lines can achieve. The lower, warmer anvils on the north end of the line and the colder, higher cloud tops at the south end reveal the tendency for older, weakening storms to be shed on the north side of the line with newer and stronger development near the south end.

Supercells Thunderstorms

Thunderstorms with deep rotating updrafts

• The last of the four major storm types is the supercell. We define a supercell as a thunderstorm with a deep rotating updraft (mesocyclone). In fact, the major difference between supercell and multicell storms is the element of rotation in supercells. As we shall see, circumstances keep some supercells from producing tornadoes, even with the presence of a mesocyclone.
• Supercells are long-lived thunderstorms which exhibit quasi-steady structure including a rotating updraft. These storms generally produce severe weather including heavy winds, large hail, heavy rainfall, and occasionally tornadoes. In fact it is these supercells that produce the strongest and longest-lived tornadoes.
• With the danger that supercells pose, it is wise to learn more about the nature of their origin and evolution. By discovering how supercells behaviour is related to the surrounding environment, meteorologists can help predict when and where such storms will actually occur -- with the ultimate goal of saving lives. While real supercells like the one in the photograph above continue to occur, computer model visualizations like the one below are being used to advance our understanding and prediction of these terrible storms. These storms tend to develop during the afternoon and early evening when the effects of heating by the sun are strongest

In summary, supercells are extremely dangerous, but excellent warnings are possible once the storm has been properly identified. The demarcation between supercell and multicell storms is most important, obviously much more so than that between single cell and multicell storms, or between multicell and squall line storms. As mentioned earlier, it has been suggested that thunderstorms simply be classified as "supercells" and "ordinary" storms. A few supercells will have the updraft located on the leading southeast (or east) flank, as we shall see in the section, Supercells Variations

Schematic Diagrams
Horizontal cross-section and westward view

This is a horizontal, low-level cross-section of “classic” supercells. The storm is characterized by a large precipitation area on radar, and a pendant or hook-shaped echo wrapping cyclonically around the updraft area. Note the position of the updraft and the gust front wave. The intense updraft suspends precipitation particles above it, with rain and hail eventually blown off of the updraft summit and downwind by the strong winds aloft. Updraft rotation results in the gust front wave, with warm surface air supplying a continual feed of moisture to the storm.

• Updraft rotation occurs when winds through the troposphere are moderate to strong, and low-level turning is significant. As inflow air in the lowest 1-3 kilometres approaches the storm from the south or southwest, the low level turning results in the development of rotation about a horizontal axis. As the air is lifted into the updraft, the rotation is "tilted" to that about a vertical axis. To see this rotation about a horizontal axis caused by wind shear, imagine rolling a tube along a table-top with the palm of your hand.
• The movement of your hand represents the strong winds above the surface, producing rotation because the winds near the ground are much weaker. This simple picture is complicated by the turning of the wind direction with height, but the concept remains similar. Lifting this "horizontal" vortex into the updraft results in cyclonic rotation.
• A westward view of the classic supercells reveals the wall cloud beneath the intense updraft core and an inflow tail cloud on the rainy downdraft side of the wall cloud. Wall clouds tend to develop beneath the north side of the supercells rain-free base, although other configurations occur.
• Observe the nearly vertical, "vaulted" appearance of the cloud boundary on the north side of the Cb and adjacent to the visible precipitation area. A sharp demarcation between downdraft and rotating updraft results in this appearance. Note the anvil overhang on the upwind (southwest) side of the storm and the overshooting top, both visual clues as to the intensity of the updraft
Overshooting Tops
Indicative of powerful updrafts

Looking east from about 40 miles away, we see a line of towering cumulus clouds and a large supercells storm in the background. Note the great amount of anvil overhang and the large overshooting dome at the summit of the updraft.

