Weather

A weather event can last for one minute to a few weeks. A region’s climate is established over about a 30-year period.

First - A few points to remember about Earth’s physical dynamics and basic atmospheric physics:

The Earth’s axial tilt of 23.5 degrees creates the seasons and significant uneven heating and cooling of the planet.

Land and water experience differential heating - land is a good conductor of heat – it heats and cools much faster than water.

Most of the incoming solar radiation (insolation) from the Sun is received at the Earth’s lower latitudes near the Equator. Direct insolation (the Sun is directly overhead at a 90-degree angle to the horizon) only occurs at latitudes on and between the Tropic of Cancer (23.5 degrees north) and the Tropic of Capricorn (23.5 degrees south).

When air is heated, as it is near the equator, it rises due to increased molecular activity. Conversely, as it cools, air descends.

If water vapor is present in the air, the vapor will condense (change from a gas to a liquid) as it cools. Atmospheric condensation coalesces into clouds and precipitation.

As air and water move across the planet, several effects and forces produce directional motion:

1) Force of Gravity

2) Centrifugal Force – Keeps air and water moving around a spherical planet

3) Force of Friction – Objects such as tall buildings, trees, etc., create a frictional force as air moves across them – similar to rocks and boulders in a stream

4) Coriolis Effect – The primary effect causes air and water to curve to the right in the northern hemisphere and to the left in the southern hemisphere. There are several online videos that explain this important phenomenon.

Uneven heating and cooling of the planet create areas of high and low pressure. Characteristics of High and low pressure in the northern hemisphere are as follows:

High Pressure – Anticyclone - air diverges and descends in a clockwise motion – associated with clear skies and fair weather.

Low Pressure – Cyclone – air converges and ascends in a counterclockwise motion – associated with cloudy skies and possible stormy weather.

Earth has several, semi-permanent areas of high and low pressure. Two of these significantly affect California by acting as steering mechanisms for storms and ocean currents.

CALIFORNIA WEATHER - TEMPERATURE AND PRECIPITATION PATTERNS

California has three major influential factors which affect temperature and precipitation:

1) Latitude – North/South

2) Ocean Air (Maritime) verses Continentality – East/West

3) Elevation – varies throughout the state

Latitude – Temperature

California includes 9.5 degrees of latitude from its southern border shared with Mexico and northern border with Oregon. This expanse of latitudes creates significant diversity in temperatures from north to south. Remember that Sun angle affects insolation. The northern border of California is 42 degrees north latitude. On the June Solstice (June 21 - the highest angle of the Sun during the year for the northern hemisphere), the Sun is at 71 degrees above the horizon (90 degrees is directly overhead). The Sun angle only reaches 25 degrees above the horizon on the December Solstice (December 21 – the lowest angle of the Sun during the year). Example: Imagine looking out across the ground in front of you to where the land meets the sky – the horizon. If a 90-degree angle is directly above your head, look up to what you would think is about 25 degrees above the horizon – that’s a low angle – not much insolation or opportunity to warm up an area.

The state’s southern border lies along the 32.5-degree line of latitude. On the June Solstice, the Sun angle is 81 degrees – very high in the sky - and 34 degrees above the horizon on December 21 (winter Solstice). Variances in insolation throughout the year contribute to the general weather pattern of cooler temperatures in Northern California. These differences are more pronounced inland, away from the coast.

Latitude – Precipitation

Precipitation (rainfall and snowfall) varies greatly throughout the state but usually increases northward. Fair weather and less precipitation dominate regions farther south. All of California receives the bulk of its rainfall during the winter except for the southeast corner of the state. As warm air rises in the tropics creating lower pressure around the equatorial regions cooling of the air occurs at higher altitudes then descends, creating higher pressure. These Sub-Tropical High Pressure Belts are located about 30 degrees north and south of the equator. In the northern hemisphere, as the descending, compressed air diverges at the ground, some of the air returns south to the equator and some of the air heads north and turns toward the east (or the right) due to the Coriolis Effect. This air, moving from west to east creates California’s prevailing wind – the Westerlies. About 60 degrees north latitude, cold polar air collides with warmer air further south creating Fronts - https://www.youtube.com/watch?v=huKYKykjcm0.

Fronts are often steered around high and low pressure systems. The Aleutian Low is a semi-permanent pressure system off the coast of Alaska that acts in steering Fronts that bring precipitation across California from the Pacific Northwest. During summer months, the Aleutian Low is generally much further north keeping Fronts (storms) away from California. In winter, the Low often migrates south allowing Cold Fronts to travel down and across the state.

The East Pacific High (EPH, Hawaiian High) is a strong cell of high pressure that imposes a significant influence on the state’s weather. In winter, the High shrinks and moves toward the western Pacific Ocean leaving room for storms to travel south and across California. In the summer, the High enlarges and moves towards the eastern Pacific keeping precipitation much further north. In addition, the EPH assists in steering ocean currents in the Pacific. It’s clockwise rotation steers cold water from the north Pacific down along the California coast.

