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2(c). Atmosphere

Warm Air not as many molecules	Cold Air more molecules

A change in air temperature can cause a change in air density. Air density changes will affect the performance of an aircraft. Without changing the thrust force, decreasing air density will lead to fewer molecules in the air. This will decrease the lift force as fewer molecules are available to generate lift. Increasing the amount of molecules in the air will increase lift at the same given rate of speed. A change in air temperature will also create local winds, which if not accounted for in the flight calculations will push a flight off course.

Altitudinal variations need to also be monitored by the pilot before and during flight. Normally in the troposphere temperature decreases as the altitude increases. This is known as the lapse rate. The average temperature decrease (or average lapse rate) in the troposphere is usually given as 2.0 Celsius per 1,000 feet. Remember this is just an average! There are times when the temperature actually increases with height. This increase in temperature with an increase in altitude is known as an inversion. That is, the lapse rate is inverted or moves opposite to what is expected. Inversions can occur at any altitude if conditions are conducive to their development. See the two examples below for a description of such conditions.

Ground-based inversion: occurs near the ground on cold, clear nights. Since the ground radiates and cools much faster than the air above it, air in contact to the ground becomes cold while just a few hundred feet above it that temperature has changed very little. This can trap fog or smoke close to the ground and decrease visibility.

Inversion aloft: Occurs a little higher in the atmosphere than ground-based inversions. A current of warm air overruns a large patch of cold air trapping it next to the Earth's surface. If rain clouds in the warm air current drop rain, it will pass through the colder air and freeze. This can develop into icing on an aircraft's wings drastically reducing the aircraft's lift force.

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