Mesoscale Weather
Small area weather conditions. These are the weather that occurs in the valleys around the bay area. Each small area will have weather distinct from its neighbors. This is very typical of California coastal regions

California Weather
Ocean water temperatures determine the weather world wide but most dramatically in California. Warm water in the Gulf Stream (El Nino) causes precipitation; cold water in the stream causes stratus and fog. California fog requires cold water and warmer air. This creates an inversion where the colder air is below the warmer. The temperature increases (instead of usual decrease) with altitude.

The cold Alaska Current follows the western coast of North America and increases the moisture content of the air in the Pacific High while decreasing its temperature. FOG. This California fog would stay along the coast were it not for the warm interior valleys and desert of California. The heat from the deserts causes low pressures from rising air, which is very attractive to the air in the Pacific High. The result is a strong surface pressure gradient creating winds that carry the fog ashore.

Bay area fog, were it not for occasional fronts, follows a four day cycle of the fog coming on shore, into the bay, into the east bay and into the Central Valley. Mesoscale weather will exist in the various areas both during the four-day fog arrival and during the four-day retreat back to sea.

  An air mass extends for over 1,000 miles. The air mass acquires its properties from the surface below. North America has three air mass areas, Arctic, Polar and Tropic. Thus air mass types are polar and tropical with sub-classes of maritime or continental. The formation of air masses is seasonal. Air masses change as they move. Where two air masses meet we have a frontal zone where the colder mass moves below the warmer. The air mass may be far ahead or far behind the surface front. Worst weather is when fronts move fastest.

Cold fronts are usually in the winter and the passage is shown by clearing, wind shifts, a rise in pressure and colder temperature. The extent of clouds depends on the stability of the air.

Fronts:
Warm Front
A warm air mass shows several days ahead of the front with a slow progression (less than 20-kph) of very high clouds as it overrides relatively cooler air below. First comes cirrus, lowering to cirrostratus, becoming still lower altostratus which changes to alto cumulus. Finally comes nimbo stratus with continuous light rain or snow. Imbedded thunderstorms. Visibility goes below minimums. Freezing rain occurs in winter. Frontal passage gives good flying for a few days. Warm and stationary worst weather.

There is a variety of warm and cold fronts. The main difference being stability. Stable air masses can have weather dependent on the moisture available. Stable warm air masses can have clouds to 25,000' with icing.

Cold Front:
A cold air mass appears quickly at over 20-kph. Any cirrus becomes altostratus and altocumulus changing quickly into stratocumulus. When the front hits strong gusty shifting winds accompanied by nimbostratus or cumulus. Produces summer thunderstorms and squall lines in front. Heavy bursts of localized rain can cause flooding. Passes quickly and leaves two days of cumulus clouds for bumpy flying below. Cold fronts change when they cross mountain ranges. Pacific cold fronts are moist before the Sierras, dry afterwards. Across the Rockies it picks up moisture from the Gulf and will intensify. Dry cold fronts will be bumpy but VFR.

Cold fronts are areas of lowest pressure and strongest winds. Wind coming over water carries moisture. Wind direction and velocity can vary dew point spread. If you stand with your back to wind point to the area of low pressure. This tells you where the fronts are located. Winds over water or blowing up slope are likely to worsen weather conditions. When forecast winds are in error then the entire weather forecast is in error. Westerly winds bring better weather. Southerly winds bring worse weather. Surface winds 100 miles to the west are likely to arrive at your location in two hours. The pilot who makes note of frequent changes in altimeter settings is in an area of strong winds.

The cold air of the world descends to the surface near the poles. Moving southward it deflects to the right. This polar front and its associated low, the Aleutian low cause much of the U. S. weather. The Aleutian Current is a north to south cold current that causes the weather along the Pacific coast.

Three elements mix to form thunderstorms, excess moisture, a lifting force, and unstable air. Thunderstorms are forecast in the area forecast (FA) and terminal forecasts (TAFs). Any time the lifting index has a negative number you can expect thunderstorms. Any thunderstorms upwind of your flight with the three elements present indicates a no-fly zone. Fly to the bright spot and blue. What your eye sees is just as valid as radar.

Thunderstorms are either air mass which are isolated and easy to avoid or severe. Severe thunderstorms have hail, 50-knot winds with possible tornadoes. Avoid flying on the upwind side and the north side of storm lines if given a choice.

Turbulence beyond the pilot's and aircraft's capability is to be expected in or near a thunderstorm. PIREPs are a pilot's best friend in turbulence since it is real time information. Avoid any reported moderate or higher level of turbulence. Proper interpretation of PIREPS, those real time and position reports by flying people, is critical to safe weather flying.

Occluded Front:
Occurs when faster cold front catches and winds around a warm front. Worst of both systems. Stratocumulus and cumulonimbus clouds lower slowly but clear quickly. If a low-pressure area is gaining strength rapidly, it will slow down. The fronts will rotate around the low. The cold front will overtake the warm front and result in an occlusion.

Stationary Fronts:
Sometimes spring fronts blend together without any clearing and bring days or a week of weather having nothing but low levels of fog, haze and smog. Following cold fronts are needed to clear out stationary fronts.

In the winter where cold air is under the warm layer and the temperature difference is over 10 C, forecasts of improving weather are inaccurate. The conditions are too stable to make a change likely.

Weather Demonstration
Place pencil horizontally between praying hands out in front of you. Left hand is northern cold air and the right is southern warm air. The low exists between your hands. Move left palm down to roll the pencil. The pencil rolls cyclonically to the left.

The cyclone draws warm air down from the south and sends it above the sinking cold air. The cold front moves to the southwest and a warm front forms east. The bigger the cyclone low the stronger the weather.

Inversions:
Any time the air temperature is warmer above than the air below is called an inversion. An inversion may be surface based or an inversion aloft. Inversions can be the cause of poor visibility and turbulence. Inversions are most common in late spring and early fall.

Surface inversions begin at sunset when the earth's surface temperature cools rapidly on a cloudless clear night. This allows the air close to the surface to cool while the air only a few feet higher remains warm. This condition can last till mid-morning. The condition is most likely to exist when winds are very light and unable to cause a mixing of the air layers. The heat from a rising sun will increase the mixing of the layers and eliminate the inversions. In regions like Alaska such inversions can be dangerous to aircraft due to frost forming on the aircraft.

The inversion aloft is caused when air masses flow in such a way as to cause the warm air to pass over the cold air. If the upper layer of air is also descending its increased pressure will cause further warming. This is a common summer condition on the west coast of the U. S. This is also called a subsidence inversion. This condition causes the stratus (layered) clouds of avection fog, smog layers, and reduced visibility.

Inversions exist by providing warmer air aloft than exists in air at lower altitudes. The temperature lapse rate is upside down. An inversion prevents vertical air motion. Large, slow-moving high-pressure systems are a common source of inversions. The top of an inversion layer is flat.

There are four pilot clues to the existence of an inversion:
1. Cloud formations can occur with as much as a 6 degree dew point spread.
2. An illusion in inversion visibility conditions makes things seem further away.
3. There is an increased possibility of low level wind shear.
4. The air will be very stable and give smooth flight above the inversion of either type.

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