Because of evaporation, the atmosphere always contains moisture in the form of water vapor. This water vapor replaces molecules of dry air, and, because water vapor weights less than dry air, any given volume of moist air weights less-is less dense- than an equal volume of dry air.
–Effects of Wind
Surface winds during takeoffs and landings have, in a sense, an opposite airplane performance to winds aloft during flight. During takeoff, a headwind shortens the takeoff run and increases the angle of climb. This increases performance and, if the density altitude is high, helps to compensate for lost performance. A headwind during flight, however, has the opposite effect on performance, in that it decreases groundspeed and consequently increases the total amount of fuel consumed for that flight.
During takeoff, a tailwind increases the takeoff run and decreases the angle of climb. Because of the risks involved, a pilot should carefully consider this decrease in performance before attempting a downwind takeoff; the result, otherwise could be disastrous. During flight, though a tailwind is a great plus, as it increases groundspeed, conserves fuel, and saves time. During landing, a headwind steepens the approach angle and shortens the landing roll, while a tailwind decreases the approach angle and increases the landing roll.
In light of the dangers, downwind operations should be carefully evaluated before being attempted.
(to be continued)