• Distant supercells frequently have this domed, "diffluent" anvil appearance, with the supercells tremendous updraft velocities and outflow resulting in marked upper-level divergence. The visual clues are strong, although we cannot be sure that this is a supercell simply from appearance. By necessity, man and machine (i.e., spotters and radar) complement each other in the severe weather detection program. This storm produced hail but no known tornadoes in eastern Oklahoma.
• This supercells featured a rock-hard, overshooting Cb top and anvil overhang, looking southeast from about 40 miles away. Note that the supercells Cb is more vertically oriented than the weaker updraft of the neigh boring towering cumulus cloud. This is a valuable clue in estimating the strength of updrafts on a day with strong vertical wind shear. This storm produced baseball hail, but no known tornadoes, along a track in southeast Oklahoma and southwestArkansas
Rotating Updrafts
Visual clues

There are ample signatures of updraft rotation in this hazy, northeastward view of a very intense supercell from 40 miles away. The circular mid-level cloud bands and the smooth, cylindrical Cb strongly hint of updraft rotation. Above the mid-level cloud band, an extremely hard Cb top is barely visible (upper right) towering into the anvil. Note the smooth, "laminar" flanking line on the extreme left. A strong, "capping" temperature inversion in the low levels probably accounted for the laminar appearance of the flank.

A close, westward view of a supercells updraft and adjacent precipitation cascade strikingly resembles the model we have just seen. Wall clouds frequently slope downward towards the precipitation area, as shown. If you are a mobile spotter and encounter a view such as this, turn around and out-run the storm by going eastward or, better yet, move away from the storm to the southeast. This is very close to the fall area of large hailstones and moving north or waiting at this location will put you in danger from large hail and tornadic winds.

High Precipitation (HP) Supercells

Very heavy rainfall, possible large hail, downbursts and tornadoes

In this photograph, the typical HP storm visual appearance is present: beaver's tail inflow bands curling into the front-flank updraft, a gray area of anvil precipitation to the north, and a dark rain and hail core to the southwest, falling from what earlier had been the rain-free base.

• In the HP stage, this storm produced large hail, gusty winds, and extremely heavy rainfall, as well as several funnel clouds. One of these is visible where the inflow bands intersect the updraft. Continuous lightning occurred with this storm, much of it in-cloud, but a sizable percentage being cloud-to-ground strikes. Indeed, HP supercells seem to be especially prolific producers of lightning
• Another HP supercells is pictured in the distant west. The characteristic inflow bands are present in front of a translucent, anvil-born precipitation area on the extreme right. Note the rotating, vaulted Cb adjacent to the anvil precipitation, and the dark precipitation shaft in the left-center, emanating from an area that would be visually rain-free in a classic supercells. This storm produced a 1/2 mile wide tornado shortly before this time. It later produced several smaller and weaker tornadoes. Many times when a tornado does form from an HP supercells, the southwest flank precipitation area literally wraps around the tornado, obscuring it from view.
Characteristics of (HP) Supercells
Radar features, weather events and severe events

Heavy precipitation supercells have some identifiable radar features, including "broad hooks" and/or large inflow notches on the east and southeast storm flank. A lemon technique tilt sequence will indicate a weak echo region (WER), overhang, and highest top in alignment on the leading flank. These storms will be quite difficult for spotters to handle because of both the lack of contrast between the updraft and surrounding rainy-downdraft areas, and lack of past training about these storms.

An HP storm in Fort Worth, Texas, produced almost 5 inches of rain within one hour, with most of the rain falling within 45 minutes. Some indications are that HP storms might be somewhat more frequent in the southeast U.S., but they do occur in most areas east of the Rockies. Quite important to storm spotters and severe weather forecasters is that HP supercells probably account for many of the "tornado embedded in rain" events, a phenomenon that occurs not only in the southeast but elsewhere, including the High Plains.

Westward View of HP Supercells
Precipitation curtain wraps around the west and southwest flanks

A westward view of a composite HP storm model shows the position of an inflow cloud band, very similar to the previously mentioned beaver's tail cloud in the classic supercell. In fact, the HP storm has an appearance similar to the classic supercell, except for the opaque precipitation curtain wrapping around the west and southwest flanks of the wall cloud and/or updraft.

