Isaac's Storm | Storm Tracker

DAY 5

THE STORM

September 6, 1900

Swells

The hurricane had begun sculpting the Gulf the moment it left Cuba and now it transmitted storm swells toward Galveston.

Waves form by absorbing energy from the wind. The longer the "fetch," or the expanse of 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 or 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.

In a cyclonic system, the wind spirals to the left, but the waves continue forward along their original paths at speeds far faster than the storm's overall forward velocity. The forward speed of the storm of 1900 was probably no greater than ten miles an hour, but it produced swells that moved at fifty miles an hour, and began reaching the Texas coast fifteen hours after their formation.

Soon after the waves left the cyclone, they changed shape. They retained their energy, but lost much of their height and their jagged crests. They became long, easy undulations, like the grease-smooth swells that Columbus spotted on his first voyage.

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.