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Humidity, Precipitation, Clouds

Water, in one form or another, is an important ingredient necessary for weather.   Water can exist on the surface of the earth and the air in all three states - solid, liquid or gas.  The amount of water in the atmosphere is a given area can have a profound effect in the weather for that area.   Water vapor is always found in the atmosphere.  It varies from place to place.  Through condensation (changing from vapor to liquid), evaporation (liquid to vapor) and sublimation (vapor to ice or ice to vapor) water vapor is continuously being added to  and subtracted from the atmosphere.

CONDENSATION    water vapor to liquid water vapor loses heat to the surroundings warms the air around it
EVAPORATIONliquid water to water vaporwater takes heat from the surroundingscools the air, surface, skin from where evaporation occurs
 SUBLIMATIONvapor to solid ice OR ice to vaporNote #1:  This is the process that makes ice cubes 'disappear' if left in the freezer for a long period of time. Note#2:  Frost forms by this process.

Humidity is the amount of water vapor in the air.  Normally, there are not more than about 12 grams of water per cubic meter of air, although as much as 40 grams per cubic meter can occur.  However, this is not the way humidity is commonly reported.  Relative humidity and dew point are the most common ways that humidity are used today.

Relative humidity is the ratio which describes the amount of water the air is holding compared to the amount it could hold at a given temperature.    A relative humidity of 50% indicates that the air is holding 50% of the total water possible at that temperature.  

As the temperature of the air rises, the air can hold more water.  If the amount of vapor stays the same the relative humidity becomes less.  As the temperature drops, the air can hold less water and the relative humidity increases.  If the temperature drops until the air cannot hold any more water, the air is saturated.  The relative humidity is 100%.  

When the relative humidity reaches 100% water vapor in the air will begin to condense.  The liquid droplets will form on tiny particles in the air and form clouds or water will condense on the surface forming dew.  The temperature at which this happens is called the dew point.  Dew point is commonly used to indicate the humidity.  A dew point of 65o or higher would make it feel quite 'sticky' outside.  Dew points under 65o feel drier and more comfortable.

Clouds are the result of condensation and sublimation of water vapor in the atmosphere.  Clouds are tiny droplets of water and/or ice crystals suspended in the air.  In order for clouds to form the temperature must reach the dew point and there must be enough solid material suspended in the air.  These are called (big science word of the day...) condensation nuclei.  These tiny particles come from salt particles, volcanic dust, meteor dust, smoke, etc.  Without these particles clouds would not form.

As warm air rises it cools.  If there is enough cooling and moisture in the air, the temperature will reach the dew point and clouds may begin to form.  The different types of clouds depend on how, when and where they were forms.  The table below gives common cloud names and their properties.

 HIGHCirrus cirrus, cirrostratus,cirrocumulus-feathery in appearance,high in altitude
 MIDDLE Alto altocumulus, altostratus
 LOW Stratus stratus, stratocumulus, nimbostratus, fog (a cloud at ground level)-formed in layers

 Nimbus nimbostratus, cumulonimbus-refers to a cloud producing rain

 Cumulus -indicate rising air movement, usually considered fair weather clouds, however, may lead to cumulonimbus (thunderstorms) cloud

Once water droplets or ice crystals get large enough, they can no longer be suspended in the air.  The force of gravity overcomes the upward force of the air currents and the droplets or crystals fall to the earth as precipitation.  The main types of precipitation are rain, snow, sleet and hail.

Rain is liquid precipitation.  Drizzle refers to very tiny droplets of water.  Freezing rain occurs when rain or drizzle freezes on contact with an object or the ground.

Snow occurs in saturated air that is below the freezing point of water.  As the ice crystals grow in size gravity overcomes the upward force of the air currents and the snow falls to the ground.

Sleet is simply frozen rain.  This type of precipitation starts out as rain but freezes as it goes through the cold air near the ground.

Hail is small balls of chunks of ice.  Hail stones achieve their size by the accumulation and freezing of successive layers of water droplets as the ball moves through a cumulonimbus cloud.  If a hailstone is cut in half it may show layers like an onion.