in the temperature comes from uneven heating of the Earth's surface by the
Sun. The warmer air expands becoming less dense than the cooler air around
it. The cooler air (which has greater density) moves toward the ground. The
rising air spreads out above, becomes cooler and eventually descends while
the cooler air below warms and rises. This process of convection plays itself
out worldwide from hemispheric circulations to local airflows. Horizontal
movement of the air is known as wind.
convection process causes changes in the air density, and those variations
cause winds. Winds flow out of higher air density areas into lower air density
areas. Because of the Coriolis effect in the Northern Hemisphere, the wind
flows clockwise around higher air density areas, called a "high,"
and counterclockwise around lower air density areas or a "low."
For general reference, highs bring clear weather and lows bring stormy weather.
Because this is not always true, pilots perform a thorough check of all available
weather data when planning a flight.
these high and low pressure areas high in the troposphere (near the tropopause) is the jet stream. On the average,
winds tend to increase speed with height in the troposphere culminating in maximum speeds near the tropopause.
These high-speed winds also become concentrated, narrow bands that wind their way through the atmosphere. The jet
stream varies from 100 to 400 miles wide and is usually found above 30,000 feet. Its general motion is from west
to east with a speed range of 150 to 300 miles per hour. It has seasonal migrations within the United States. Closer
to the Earth's surface the jet stream causes local disruption in the airflow and some additional turbulence. However,
during high altitude flights riding with a jet stream increases overall flight speed.
There exist general circulation patterns and seasonal surface pressure systems worldwide, but of equal importance
to the airplane pilot are those influences closer to the surface such as friction, local wind patterns and wind
shear which all affect flight. Friction occurs between the wind and the terrain surface acting in opposition to
the wind's direction. Friction slows the windspeed. The rougher the terrain the greater the friction. The greater
the windspeed, the greater the friction. Knowing the terrain over which the flight is to take place will help the
pilot account for patches of rough air along the way.
Terrain features such as mountains, valleys and shorelines also generate local wind patterns of which low flying
pilots need to be aware. Land and sea breezes are important when flying above coastal regions. During the day,
the land is warmer than the sea; therefore the wind blows from the cooler water toward the warmer land. This is
called a sea breeze. At night, the wind reverses as the land cools more quickly than the water generating a land
The interactive diagram above shows the wind blowing from the land to the sea at night, and the
wind blowing from the sea to the land during the day.
The mountain and valley breezes are also diurnal. The radiated ground heats air next to a mountain slope in
the daytime. Colder, denser air farther away from the mountain slope located at the same altitude as the warmer
air settles down upon the warmer air forcing it to move up the mountain slope. This is referred to as a valley
wind because it flows up the mountain slope out of the valley. At night, the opposite movement occurs. The air
on the mountain slope is cooled, becomes heavier than the surrounding air and follows the mountain slope down into
the valley. Mountain winds are usually stronger than valley winds.
The interactive diagram above shows the wind blowing up the face of a mountain during the day, and down the face of
the mountain at night.
Rising and descending air currents affect local air circulation. Surfaces such as planted fields, meadows and
water tend to retain heat and cause descending air currents. Meanwhile rocky or sandy terrain, plowed fields and
barren land reflect heat and cause ascending air currents. These will cause a landing aircraft to overshoot or
undershoot the runway if not accounted for.
Normal Glide Path
Convection Effect on Path
The interactive diagram above shows an aircraft ona normal glide path reaching the threshold on the runway.
The diagram then shows the glide path altered by convection. In this case, the glide path ends short of the
shear is encountered in an area where two winds moving in opposite directions
"rub" or mix together. This shear zone creates small eddies and
whirling masses of air that move in various directions. This generates tremendous
turbulence. Some wind shears are predictable, but others may occur unexpectedly.
Getting caught in wind shear can be devastating to an aircraft, especially
if the wind shear occurs close to the ground. Currently airports are currently
being outfitted with wind shear alarms that warn controllers and pilots of
the potential windshear existence within runway takeoff and landing corridors.
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