How does humidity effect the way that an airplane flies?

Asked by: Jesse Thune

Answer

Humidity affects the way an airplane flies because of the change in pressure that accompanies changes in humidity. As the humidity goes up, the air pressure for a given volume of air goes down. This means the wings have fewer air molecules to affect as they are pushed through the airmass. Fewer molecules = less lift. The other problem is that jet engines do not like humidity either. Jet engines are built for cold, dry air, and humid air has fewer oxygen molecules to burn per unit volume. Therefore the engine combusts a little bit less and puts out slightly less thrust. There are four factors that decrease the performance of a jet airplane - heavy, hot, high, and humid. Notice that three of those factors all have the net effect of lowering the density of the air. So there you have it. Humidity decreases the performance of most aircraft, not only because of it's effect on the wings, but also the effect on the engines.
Answered by: Frank DiBonaventuro, B.S., Air Force officer, Physics Grad, The Citadel

Humidity has a major affect on the way planes fly. This is due to the weight of the air when it is humid. When air is humid, it is actually lighter then dry air, contrary to common belief. That is because the water (H₂O) weighs less then the N₂ or O₂ that it replaces. So if you take the fundamentals of lift, which is that the curved part of the wing (the top) will cause air to move by it quicker, causing the bottom of the wing to have a higher pressure (with slower moving air), causing lift. If you take humid air (less dense), then the plane can no longer create the amount of lift it could when the are is dry (more dense). This causes pilots to have longer runways to gain speed before enough air is passing the wings per second to create enough lift, it also forces pilots to fly faster then would be required if the air were dry.
Answered by: Steve Smith, None, High School Student