2012: Advent Atmosphere: Hadley Cells

by on December 12, 2012

As the ocean has currents, so does the air. But where the ocean has to move around the continents, air can move over them. The land isn’t flat, but in most places it’s a pretty good approximation of it. Air currents are primarily shaped not by the Earth’s surface, but by its spin.

You’ve heard of Coriolis forces. That’s the name for the lag you get when the Earth – or any planet – spins around under its atmosphere. The air isn’t tied down to the ground. It follows on slightly behind. The prevailing wind around the Earth is from east to west, because the ground moves faster than the air does.

That’s the first effect in shaping the major currents of the atmosphere. The second is temperature. Air at the Equator gets warm and expands. It moves towards the poles, where it cools again and starts to flow back, pushed along by the new hot air following behind it.

A Hadley cell is a region where air flows west along the equator, then away from it, then east, then back toward the equator again. There are three bands of Hadley cells in each hemisphere, stacked north to south and south to north.

So the prevailing wind where you are is shaped partly by these two forces that push the air around, as well as by the local geography and ocean currents. All this north-south and east-west movement mean the atmosphere is pretty well mixed horizontally. Things you put into the air here end up over there, and pretty quickly too. Vertically, things look rather different.

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