• This is a westward view of an HP storm in extreme northeast Colorado. Cyclonically-curving inflow bands are visible in the upper portions of the photo, feeding into the updraft area. The storm had a very well-developed wall cloud, with precipitation wrapping around the north, west, and southwest flanks of the lowered cloud base. Note the subtle gust front and shelf cloud extending southward from the wall cloud.
• Spotters will have a difficult time with the HP supercell, since there can be poor visual contrast between the wall cloud and precipitation behind it. The strongest visual clues in identifying this type of supercell usually are the curving inflow bands and mid-level cloud bands which wrap around the updraft, both suggestive of storm-scale rotation. This dramatic storm produced large hail but no known tornadoes.

Flow Field of Tornadic HP Supercells
Inflow and outflow

A few HP storms do produce violent tornadoes. When they occur, the tornadoes often will be wrapped in precipitation and quite difficult to observe. The photographer heard a roaring sound, and ran outside where he had this westward view. Behind the super-imposed inflow arrows is a wall cloud, with a rain area and RFD wrapping from left to right around the wall cloud's southeast flank. This rain shaft is visual manifestation of the radar hook (an unusually "fat" radar hook in this case) wrapping around the wall cloud and developing tornado.

We are in a position of strong inflow, as noted by the northward-bending trees (in the image below). However, a gust front (blue, descending arrows) is accompanying the precipitation and approaching the photographic position. The brunt of the HP storm's precipitation area is out of the photo and to the right (northeast of the wall cloud). Still hearing a roaring sound, the photographer shifted his view a bit towards the northwest. Almost hidden behind the advancing rotating rain curtain is a large and devastating tornado!
The rain curtains that wrap around an HP supercell's tornado often change very quickly in appearance. Minutes later, the tornado is not quite as obscured by the precipitation. View these three slides a second time and observe the advance of the rain curtain and gust front. The tornado was continuing to receive a narrow corridor of inflow from the northeast at this time, as it approached Drumwright, Oklahoma.

Low Precipitation (LP) Supercells
Lacking in liquid rainfall content

At the opposite end of the supercell scale is the Low Precipitation (LP) supercell. For years, storm chasers have observed LP storms in the Plains' states, usually in conjunction with a dry line or low pressure trough dividing dry, warm air to the west from very humid air to the east. These rotating storms typically are quite small and lacking in liquid rainfall content.

This northward view of an LP storm in western Oklahoma shows both the small size and the powerful nature of the updraft. This storm was shrinking to an even smaller size at this time, which is how most LP storms meet their demise. Note that the updraft tower is scarcely any wider than the wall cloud. The storm earlier produced golf ball size hail and, although it rotated vigorously, it did not produce any tornadoes.
Low-precipitation supercells probably rarely occur, if at all, east of the Mississippi River. They frequently produce large hail, funnel clouds, and wall clouds, and occasionally spawn weak or even strong tornadoes. Radar identification of the storm as a supercell is difficult, especially at great range, because of the relatively small size and dry nature of the storm. Similar to the classic supercell, but unlike the HP storm, severe weather usually occurs in the southwest quadrant of the LP storm.

Flash Floods and Hail

Property and personal devastation Cases involving either slow-moving thunderstorms or a series of storms which move repeatedly across the same area (sometimes called train-echo storms) frequently result in flash flooding. The total number of flash flood deaths has exceeded tornado fatalities during the last several decades

Hurricanes
A tropical cyclone with winds > 64 knots

Hurricanes are tropical cyclones with winds that exceed 64 knots (74 mi/hr) and circulate counter-clockwise about their centers in the Northern Hemisphere (clockwise in the Southern Hemisphere)
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The Eye

The center of the storm

The most recognizable feature found within a hurricane is the eye. They are found at the center and are between 20-50km in diameter. The eye is the focus of the hurricane, the point about which the rest of the storm rotates and where the lowest surface pressures are found in the storm. The image below is of a hurricane (called cyclone in the Southern Hemisphere). Note the eye at the center
The eye is so calm because the now strong surface winds that converge towards the center never reach it. The Coriolis force deflects the wind slightly away from the center, causing the wind to rotate around the center of the hurricane (the eye wall), leaving the exact center (the eye) calm.

