Today’s post on temperature inversions comes to us from The Pilot’s Manual: Ground School (PM-2).
Temperature normally decreases with altitude. In the standard atmosphere the temperature is assumed to decrease by approximately 1.98°C for each 1,000 feet climbed in a stationary air mass. In practice, we can assume a decrease, or temperature lapse rate, of 2°C per 1,000 feet. In some layers of air in the actual atmosphere, however, air temperature may increase with altitude (an inverted temperature structure), and a temperature inversion is said to exist. This often happens near ground level on cold, clear nights when the earth’s surface loses heat by terrestrial radiation and cools down. The air near the ground is cooled by conduction, and tends to sink and not mix with air at the higher levels. This leads to the air at ground level being cooler than the air at altitude, and a temperature inversion will exist.
Air that has no tendency to rise is called stable air, as is the case in a temperature inversion, and this generally means smooth flying conditions. Visibility may be a problem, however, because there will be no upward convective currents to carry particles in the air away, so any fog, haze, smoke, smog or low clouds will stay beneath the inversion layer and restrict visibility.
A phenomenon known as windshear (in which the wind strength or direction changes from place-to-place) may exist at the upper boundary of the inversion if there are overlying strong winds. An airplane may experience an airspeed change or some turbulence as it flies through the inversion level from one air mass to another.
Over desert areas, the upper level of an inversion can sometimes be identified by a layer of dust with clear air above it. Inversions also occur at altitude in warm fronts, when a warm current of air overruns a lower colder layer. A danger for pilots in this situation is freezing rain, which is liquid rain falling out of warmer air above into below-freezing air beneath, where it can quickly form a great deal of ice on an airplane’s structure.