Ocean Air verses Continental Air – Temperature

Areas of Cismontane California, particularly the Windward side of coastal mountains and valleys, generally experience moist air compared with the much drier air of Transmontane regions including the Leeward side of mountain ranges and deserts. Moist air modifies temperatures in the winter much like a blanket – water vapor retains heat and keeps an area warmer. It is rare that coastal regions in California experience temperatures near freezing – even in the northwest corner of the state. Moisture also acts to maintain overall cooler temperatures during summer months in regions near the coast. In drier areas, temperatures are more extreme – above 100 degrees and well below freezing are common readings.

- Of special note, California mountain ranges, particularly the Sierra Nevada, separate regions with some of the greatest temperature contrasts found anywhere on Earth.

Air masses are large areas of air that share common characteristics – humidity (atmospheric water vapor) and temperature. Masses can be warm, cold, moist or dry. Atmospheric winds move air masses across California from their points of origin. Air masses that originate over the Pacific are usually full of water vapor whereas air that begins from desert regions are dry and mostly devoid of moisture. A Front results when two air masses of different characteristics collide.

Ocean Air verses Continental Air – Precipitation

The Pacific Ocean serves as the primary moisture source for California. Air masses containing large amounts of water vapor move inland from the ocean, rise over the coastal mountains, across the Central Valley, up and over the Sierra Nevada and continue this pattern across the Basin and Range mountains and valleys to the east. Water vapor condenses, becomes clouds and possible precipitation as it rises in altitude. All temperatures have a Dew Point – the point where water vapor MUST become liquid. Warmer air is capable of ‘holding’ more vapor than cooler air. As moist Pacific air rises up and over the Sierras, clouds and precipitation develop along the Windward (west facing) slopes. The vegetation, including the giant Sequoia’s, are evidence of what is known as Orographic Precipitation. As the air rises, condensation occurs = clouds, rain and possible snow at higher elevations.

Once an air mass crosses the highest elevations and begins its descent on the Leeward (east facing) slopes, its water vapor has almost been depleted. This effect is referred to as the Rainshadow – the dry Leeward slopes of mountains. The Owens Valley, and those valleys to the east, all exist in the Rainshadow of the Sierras.

Elevation – Temperature

In general, air aloft is cooler. The adiabatic lapse rate is a consistent atmospheric temperature change of 5 degrees Fahrenheit per 1000 feet in the Troposphere (the Troposphere is the lowest layer of the atmosphere where life exists, and weather happens). There are exceptions:

1) Air masses can have warm layers aloft.

2) Inversions:

a. As cold air is more dense, warm air in mountain regions can cool at night and migrate down slopes onto valley floors. This cold air can linger while higher mountain locations reheat during daytime hours.

b. Widespread Upper Level Inversion – Common summer pattern in Los Angeles – High pressure becomes stationary above the Los Angeles Basin in the summer. Atmospheric compression, resulting from high pressure, creates heat at the surface. Cool, moist ocean air forces inland across the Basin but is unable to rise and escape up the slopes of the San Gabriel Mountains to the east. The moist air is held in place and fills with exhaust from cars, factories and other anthropogenic (human) emissions. This cooler, pollution-filled air is known as Smog. This brown, hazy and unhealthy air is most visible during the summer. Eventually the inversion layer collapses allowing the cooler smog-filled layer to escape.

Elevation – Precipitation

California experiences great precipitation variations across the state. Orographic precipitation applies at all elevations. Generally, above 8000 feet in elevation, precipitation decreases. Air masses lack available moisture above this point as most moisture has been depleted at lower elevations. The Orographic Effect can be compared with a sponge that is slowly squeezed, eventually losing all its available water.

SEASONAL WEATHER PATTERNS

Fog (a stratus cloud at ground level) is a common occurrence throughout California and is often associated with different seasons. There are 3 common types of fog:

1) Radiation Fog – When cold, drier air from high pressure moves in over areas where the ground is moist, particularly in areas that have recently experienced rainfall, condensation occurs rapidly as water vapor encounters colder air aloft.

2) Advection Fog – As moist air masses move inland over cold surfaces, particularly cities such as San Francisco, condensation occurs at ground level as the cold and moist air collide.

3) Tule Fog – The Central Valley of California has natural rivers, streams and human-made waterways which add humidity to the lower levels of the atmosphere. Tule Fog is a type of Radiation Fog that is named for a Sedge Grass that lines many of these waterways. Winter months are prone to this thick fog which can reduce visibility to less than a few feet for several weeks across much of the Central Valley.

Winter Patterns

As the northern hemisphere is tilted away from the Sun resulting in decreased incoming solar radiation (insolation), the East Pacific High (EPH) usually weakens. The Polar Jet Stream (a strong, fast moving upper atmospheric stream of wind that flows around the Earth’s higher latitudes from west to east) migrates south and transports low pressure systems (storms) from west to east https://www.youtube.com/watch?v=huweohIh_Bw. Jet streams contain waves of air (Rossby Waves), like waves on the ocean. The Aleutian Low serves as a point of origin for Cold Fronts that produce many of California’s winter storms. These Fronts are carried along the winds of the Jet Stream to the eastern US and out across the Atlantic.