An eye becomes visible when some of the rising air in the eye wall is forced towards the center of the storm instead of outward -- where most of it goes. This air is coming inward towards the center from all directions. This convergence causes the air to actually sink in the eye. This sinking creates a warmer environment and the clouds evaporate leaving a clear area in the center

Spiral Bands
Where more rain is found

Movement of Hurricanes

Steered by the global winds

The global wind pattern is also known as the "general circulation" and the surface winds of each hemisphere are divided into three wind belts:

Polar Easterlies: From 60-90 degrees latitude.
Prevailing Westerlies: From 30-60 degrees latitude (aka Westerlies).
Tropical Easterlies: From 0-30 degrees latitude (aka Trade Winds).

The easterly trade winds of both hemispheres converge at an area near the equator called the "Intertropical Convergence Zone (ITCZ)", producing a narrow band of clouds and thunderstorms that encircle portions of the globe.

The path of a hurricane greatly depends upon the wind belt in which it is located. A hurricane originating in the eastern tropical Atlantic, for example, is driven westward by easterly trade winds in the tropics. Eventually, these storms turn north-westward around the subtropical high and migrate into higher latitudes. As a result, the Gulf of Mexico and East Coast of the United States are at risk to experience one or more hurricanes each year In time, hurricanes move into the middle latitudes and are driven north-eastward by the westerlies, occasionally merging with midlatitude frontal systems. Hurricanes draw their energy from the warm surface water of the tropics, which explains why hurricanes dissipate rapidly once they move over cold water or large land masses.

Thunderstorm Safety
Rapid falls in pressure can indicate the approach of severe thunderstorms.

A Severe thunderstorm WARNING is an urgent announcement that a severe thunderstorm has been reported or is imminent and warns you to take cover. Severe thunderstorm warnings are issued by local NWS offices.

The strong wind gusts of severe thunderstorms can damage buildings, knock down trees, and create a hazard due to wind-blown debris. Therefore:
• Seek shelter but avoid trees as these are targets for lightning.
• If indoors, stay away from windows and go to the safest location on the lowest level of your home.
• When boating, always stay tuned to the latest weather reports and return to a safe harbor before the strong winds arrive.
Barometers using mercury are heavy and fragile. The idea of "dry" barometer was conceived by Gottfried Wilhelm Leibniz around 1700. The idea was to detect pressure changes using sealed bellows. The first working version of an aneroid (without water) barometer was built in 1843 by French scientist Lucien Vidie.

This made the barometer very portable and it became commonly use meteorological instrument. It was still calibrated to the mercurial barometer with readings in inches of mercury. Even as late at the 1990s, National Weather Service offices still calibrated and verified the accuracy of the aneroid barometer with the mercurial barometer.

Procedure

1. Cover the top of the coffee can tightly with the plastic wrap, using the rubber band to hold it in place. (The cover should be a taut, airtight fit.)
2. Position the straw so that it lays across two thirds of the cover with the remaining length of the straw suspended over air. Tape in place.
3. At a 90° angle, fold one short end of the index card at about one inch from that end. Tape the folded end of the index card to the can behind the straw in such a way that allows you make marks on the card every day.
4. Record the level of the straw onto the card.
5. For the next 10 days, at the same time each day, record the level of the straw while paying close attention to how changes in the weather affect the straw's level.

Severe Thunderstorm Safety

• One measure of the severity of a thunderstorm is the wind speed. In addition to the size of hail, the National Weather Service defines a severe thunderstorm as one containing wind speed of 58 mph (50 kts) or greater.
• The force of all of the molecules moving at 58 mph (50 kts), or more, can create hazardous weather conditions such a blowing down phone and power lines, trees, and make driving hazardous. When the National Weather Service issues a Severe Thunderstorm Warning it means a thunderstorms with wind gusts to 58 mph (50 kts) or greater and/or hail size of 3/4" or greater is occur or about to occur near you.
•  Discuss severe thunderstorm safety with your family. Everyone should know what to do in case all family members are not together. Discussing disaster response ahead of time helps reduce fear and lets everyone know what to do should a severe thunderstorm occur.
• Postpone outdoor activities if thunderstorms are likely. Many people take shelter from the rain, but most people struck by lightning are not in the rain! Postponing activities is your best way to avoid being caught in a dangerous situation

Relampagos

Relâmpago pode existir em formas variadas. O tipo mais observado é o relâmpago ziguezague, quando um lider contínuo movendo-se para o solo desvia-se do trajeto original feito pelo lider escalonado. Isto causa o relâmpago parecer-se curvo. A foto acima é de uma descarga ziguezague para o ar, fotografado debaixo de uma trovoada super-célula de baixa precipitação. Relâmpago fita forma-se quando o vento move o canal ionizado entre cada descarga de retorno, causando o relâmpago parecer-se com uma fita suspensa da nuvem.

