air pockets

An airplane flying through the air will often hit a "bump," or air pocket. These bumps are sometimes frightening to inexperienced travelers by air, but they do not bother experienced travelers, who know there is nothing dangerous about them. A bump can either lift the plane in the air, or cause it to fall suddenly through the air, because it can be caused by either a rising or a falling current of air. Just as cream floats to the top of milk, because it is lighter than milk, hot air rises because it is lighter than cool air.

 When the ground below the plane is very hot, the warm air from the ground will rise, which causes the cool air to rush downward and carry the plane down with it. Also, moving air tends to follow the earth's surface. As a plane passes the top of a mountain, it is likely to hit a descending current of air going down the side of the mountain, and to bump downward with this current. Descending currents of air, causing air pockets, will also be found near thunderstorms, because warm air will be rising. The plane neither falls nor rises very far when it strikes an air pocket, and is immediately leveled out to continue on its way.