Friday, August 31, 1900
Spiderwebs and Ice
The storm entered the Caribbean Sea early on Friday morning, August 31, in a confetti of sparks and thunder, with increased winds that raised from the sea patches of dense foam and streaks of spindrift. In the cloudlight of morning the sea was a dead gray scabbed with green. Rain began falling on St. Kitts, an island west by northwest of Antigua. What made this rain unusual was the fact it did not deplete the clouds overhead. The storm only got bigger.
As vapor rose through the clouds and began to condense, it deposited its moisture on tiny bits of airborne debris, ranging from submicroscopic "Aitken" nuclei to pollen, spiderwebs, volcanic ash, steamship exhaust, Saharan dust, even the pulverized ferrous salts of meteors disintegrated in the atmosphere. Somewhere over St. Kitts, a giant plume of water, ice, and aerosol debris rocketed through the troposphere getting colder and colder until it penetrated the stratosphere, where it entered a realm of new warmth caused by direct radiation from the sun. Suddenly the plume was colder than the air around it. It lost buoyancy. It arced against the hard blue of the stratosphere and fell back toward the earth.
This descending air met air still rising from below. Falling droplets met ascending droplets. The collisions formed bigger drops and the bigger they grew, the faster they fell. Now they overtook other falling droplets and grew bigger still. A raindrop four-hundredths of an inch in diameter falls at nine miles an hour; a droplet six times as large falls at twenty. Billions of droplets now got bigger and bigger until they achieved terminal velocities capable of propelling them all the way to the ground.
Under ordinary circumstances, the process of rain production depletes clouds. The "sink rate," or the rate at which water leaves a cloud, exceeds the supply of moisture arriving from the air and sea below, causing clouds to dissipate like ghosts returning to the afterworld. But hurricanes defeat this cycle. They use wind to harvest moisture and deliver it to their centers. As the wind races along the surface of the sea, it increases the rate of evaporation and captures spindrift and foam. The faster the wind blows, the more vapor it picks up and the more energy it transfers to the storm. The resulting surge of condensation and heat in the storm's core causes even greater volumes of air to rush into the sky. Pressure falls again. Wind velocities increase. The cycle repeats itself.
The result can be rainfall more akin to the flow from a faucet than from a cloud.
In 1979 a tropical storm named Claudette blew off the Gulf of Mexico near Galveston and deluged the town of Alvin, Texas, with forty-two inches of rain in twenty-four hours, still the U.S. record for sheer intensity. A Philippine typhoon holds the world's record, dropping 73.62 inches in twenty-four hours. Total accumulations have been higher, however. Ninety-six and a half inches of rain once fell on Silver Hill, Jamaica, over four days. That's eight feet. In 1899 a hurricane dropped an estimated 2.6 billion tons of water on Puerto Rico. Hurricane Camille, which came ashore on the Gulf Coast in August 1969, was still flush with water two days later when it reached Virginia. With no advance warning from the Weather Bureau, it jettisoned thirty inches of rain in six hours. Hillsides turned to mud, then to an earthen slurry that flowed at highway speeds. In Virginia alone, 109 people lost their lives.
Camille's rain fell with such ferocity it was said to have filled the overhead nostrils of birds and drowned them from the trees.
Excerpted from ISAAC'S STORM. Copyright © 1999 by Erik Larson.