Tutorial

3. Wind
Variation 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.

This 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.

Running through 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 breeze.

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.

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.

Wind 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 being outfitted with wind shear alarms that warn controllers and pilots of the potential windshear existence within runway takeoff and landing corridors.