No Hurricanes for Alaska

Northerners love to brag about the horrors they must endure. We start with cold and snow, tell a few bear stories, go on through active volcanoes and earthquakes, and can always toss in the size of our forest fires. We imply that we have to be very tough people to deal with this tough land--until some city-bred Easterner smilingly asks us about the last hurricane to hit our local coasts.

Now, it's easy for us to come up with hurricane-force winds. Those are formally defined as winds moving at or faster than 120 kilometers an hour (73 mph, or 64 knots). In a typical winter, the Aleutian passes can easily muster that much wind--far too frequently for the comfort of those who fish offshore. For that matter, whenever there's a strong high-pressure zone on one side of our mountains and a low on the other, a great deal of air can howl in a hurry through the valleys. A homebuilder who wants a secure roof in Valdez, for example, should design it to withstand winds of 120 mph, because cold gusts surging down Mineral Creek from the mountain snowfields above can reach such speeds.

In the western Pacific Ocean, hurricane-type storms are called typhoons, from the Chinese words for supreme wind. Whether called typhoons or hurricanes, we don't have them. We don't have them for the same reason that we don't have white coralline beaches: our seas are too cold.

It takes tropical ocean temperatures to provide the energy for a really big cyclonic storm, and a big cyclone is what a hurricane or typhoon is. The heat in the water--a huge supply of heat--provides the energy for the storms.

As summer ends in the northern hemisphere, atmospheric patterns that have been fairly stable through the season begin to respond to the waning of energy reaching the earth from the sun. Parcels of air break away from the belt of low pressure that surrounds the equator. The low-pressure cells wander northward, propelled by the earth's rotation, gathering energy from the warm ocean as they go. The warmer the water, the more energy available to power the storm.

Just as water flows from a high spot to a lower one, air flows from a zone of high pressure to one that is lower. The heart of a hurricane--the famous eye, where all seems comparatively calm--can be profoundly low. In a good Alaska winter storm, the air pressure may register on a barometer as about 28 inches of mercury. Air pressure in the recent Hurricane Gilbert descended to only a little over 26.

With such conditions, the neighboring air pours in like water rushing down a hole. Because of the earth's spinning, the inflowing air takes a curved path as it streams toward the depression. The spin is counterclockwise in the northern hemisphere, clockwise in the southern. Enough air rushing in, enough evaporation to keep the flow moving, and the storm becomes a hurricane.

The entering air doesn't equalize the air pressure--it speeds up the evaporation. Impressive amounts of warm water vapor get trapped in a hurricane, later to be dumped as great quantities of rain when the storm crosses land: hurricanes have dropped as much as 20 inches of rain in just a few hours. Once over solid ground, a hurricane is disconnected from its heat engine and loses force swiftly.

In effect, the tropical cyclones are nature's way of moving heat from the equator to the temperate zones. The heat carried into the atmosphere by evaporation in the low latitudes is released by condensation in the higher latitudes. With hurricanes, nature has found a very dramatic way to accomplish the heat transfer.