Weather Whys; Hadley Cell

This is weather wise. On a local scale, the circulation pattern of the atmosphere may seem to be changing continuously. From day to day, a city may experience winds of varying speeds coming from widely different directions. Winds can be warm and moist one day, then cold and dry the next. But looking at the big picture, there are certain wind and circulation patterns which are on the average, relatively constant. In other words, if you average the wind speeds and directions over a long period of time, those patterns should show up clearly. One such pattern is called the Hadley Cell. It was first theorized by the 18th century meteorologist George Hadley. He figured that since warm air rises and cold air sinks, there should be rising air at the equator and sinking air at the poles. The upward flow of air at the equator removes a lot of air and so a region of low pressure forms at the equator.

Similarly, the downward flow at the poles brings in air molecules and builds an area of high pressure. Now in the absence of other effects, air tends to flow from regions of high pressure to regions of low pressure. So in the Hadley Cell pattern, air at the ground flows from the polar regions to the equator. There are two of those cells, one each in the southern and northern hemispheres. That simple model represented a great advance in the understanding of meteorology, but it was not complete. For one thing, Hadley didn't take into consideration the effects of the Earth's rotation, and that changes things quite a bit. Of course, the real circulation in the atmosphere is much more complex than any simple model can show. But understanding the basic pattern is essential to unraveling the complexities we still don't understand. Otherwise, as a production of the Center for Analysis and Prediction of Storms at the University of Oklahoma and is made possible with a grant from the National Science Foundation, I'm Drew Barlow.