Relâmpago conta forma-se quando o canal de relâmpago quebra-se ou parece quebrar-se, causando o relâmpago parecer-se como uma série de contas amarradas numa corda. A causa desta forma de relâmpago não é conhecida. Entretanto, uma teoria propos que isto ocorre quando a descarga de relâmpago é parcialmente interceptada por nuvens ou chuvas. Relâmpago lampejo é o clarão súbito da nuvem e do céu, quando a trajetória de descarga não é vista. Este caso forma-se quando uma descarga ocorre dentro de uma nuvem ou outras nuvens intercepta o flasho.

Fogo de St. Elmo é uma forma de eletricidade vista mais sobre os topos de objetos altos. Estes podem brilhar porque as cargas positivas permanecem e as cargas negativas desaparecem quando a trovoada passa acima. Torres, picos de montanhas e também chifres de gados as vezes experimentam este tipo de atividade. O topo de uma trovoada pode brilhar com o Fogo de St. Elmo quando as cargas positivas se concentram ali. Pilotos de aeronaves frequentemente observam o Fogo de St. Elmo se eles voam através de trovoadas. Relâmpagos que atingem aeronaves podem preceder o Fogo de St. Elmo por cinco minutos ou mais.

Relâmpago bola consiste de uma esfera luminosa de 1 cm a 2 m em diâmetro que normalmente ocorre perto das trovoadas. Elas podem cair do céu e explodir ou rolar a ribanceira até colidir com algum objeto e explodir. Elas tem a aparecência de uma bola elétrica e são usualmente vermelhas, amarelas, azuis, laranjas ou brancas. Relâmpagos bola tem entrado em casas pelas janelas ou tomadas elétricais e fluem ou rolam nos comodos. Foi reportado que uma esfera luminosa de meio metro de diâmetro rolou no corredor de encontro a uma pessoa que saiu do caminho para ela passar. A causa deste forma de relâmpago remanesce uma enigma

Red sprites and blue jets

• Relâmpagos produzem fenômenos transientes na atmosfera superior acima de trovoadas. Estes fenômenos são fracas luzes quase invisíveis ao olho humano. Reportagens destas emissões existem a mais de um seculo, com algumas observações de pilotos. Reportagens iniciais referiram-se a estes eventos por vários nomes, incluindo "descargas ascendentes," "descargas nuvem-estratosfera" e "descargas nuvem-ionosfera." As primeiras imagens foram capturadas por veículo espacial nos 1990's. Depois disto, milhares de imagens foram capturadas pelas aeronaves e câmaras sensíveis no solo. Intensos esfoços experimentais e teoricos estão ocorrendo para determinar como estes fenômenos formam uma parte do ambiente terrestre elétrico.
• Sprites são flashos luminosos massiços e fracos que aparecem diretamente acima de trovoadas e são relacionados com descargas nuvem-solo e intra-nuvem. As suas estruturas são poucas e isoladas ou multiplas pintas verticalmente alongadas que estendem-se dos topos das nuvens para altitudes de quase 95 km. Sprites são geralmente vermelhos e raramente ocorrem isolados, mas em grupos de dois, três ou mais. Evidencias neste momento sugestem que sprites ocorrem nas áreas decadentes de trovoadas ativas e duram apenas poucos milisegundos. Sprites são quase invisiveis aos olhos humanos, mas com imagens intensificadas de televisões obtidas do solo ou aeronaves, eles aparecem como fantásticas estruturas complexas que assumem formas variadas
• When large cloud to ground lightning discharges occur below an extensive Cb cluster, which has a spreading stratiform anvil, other discharges are generated above the anvil. These discharges are in the form of flashes of light lasting just a few milliseconds and probably not observable by the untrained, naked eye but readily recorded on low light video.
• Red sprites are very large but weak flashes of light emitted by excited nitrogen atoms and equivalent in intensity to a moderate auroral arc. They extend from the anvil to the mesopause at an altitude up to 90 km. The brightest parts exist between 60 – 75 km, red in colour and with a faint red glow extending above. Blue filaments may appear below the brightest region. Sprites usually occur in clusters which may extend 50 km horizontally. Blue jets are ejected above the Cb core and flash upward in narrow cones which fade out at about 50 km. These optical emissions are not aligned with the local magnetic field

