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Blowin' in the Wind, Issue #036 How the Jet Stream Works - November 1, 2006
November 01, 2006
How the Jet Stream worksWant to know how the jet stream works? First of all where is it? High up. Normally 6 or 7 miles above the ground. Where do they go? West to east, generally, with huge meanderings northward and southward. They go this way in both hemispheres, northern and southern.
The biggest jet streams are near polar areas, where the sharpest change in temperature takes place on the ground. For instance, on the North American weather map jet stream lines are often seen in Canada, and we call it a sub-polar jet stream. Smaller jets exist elsewhere. There is a connection between that sharp temperature change at ground-level and the jet's strength.
Jet stream explanation.What is a jet stream and what causes these things? Well, jet streams are just wind. What causes all winds? A difference in pressure between one place and another. For example, the pressure inside a balloon is slightly higher than it is outside. And if you allow the air to escape through the opening, that pressure difference causes the air to move outward, creating a small wind.
Geostrophic flowThe air flows from higher pressure to lower pressure. Ideally. What happens on earth, though, is the big winds have their direction changed by this mysterious action called the Coriolis Force. It reorients the flow so that the wind now blows along the line separating large areas with pressure differences instead of across it.
So why is there a pressure difference 7 miles high, anyway? It comes from the difference between cold air and warm (or moist) air. Cold air is heavier. If you start at typical ground pressure, say 95 to 100 kPa, the pressure in cold air drops very quickly as you go up through it. More so than in the warm air, and that pressure difference becomes more dramatic as you go higher up.
Now what happens if you have very cold air right next to warm air? First of all, you have a sharp temperature change on the ground. If you were to drive, say, 200 miles and find it much colder so that you now need a coat, that's the type of temperature change we could get. Secondly, you have two adjacient bodies of air (air masses, we call them) with a sudden change in temperature and pressure between them.
You now know that there should become a strong wind between them. Jet streams in the air. It would travel in the direction so that air with lower pressure (the cold) is to the left (in the northern hemisphere) according to the Buys Ballot Law. This wind gets stronger at increasing altitudes, until you ascend into the stratosphere.
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