Altimeters are devices on an airplane that indicate the aircraft's actual height above sea level, called the true altitude. They read the changes in barometric pressure by calculating the weight of the air pushing down on the aircraft from above. Warm air increases air pressure, and cool air lowers pressure, but the altimeter can't calculate temperature and air density. The standard atmosphere is an internationally agreed-upon variation of atmospheric conditions representative of the average yearly conditions at middle altitudes. Changes to instruments and flight plans can be made when the pilot compares his aircraft's performance in particular conditions to this standard.
Warmer Than Average
When the temperature is higher than the standard atmosphere, the plane's altimeter reading will be lower than the true altitude. Warmer temperatures cause air molecules to move farther apart and become less dense. The aircraft will require more runway length and increased runway clearance due to reduced power, thrust, lift and climb rate caused by the less dense atmosphere. The plane reacts as though it is flying at a higher altitude.
Cooler Than Average
Temperatures in the troposphere usually decrease with increasing altitude. Temperatures lower than the standard atmosphere result in denser atmosphere, because there are more molecules, and they are closer together. Thrust, lift and climb rate are all increased. The plane's altimeter will read higher than the true altitude, and the plane will react as though it is flying at a lower altitude. The pilot must be aware that he is closer to the ground than the altimeter indicates.
Warmer Than Average en Route
When temperature rises en route, the plane will actually be higher in the atmosphere than the altimeter indicates. This leads to a reduction in performance and power. Reduced air density leads to reduced air intake of the engine and a loss of engine power. The propellers and engine don't have as much air to move, reducing thrust. Less air means reduced lift due to less force on the wings. Arid areas of rocky or sandy deserts and large continents produce the largest temperature changes. Abrupt changes in temperature are encountered along the shorelines of large lakes and oceans.
Cooler Than Average en Route
The airplane will be lower than the altimeter reading when the temperature falls en route. The term, "Look out below" applies to this situation, because the plane is closer to the ground than indicated. The aircraft performance increases due to the increase in air molecules. Thrust, lift and climb rate increase.
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