St. Elmo's fire

• St. Elmo's fire is a plasma (i.e. a hot, ionized gas) that forms around the tips of raised, pointed conductors during thunderstorms. It is known as a corona discharge or point discharge to physicists. The few people that have had the privilege of viewing an actual St. Elmo's fire have given various descriptions. It has been seen with different physical characteristics depending on the conditions of the viewing. It could be blue to bluish-white, silent to emitting a hissing sound, and ghostly to solid.
• St. Elmo's fire occurs during thunderstorms - generally after the most severe part of the storm has passed - when the air reaches a very high voltage. These conditions are necessary to accumulate a charge large enough to create the phenomenon. It is always found attached to a grounded conductor with a sharp point; the most common are masts of sailing ships, church steeples, airplane wings or propellers, or even horns of cattle. The non-attached version of St. Elmo's fire is known as Ball Lightning
• < outward from thunderstorms Thunderstorm winds also cause widespread damage and occasional fatalities. Thunderstorm
"straight-line" winds originate from rain-cooled air that descends with accompanying precipitation.

Thunder Rumbles

• When a lightning bolt flashes through the sky we see it instantly. The following thunder, the sound of the lightning, takes a few seconds longer to reach us, as light reavels much faster than sound. Thunder never just goes "Boom!" and then stops, rather one hear a loud clap followed by several seconds of rumble. Why does thunder rumble? Lightning heats air to more than 20,000°C, much hotter than the surface of the sun is. The sudden heating causes the air to expand as the flash passes trough the atmosphere and immediate cooling contracts the air again. Quick expansion and contraction of air around lightning starts air molecules moving back and forth, creating sound waves. As this is happening extremely fast we will hear the 'clap' of a thunder.
• Now imagine a lightning bolt. They can be several miles long, no matter if it is a cloud to cloud or cloud to ground discharge. Let’s say the nearest part of the bolt is one mile away. Sound travels about a mile in 5 seconds, thus you will hear the first part - the clap of the lightning - five seconds after the flash. If the farthest part of the bolt is three miles away, it will take 15 seconds to hear that part of the bolt. From second 5 to second 15 after the flash you will hear every different part (clap) in between, resulting in a rumbling sound. The end of the rumble is the farthest part of the bolt.
• Often the thunder gets softer, then louder, then softer, and so on. It makes sense that as one hear parts of the lightning that are farther away that the sound would get softer, just as other sounds are louder when they are near and softer when the source of the sound is farther away. This has to do with the shape of a lightning bolt. Lightning bolts are not straight, rather they "zig zag" forth and back in different angles, towards and away from the observer point.

