It is sometimes more difficult to get airplanes off the ground in extremely hot weather. But the problem is not the temperature itself or the humidity. It is the air density that can get in the way. Hotter air gets thinner. Thin air makes it more difficult for planes to lift and land safely. This is especially the case with smaller planes. That is why more than 50 flights were canceled in the southwestern United States earlier this week. Temperatures there have climbed to 49 degrees Celsius. Airplanes take off and stay in the air because of lift -- the force from the movement of air under the plane’s wings that push it upward.