A visible light satellite image Sunday shows powerful hurricane Isabel churning in the Atlantic. The storm's forward speed increased from 9 mph Friday to 12 mph Sunday as it took a bead on the East Coast of the United States.

You could say Isabel was building a head of steam.

Hurricanes gain their strength from warm ocean water, as discussed in Friday's Image of the Day. But why do they weaken when they come ashore? Some people think it's due to friction. That's a myth.

The truth is a hurricane loses its fuel, the warm water, when it moves over land. In hours, a Category 4 or 5 storm can weaken to tropical storm status, its winds no more than a bother. But the moisture picked up while offshore can continue to pour down copiously for days.

A hurricane over land is robbed not just of heat but of moisture source. That cuts down on the ability to produce thunderstorms near the center. This reduces convection, which also serves to weaken the storm.

Studies have found that curious things happen at landfall that are related to the friction myth, however.

Upon landfall, the top sustained winds -- those that last a minute or longer -- can drop due to a dampening effect caused by rough terrain -- bushes, trees and houses. But a corresponding increase in brief gusts sort of makes up for this, danger-wise. The gusts are enhanced by turbulence caused by the rough terrain, bringing faster winds to the surface in bursts that last a few seconds.