Aspectos importantes dos parâmetros meteorológicos:
• (a) A temperatura do ar e a umidade indicam as propriedades da massa de ar presente e sua alteração brusca pode ser a chegada de uma frente com outra massa de ar;
• (b) a pressão atmosférica indica o grau de aquecimento da superfície e o comportamento da temperatura do ar e, portanto, as características da massa de ar presente. Uma alteração brusca da pressão pode significar a chegada de outra massa de ar;~
• (c) a TSM associada à informação da temperatura do ar indica como está se comportando a interação atmosfera-oceano. Se a diferença for acentuada, pode provocar a alteração nas características da massa de ar presente. Quando a TSM é mais fria, pode afetar a visibilidade, se houver formação de nevoeiro; e quando a TSM for mais quente, pode instabilizar o ar, favorecendo a convecção e formação de nuvens Cumulus. Nas regiões costeiras, a diferença entre a temperatura da superfície do solo e a TSM tem influência sobre a circulação local do ar e a ocorrência de brisas;
• (d) a observação do vento na região, associada à verificação da carta sinótica de pressão à superfície, mostra ao navegante sua posição em relação ao sistema de pressão, indicando sua situação em relação à depressão e também ao anticiclone;
• (e) o navegante aproado ao vento terá no hemisfério sul (HS) o centro de baixa pressão à sua esquerda (bombordo) e o centro de alta pressão à sua direita (boreste). No hemisfério norte (HN) ocorre o contrário;
• (f) a intensidade do vento está relacionada ao gradiente horizontal de pressão, que é função do gradiente horizontal de temperatura. O navegante constata que quanto mais forte for o gradiente, maior será a velocidade do vento observado na região em questão;
• (g) a umidade relativa presente sendo elevada indica que a saturação do ar pode ser obtida com um pequeno resfriamento. Nesta situação, o navegante deve estar atento aos outros parâmetros que favorecem a formação de nevoeiros e conseqüentemente afetam a visibilidade; e
• (h) o navegante deve ter o hábito de observar o céu. Inúmeras nuvens Cirrus aparecendo de uma mesma direção podem ser consideradas Cirrus pré-frontais e podem representar indícios de condições severas de tempo nas proximidades da frente.
Aspectos importantes dos sistemas frontais:

Outro resultado importante que o navegante pode obter com a verificação do tempo presente é a identificação dos sistemas frontais. Pela observação da direção do vento na superfície próximo à frente e da tendência barométrica, o navegante pode classificar a frente que está na região em questão:
• (a) Se o vento na superfície no lado do ar frio se apresenta na direção da frente, esta pode ser considerada como frente fria;
• (b) se o vento na superfície no lado do ar frio for paralelo à frente, esta deverá ser designada como frente quase estacionária;
• (c) se o vento na superfície no lado do ar frio tiver uma componente na direção oposta à da frente, esta pode ser considerada como frente quente;
• (d) se a pressão está parando de cair ou passando a subir no lado do ar frio, significa que o cavado está se deslocando na direção do ar mais quente. Em conseqüência, a frente pode ser considerada frente fria;
• (e) se a tendência barométrica é praticamente a mesma nos dois lados da frente, podese considerar que ela está quase estacionária;
• (f) se a pressão está parando de subir ou passando a cair no lado do ar frio, o cavado está se deslocando na direção do ar frio, portanto a frente pode ser considerada frente quente;
• (g) se na costa brasileira o vento local predominante apresentar uma mudança brusca de direção do quadrante norte para o quadrante sul, indica que a frente que chegou é do tipo fria; e
• (h) se os ventos forem fortes com precipitações torrenciais, indicam frente fria de deslocamento rápido, ou seja, a velocidade de deslocamento acima de 20 nós.
Utilizando os conceitos apresentados nos itens anteriores e realizando observações doselementos meteorológicos, o navegante poderá efetuar a previsão do tempo a bordo.
• A posição e o caráter do movimento das depressões e frentes devem ser cuidadosamente acompanhados, procurando-se estimar suas trajetórias e posições futuras.
• A tendência barométrica é outra informação essencial para o prognóstico da atmosfera.
• A migração de massas de ar causa a variação dinâmica da pressão atmosférica. Logo, o registro horário das leituras barométricas fornece o dinamismo do ar atmosférico, favorecendo a previsão de chegada dos sistemas de pressão e frontal num determinado local.
• A bordo, para previsão dos sistemas de pressão, é conveniente traçar um gráfico da tendência barométrica, onde são registrados, no eixo das ordenadas, os valores da pressão atmosférica, em milibares (hectopascais) e, no eixo das abcissas, as horas. No exemplo da figura , estão registrados no gráfico os valores da pressão nos horários sinóticos (00h, 03h, 06h, 09h, 12h, 15h, 18h, 21h e 24h HMG). Para cada observação foram registradas, também, a temperatura do ar e da água do mar, a direção e intensidade do vento.
A variação da temperatura é, também, uma informação importante. A compressão da massa de ar quente provocada pela força do ar frio produz um aumento significativo de temperatura pouco antes da chegada de um sistema frontal frio. Antes da passagem de uma frente quente, a temperatura permanece estável, ou declina um pouco, para subir acentuadamente após a passagem da frente.

