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How Do Jet Streams Form?

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We have all heard about this weather phenomenon, in fact, it is often one of the first things people remember about the weather. What I am talking about is the jet stream – the fast-moving ribbon of air that flows high above our heads. While pretty much everyone has heard about this, and most of us have some kind of idea just what it is, most of us don’t really know what creates it and what part it plays in our big weather picture.

The last couple of issues we discussed Rossby Waves and how they are large undulations in the boundary between the cold air in the north and the warm air to our south. It is along this boundary that we find the Polar Jet, one of the two main jet streams. The second is found to our south (usually) and is known as the Subtropical Jet. This jet stream forms between the boundary of warm air flowing northward from the equatorial low and the descending air associated with the subtropical high.

Jet streams were first discovered during World War II, but their existence was suspected well before that. The Polar Jet is located about 10km high in the atmosphere near the tropopause, or the upper edge of the main weather-producing part of the atmosphere. The Subtropical Jet is typically found at a height of 13km above the subtropical highs. Both jet streams are typically several hundreds of kilometres wide and a few kilometres thick. Wind speeds will vary with the low end being around 150kph and the extreme high end coming in at nearly 450kph.

The jet streams form where we have sharp, or quick, changes in air temperatures. These rapid changes in temperatures create rapid changes in pressure. This rapid change over a relatively short distance means there is a steep pressure gradient. When the upper atmospheric wind encounters this steep pressure gradient it quickly intensifies, creating the jet stream. This steep pressure gradient is not the only thing that allows jet streams to form. We also need a fairly strong wind already blowing – the upper atmospheric wind. One of the reasons we have strong upper atmospheric winds comes from something known as the conservation of angular momentum.

WE’RE MOVING QUICKLY

Consider that at the Equator the Earth rotates at approximately 1670kph. If it is a windless day, the air over the equator is still moving along with the Earth and if the Earth would suddenly stop the air would continue to move eastwards (westerly wind) until friction slowed it down, because the air has momentum. Now it is time to introduce a little math.

The formula for calculating angular momentum is: A = mvr where: m equals mass, v equals velocity, and r = radial distance. The mass we are talking about is the mass of the air which we will take as basically remaining constant no matter where we are. Velocity is how fast the air will be moving, and radial distance is the distance between the parcel of air and Earth’s rotational axis. It is this final part of the equation that helps to explain why we have strong upper atmospheric winds.

If you remember back to our lesson on the general flow of our atmosphere you will remember that air rises at the equator and then flows northward in the upper atmosphere. As this air moves northward it needs to conserve its angular momentum. That is, our value for A in our equation wants to stay the same. The mass of the air will also remain the same but the radial distance is decreasing. So in order for A to remain the same something will have to happen with the velocity, it will increase. For example, if we say all the values in this formula are equal to one then it would look something like this: 1 = 1 x 1 x 1. Now if we cut the radial distance in half it would look like this: 0.5 = 1 x 1 x 0.5. Our angular momentum has not remained constant! In order for it to do so we would have to change our velocity to two, or double it! (1 = 1 x 2 x 0.5)

OK, the math lesson is over. We now know that the two main jet streams are formed by a combination of a steep pressure gradient brought about by rapid changes in temperature along with the conservation of angular momentum. In our next lesson we will continue our look at jet streams.

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