What are air pockets?
When the first planes took to the air, the trips were full of surprises. Sudden storms would toss the fragile wings this way and that. The pilot, with no roof over his head, was often drenched with rain. There were also plunging dips that came without warning. A little plane would seem to fall down through a hole in the air. In the old days, these sudden swoops were called air pockets.
A plane ride, even in a big modern jet, may be smooth as silk or bumpy as the waves on the ocean. When a bumpy spell begins, a sign saying, Fasten your safety belts, flashes on. So you promptly buckle yourself down into your seat. The bumps may be small ups and downs. Or they may be big swoops and you think that the plane keeps falling into big holes in the air.
Some people say that these roller coasting swoops are caused by air pockets. But the pilot might tell you that his plane is merely riding through a thermal shell. The word thermal, of course, means heat and a shell may be a circle. A thermal shell is a very complicated system of moving air. It gives plane passengers a bumpy and uncomfortable trip, but the birds love it. Some birds move up and down on the air currents and soar for hours without flapping a wing.
We all know, of course, that the wind blows from all directions. But we take it for granted that it always blows horizontally, or level with the ground. But this is not so at all. There are kinds of currents in the air blowing up and over, round and down and up again. We do not notice them because they do not push us around as the normal winds do. There are many of these updrafts and down drafts where the planes fly high above the earth.
A thermal shell starts when a current of warm air rises up from below. This may happen because the sun heats a valley or a patch of sandy desert. The updraft of warm air cools as it rises. Then it spreads out and curves over and down. When a plane meets a thermal shell, it is first buffeted from above by a current of falling air. Next it runs into a current of rising air which pushes it up from below. Sometimes the air is forced to rise up or slide down over a hill or a forest. These air currents also make a plane tip up and dip down into what seems like a pocket or hole in the air.
A soaring bird makes the best of these tricky air currents. Over the ocean, the tossing waves make the wind blow slower and the wind aloft blows faster. A sea gull soars up into the wind, then turns and slides down. The downward swoop gives him enough speed energy to turn and soar up into .the wind again. The soaring sea gull usee the changing air currents to drift around and around without flapping his wings.