A variação da umidade do ar deve ser acompanhada pelo registro horário da temperatura do ponto de orvalho. A diferença entre a temperatura do ar seco e a do ponto de orvalho indica o teor de umidade existente no ar. Quanto menor for a diferença entre essas duas temperaturas maior é o teor de umidade e maiores as probabilidades de nebulosidade e precipitações.

A plotagem horária do vento é o meio ideal para se detectar a aproximação de um sistema frontal, ou sistema de pressão, porque ficam registradas as suas mudanças de direção e intensidade. Se a direção do vento sofre deflexões contínuas de sentido horário no Hemisfério Norte e anti-horário no Hemisfério Sul, isto significa que um sistema frontal ou ciclônico está se aproximando, desde que a pressão esteja caindo significativamente. Ventos fortes com precipitações torrenciais indicam frentes frias de deslocamento rápido (velocidade acima de 20 nós) ou ciclones dinâmicos.

O controle da tendência da umidade relativa é de especial interesse quando se observa advecção (movimento horizontal) de ar quente e úmido sobre superfície de ar mais frio. Se a variação da umidade relativa mostrar possibilidade de saturação do ar, poderá ser formado nevoeiro.

O marulho é produzido por ventos passados ou distantes. Pode ser utilizado, portanto, como indicador na direção onde se encontram fontes geradoras de fortes ondulações do mar (vagas), como ciclones e sistemas frontais de deslocamento rápido, que sofreram retenção temporária (frentes frias que se deslocam em saltos). No Hemisfério Sul, a depressão está sempre do lado esquerdo da direção de onde vem o marulho.

As nuvens são conseqüência do estado do ar e, por isto, devem ser usadas como sinais precursores de fenômenos meteorológicos de atividades moderadas a fortes. Cirrus em forma de garras indicam fortes ventos em altitude e aproximação de sistemas frontais e ciclônicos.

Os quadros e tabelas práticas a seguir apresentados também auxiliam na previsão do tempo a bordo.

OUTRAS REGRAS PRÁTICAS PARA PREVISÃO DO TEMPO

O TEMPO BOM GERALMENTE PERMANECE QUANDO:

• O nevoeiro de verão dissipa-se antes do meio-dia;
• as bases das nuvens ao longo das montanhas aumentam em altura;
• as nuvens tendem a diminuir em número;
• o barômetro está constante ou subindo lentamente;
• o Sol poente parece uma bola de fogo e o céu está claro (céu avermelhado no ocaso);
• a Lua brilha muito e o vento é leve; e
• há forte orvalho ou geada à noite.
O TEMPO GERALMENTE MUDA PARA PIOR QUANDO:
• Nuvens cirrus transformam-se em cirrostratus, abaixam-se e tornam-se mais espessas, criando uma aparência de “céu pedrento”;
• nuvens que se movem rapidamente aumentam em número e abaixam em altura;
•  nuvens movem-se em diferentes direções, desencontradamente no céu, em diferentes alturas;
• altocumulus ou altostratus escurecem o céu e o horizonte a oeste (isto é, nuvens médias aparecem no horizonte a oeste) e o barômetro cai rapidamente;
•  o vento sopra forte de manhã cedo;
• o barômetro cai rápida e continuadamente;
•  ocorre um aguaceiro durante a noite;
•  o céu fica avermelhado no nascer do Sol;
•  uma frente fria, quente ou oclusa se aproxima;
• o vento N ou NE passa a soprar do S ou SE; e
• a temperatura está anormal para a época do ano.
O TEMPO GERALMENTE VAI MELHORAR QUANDO:
• As bases das nuvens aumentam em altura;
•  um céu encoberto mostra sinais de clarear;
•  o vento ronda de S ou SW para NE ou N;
•  o barômetro sobe continuamente; e
•  três a seis horas depois da passagem de uma frente fria.