Isaac's Storm - Erik Larson [8]
The waters of the Gulf got hot.
OVER THE NIGER, the colliding winds veered and arced. Thunderstorms of great violence purpled the sky. A huge parcel of air began circling slowly, far too high for anyone on the ground to notice. The powerful Saharan wind swept it west toward the Atlantic as a wave of turbulence, thunderstorms, and driving rain.
Within this “easterly wave,” moisture-freighted air rose high into the troposphere, the first layer of sky and the realm where all weather occurs. The air cooled rapidly as it pierced colder and colder layers of atmosphere and encountered lower and lower pressure. The lower the pressure, the more the air expanded. As it expanded it cooled. It continued to rise but less than a mile above the earth crossed a threshold, and a phase change occurred. The air got so cold, it could no longer retain the water it carried. The vapor condensed en masse, as if at the tap of a conductor’s baton. The resulting droplets were so tiny they remained suspended in the rising air.
The updrafts pushed the droplets higher and higher at up to one hundred miles an hour. At four miles above the ground the droplets froze, and the rising air became filled with snowflakes and shards of ice. Men on the ground saw blossoms of cotton with flat gray bottoms that marked the altitude where condensation had begun. Children saw camels, rabbits, and cannon fire. The clouds bloomed before their eyes. Cells within grew and quickly expired. Some cells smoked into the sky like Christmas rockets. Others became massive Gibraltars of condensed water, Cumulus congestus; some rose higher, Cumulonimbus calvus. In the pillars that reached the top of the troposphere, temperatures fell to 100 degrees below zero. Tiny hexagonal mirrors of ice drifted from the peaks in lovely translucent veils, or “virga.”
Something powerful and ultimately deadly occurred within these clouds. As the water rose and cooled and condensed, it also released heat. In the sky over Africa in August 1900, trillions upon trillions of water molecules began breathing tiny fires. This heat propelled the air even higher into the atmosphere until the cloudtops flattened to form Cumulonimbus capillatus incus. Incus meaning “anvil,” the name too of an anvil-shaped bone in the human ear. These were thunderheads. “Convection.” Higher up, the strongest clouds penetrated the stratosphere. Soon an army of great thunderheads was marching west along the horizon, watched closely by the captains of British ships sailing down the African coast with fresh troops for the Boer War. Seventy to eighty such waves drifted from West Africa into the Atlantic every summer, some dangerous, most not. The captains knew them less as weather, more as geography—something to watch to fill the long hours at sea. At dawn and dusk, the distant clouds warmed the sky with color. Rain smudged from their bottoms in fallstreaks. Frozen virga drifted from their glaciated tops. When the light was just right or a squall was near, the clouds formed an escarpment of black. Frigate birds sidelit by the sun drifted in the foreground and flecked the sky with diamond.
Ships directly in the path of the August wave got a different view. Each wave had a “period” of