Waves form by absorbing energy from the wind. The longer the "fetch," or the expanse of the sea over which the wind can blow without obstruction, the taller a wave gets. The taller it gets, the more efficiently it absorbs additional energy. Generally, its maximum height will equal half the speed of the wind. Thus a wind of 150 miles an hour can produce waves up to 75 feet tall. Other conditions, such as the chance superimposition of two are more waves, can cause waves to grow even bigger. The tallest wave on record was 112 feet, but occurred amid steady winds of only 75 miles an hour.
As soon as they reached the Texas coast, however, they changed shape again. Whenever a deep-sea swell enters shallow water its leading edge slows. Water piles up behind it. The wave grows again. It is this effect that makes earthquake-spawned tsunamis so deceptive and so deadly. A tsunami travels across the ocean as a small hump of water but at speeds as high as five hundred miles an hour. When it reaches land, it explodes.
At 7:30 P.M., the wind shifted again, this time from the east to south. And again it accelerated. It moved through the city like a mailman delivering dynamite. Sustained winds must have reached 150 miles an hour, gusts perhaps 200 or more.The sea followed.Galveston became Atlantis.
While I didn't love Isaac's Storm, I'm glad I finally got around to reading it and learning not only about the history of this particular Galveston hurricane, but also a little bit about meteorology.