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September 11, 2008
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Steering deadly storms

By Alan Stahler

Special to The Union

A typical hurricane packs a punch of some three trillion watts - as much power as could be generated by three thousand large coal or nuclear plants.

A truly monstrous hurricane can deliver ten times as much.

A hurricane over land is a bull in a china shop. As the bull charges from one shop to another, meteorologists try to predict where it will hit next.

Earth's tropics absorb more sunlight than do the poles. The tropics would grow infinitely hot, but for the fact that warm air rises. Heated by the land and water beneath it, warm air rises up from the tropics and flows toward the poles. Cooler air flows out from the poles to replace the air leaving the tropics.

Earth's spin puts an east-west twist on these flows, turning them, for instance, into the prevailing westerlies that blow storms out of the Pacific and into California.

As great currents of air blow over the oceans, they set the waters in motion. The Gulf Stream carries warm sub-tropical water up the eastern seaboard, moderating temperatures on the east coast and in Europe. The California Current brings cold Alaskan waters down the west coast.

In tropical and sub-tropical latitudes, atmospheric circulation manifests as the east-to-west Trade Winds, which carried sailing ships from Europe to the new world.

The Trades also carry hurricanes east-to-west, across the Atlantic.

Not all of the air warmed by the sea escapes to the poles. As warm, moist air rises, it cools; as it cools, water vapor condenses to form clouds (much as water vapor in your breath condenses when you blow into a freezer).

It takes energy to turn liquid water to vapor (water won't boil unless you put it on the stove). When water vapor condenses Ð when it changes from gas back to liquid Ð it releases that very same energy. Newly-formed cloud droplets thus warm the air around them.

Re-warmed by the energy of condensing cloud droplets, the air in the cloud resumes its ascent.

Warm air rises, and cools ... water vapor in the air condenses, forming clouds, and re-warming the air ... a cycle emerges, and a towering thundercloud climbs into the sky.

Thunderstorms are a daily occurrence in the tropics. Most of the time, they grow, rage, and die. But if conditions are right , multiple thunderstorms may merge into a tropical storm.

If such conditions endure, the tropical storm may grow into a hurricane.

Just burning wood, or coal, or gasoline won't by itself get you from here to there. For that, you need an engine - a machine that transforms the energy of heat into the energy of motion. A car engine is such a heat engine.

A hurricane, too, is a heat engine, transforming the ocean's warmth (and the energy carried within its water vapor) into the fierce motion of wind.

It's hard to think of something composed mostly of air as an object, yet that is what a hurricane/heat engine is: an object that can be pushed from place to place by the wind.

Like leaves carried this way and that by currents in a stream, hurricanes are driven by the currents of air in which they're embedded. The Trade Winds carry the storms across the Atlantic. Approaching North America, the hurricane may feel a northward thrust from the Bermuda High, a huge mountain of air that spins clockwise over the Atlantic ... and causes hurricanes to hook to the north.

Just as a wildfire creates its own weather as it sucks in the surrounding air, hurricanes spin up the air around themselves, creating whirlpools of air that, in turn, can nudge the hurricane this way or that.

High and low pressure systems, such as those that bring storms to California, also push or a pull on a hurricane. Three years ago, a low over the Midwest sucked hurricane Katrina inland ... and into New Orleans.

Since it is the sun that energizes Earth's atmosphere, our atmosphere's motions respond to where the sun is in the sky - to the seasons. Hurricane paths evolve as summer and fall progress. The Gulf is most likely to be hit in September.

I spoke recently with Kerry Emanuel, professor of meteorology at MIT, and author of "Divine Wind," a book on hurricanes. Our conversation will air on KVMR next Tuesday at 1 p.m.

Alan's radio program, "Soundings," can be heard on KVMR, 89.5 FM, on alternate Tuesdays, at noon. He offers private classes in science, math and writing, and may be reached at 470.8937, or at stahler@kvmr.org


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The Union Updated Sep 11, 2008 01:53PM Published Sep 11, 2008 03:00AM Copyright 2008 The Union. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.