Alan Stahler: Cooling the tropics, warming the poles |

Alan Stahler: Cooling the tropics, warming the poles

Alan Stahler

The sun is God's own campfire. Without the sun, our planet would be cold — hundreds of degrees below zero cold.

There can be too much of a good thing, though. The sun is overhead, more or less, in the tropics, and that makes the sun especially efficient at warming tropical climes. Earth's tropical regions must export some of that heat lest they roast. Without that heat, the extra-tropical regions, regions outside the tropics, would freeze.

If you think of all that tropical solar energy as pollution — well, the solution to pollution is dilution. That catchy phrase used to be bandied about by the waste management community — until it was realized that there are limits to how much trash we can pump into the oceans and into the air. But that's just how our planet deals with tropical sunshine; it sends the energy out, cooling the tropics and warming the poles.

This past weekend, we in Northern California had a front row seat to watch that heat-moving process in action.

Say the word cyclone and we usually think of hurricanes. Hurricanes are, indeed, tropical cyclones. But we here outside the tropics have our own cyclones — extra-tropical cyclones — counterclockwise-swirling systems of winds that form out over the Pacific, then blow onto the continent. Winds on the leading (eastern) edge of the storm blow, roughly, south-to-north; winds on the trailing (western) edge blow north-to-south.

Suppose you're cooking some soup, and you toss in some herbs. You want to taste those herbs through-out the soup, so you stir it, pulling herbs from one side of the pot to the other.

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An extra-tropical cyclone — a cyclone outside the tropics — thus stirs the atmosphere, dragging warm air up from the south, cold air down from the north.

That warm air coming up from the south has been sitting over the ocean, absorbing water vapor that's evaporated off the ocean.

Evaporation is like slow-motion boiling. It takes heat energy to evaporate water, to change it from liquid to gas. Like boiling, evaporation takes energy — it's the same process that cools us off when sweat evaporates. Here, it's the ocean that's sweating.

As water vapor wafts up off the ocean, it carries that energy with it. With all that energy in it, water vapor is like gasoline — except it doesn't have to burn to release energy. All it needs to do revert from gas back to liquid.

This is what water vapor was doing, over our heads, most of last weekend — condensing from vapor into cloud droplets, which then evolved into raindrops. As it evolved from gaseous vapor to liquid droplets, the water released tremendous amounts of heat energy — heat energy it had absorbed farther south, heat energy that's now farther north, closer to the poles.

Al Stahler's science programs can be heard on KVMR (89.5 FM). He brings an enjoyment of science and nature to students of all ages – kid to adult – and may be reached at

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