Answers: Rising and Sinking Air
Q: In your AOPA Flight Training Magazine, January 2010 article you say that the curving path of upper air winds cause air to sink in some areas, creating high pressure at the surface, and to rise in other areas creating or strengthening areas of low pressure at the surface. My question: How does adding rising air to an area, which I understand would increase the column of air and increase the surface pressure, cause a low pressure at the surface since surface pressure is a measure of the mass of air above a location?
Edgar, Sacramento, Calif.
A: In the Flight Training article I should have added that when air rises from a surface area of low pressure it flows away as part of the upper-air winds as fast as it rises.
To see what goes on, you can think of the upper air winds pulling air up from the surface and carrying the air away as fast as it rises.
In brief, as upper-air winds flow around the earth in curvy paths they converge in some areas; that is, the winds squeeze together. In effect, the air is “piling up” in an area of convergence, which forces some air to descend to form or strengthen an area of surface high pressure below.
In other areas winds aloft diverge; that is they spread out. Such divergence causes air to rise from the surface, which creates an area of low pressure at the surface. The air doesn’t “pile up” aloft in an area of divergence because it flows away with the upper-air winds.
I have more about this in Chapter 5 of my latest book, The AMS Weather Book, including a graphic on Page 112 showing how it works. Unfortunately I can’t use book images on the Web.
But, I found a diagram on a Harvard University atmospheric science Web page on extratropical cyclones that should help you visualize what happens. When you go to the Harvard Web page scroll down past the 500 mb chart, and the quasi-geostrophic vorticity equation and you’ll see a nice, three-dimensional drawing of the upper air and surface winds of an extratropical cyclone and how they are connected.
Vorticity refers to the spinning motion of the air. These spinning motions help create convergence and divergence.
The only change I would make to Harvard diagram is that I would show the wind stream becoming wider where it’s labeled “divergence” and becoming narrower where it’s labeled “convergence.
Hope this answers your question.
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