Nowadays, the word “jet” is very much in the news, and we might imagine that the “jet stream” has something to do with jet planes. This isn’t the case at all! The jet stream is part of the system of winds that surround the earth, so let’s start with the subject of winds.
Winds are currents of air that move parallel to the surface of the earth and close to it. The movement of winds is chiefly caused by the existence of areas of different pressure, and winds always blow from areas of high pressure to areas of low pressure.
If we look at it from an overall point of view (not from local regions) we can say that, in general, cold air is transported from the Poles toward the Equator and warm air from the Equator toward the Poles.
This flow doesn’t take place in smooth currents but forms a system of rather turbulent streams. There are various conditions that decide what happens in each specific region. There may be local sources of heat which affect the pressure.
The way the land and water and mountains are distributed can decide which way the local winds blow. Finally, there is the existence of semi-permanent high-pressure areas in certain places.
This means that in these regions there tends to be a high-pressure area most of the time. They are called anticyclones and wind currents locally are decided by them.
Now, this gives us a general idea of how winds blow and some of the things that affect them. But all of this concerns winds that are quite close to the earth, in the lower layers of the atmosphere.
As we know, the earth is surrounded by an atmosphere that may go as high as 1.500 miles up. In the higher levels of the atmosphere, different wind conditions exist. Here they usually move with much greater speed than near the surface.
At a height of about 30.000 feet, currents of air move with such speed that they have a special name – the jet stream! The speed of the jet stream ranges from 100 to 200 miles per hour.