Occasionally the EPH stalls and blocks the Jet Stream from carrying storms south into California. This pattern can result in dry winter conditions across the state.

California - Four Types of Winter Storms:

1) Mid-Latitude Cyclone – These massive, complex areas of low pressure originate in the northwest Pacific at the Aleutian Low and travel from west to east. Most of these storms are steered across higher latitudes, including northern parts of California. Southern California often receives the outer bands (tail end) of these rain events. Occasionally, storms are transported on the Jet Stream if it ‘dips’ further south into and across Southern California.

2) Westerly Storms – Low pressure occurs out over the Pacific directly to the west of California and channels moist air into the state. These storms are usually warmer than winter Mid-Latitude Cyclones and produce lighter rainfall totals. A Trailing Cold Front often follows these storms resulting in unstable air and brief periods of low snow levels.

3) Atmospheric Rivers – These massive precipitation events originate from low pressure areas in the southern Pacific. Like a giant gear spinning counterclockwise, warm, moisture-rich air from the tropics is channeled up and across California. Several catastrophic flooding events have occurred in the state as a result of Atmospheric Rivers. Due to their warm origin, these storms do not necessarily produce heavy amounts of snow in the Sierras – usually more rain than snow.

4) Inside Sliders – When the Jet Stream flows from north to south across Montane and Transmontane California it can bring very cold air from Alaska down to Southern California. These storms are not associated with much precipitation, but they can produce a ‘dusting’ of snow at lower elevations such as the Antelope Valley and Transverse Ranges.

Spring Patterns

The East Pacific High strengthens as the Earth’s axial tilt is equalized along the equator and insolation intensity increases further north. A strong pressure gradient (high pressure always move towards lower pressure – the greater the difference between the two, the faster the air moves – this air motion is what we feel as wind) forms as inland valleys begin to heat quickly while ocean water still retains cold temperatures from winter months. Warm air inland rises as it warms. Cool air from the coast rushes in to replace the rising warm air. This movement of air from the coast to the inland valleys during daytime heating is known as a Sea Breeze. Wind is named for the direction in which it originates. At night, as land cools rapidly, the layer of warm air above the Sun-heated sea surface rises. The cooler inland air moves to replace the rising air above the ocean resulting in a change of wind direction. This offshore-blowing wind is known as a Land Breeze.

The California Current is a cold ocean current that moves south from Alaska and down along the coast towards Point Conception. Some of the cold water then spreads into the Santa Monica Bay and coastal waters further south. When large moisture-laden air masses begin to migrate inland during daytime heating, the vapor-filled air passes over the cold California Current. Due to the temperature contrast, the vapor condenses over the cold water and produces Advection Fog. This fog can migrate inland over coastal areas and often move inland to valleys further east. As daytime heating continues, the fog usually ‘burns off’ by late morning or early afternoon hours.

Summer Patterns

Heat waves are common across California during the summer as the EPH enlarges, becomes stationary and often extends in and across parts of coastal and inland valley regions. Temperatures can peak well above 100 degrees for extended periods with some desert locations experiencing temperatures above 120 degrees.

A large area of high pressure becomes relatively stationary during the summer over the Four Corners Region of the US https://en.wikipedia.org/wiki/Four_Corners – often centered near Texas. As the air circulates clockwise around the high, extremely moist, tropical air from the Gulf of California is lifted and carried into the southeastern corner of California. As the tropical air encounters drier, cooler air, unstable atmospheric conditions occur resulting in the formation of Cumulonimbus Clouds. These massive, tall clouds can produce heavy rainfall, thunder and lightning, and hail. Occasionally this high pressure system enlarges enough to channel moisture from the Gulf of Mexico across the desert Southwest including Southern California. This Monsoon weather event delivers several inches of rain in the Mojave Desert and Imperial Valley of California. This is the only area of the state that receives most of its rainfall during the summer.

Fall Patterns

Weather patterns change dramatically during the fall in California. The EPH weakens as the Sun angle becomes much lower, reducing insolation. Ocean water temperatures remain warmer from summer heating while land surfaces cool rapidly. Cold air masses and Fronts begin to ascend across lower latitudes and into northern parts of California.

Low pressure, Mid-Latitude Cyclones begin to travel across the northern latitudes of the US followed by colder areas of high pressure. When high pressure cells stall and settle over the Great Basin https://en.wikipedia.org/wiki/Great_Basin, the clockwise air flow that is generated compresses and heats as it travels west into California. These Santa Ana Winds increase in velocity as the air channels through canyons and passes heading toward lower air pressure over the Pacific. Following months of summer drought (little to no precipitation), dry vegetation, particularly along the hillsides of the Transverse and Coast Ranges, and Sierra Nevada Mountains provides ample fuel. Sparks of any kind can easily ignite, resulting in devastating wildfires.

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