Isaac's Storm - Erik Larson [20]
He fired the cannon. As the ball soared into the sky, he hurried to the muzzle and sat on it. Skeptics in the audience no doubt stepped back a respectful distance, wary not only of the descending ball but also of the likely splash of viscera. How the seconds must have dragged as that ball whined into its descent, the smile on Furtenbach’s face growing fixed, the more squeamish members of the audience raising their hands to cover their eyes but peeking of course through the latticework of fingers.…
Thwump.
Silence.
Furtenbach slid from the muzzle, his head and smile intact. To the west—a small crater. Proof at last. The earth did spin.
It was Edmund Halley, of comet fame, who recognized that this rotation might have a powerful effect on the earth’s weather. Seeking to explain the trade winds, Halley argued that the sun’s rays fell most consistently upon the equator. As the sun moved over the earth, it caused successive parcels of air to rise. Other, cooler air rushed in to fill the space and followed the sun around the globe in a steady rush of wind.
A compelling theory, but it had a significant hole: It could not explain why the prevailing easterlies of the trade belt suddenly gave way north of the Horse Latitudes to winds blowing in exactly the opposite direction.
What Halley failed to take into account was the shape of the earth: the fact that the world moves more slowly in New York City—although no New Yorker would ever concede it—than in Key West. In 1735, George Hadley, often confused with Halley, crafted an explanation of the trades that was so compellingly simple it remained the accepted theory even through Isaac Cline’s Saturday.
HADLEY RECOGNIZED THAT an object anchored near the north pole and another near the equator traveled through space at different speeds: Both objects, being attached to the same planet, had to complete one rotation within the same period of time, but the object at the equator had to cover a much greater distance and therefore had to move a lot faster. The air at each location, Hadley saw, also moved at these differing velocities.
He agreed with Halley that as the sun heated the equator, it caused air to rise, and cooler air flowed in to take its place. But Hadley proposed that the cool replacement air would retain its polar velocity. The farther south it went, the slower it would seem to travel relative to the ground below. Anyone encountering this slow-moving mass of air would experience it as a wind that veered to the right of its direction, or toward the west. These were the trade winds.
Conversely, Hadley saw, air migrating north would seem to accelerate relative to the ground. As it cooled, it would descend but retain its faster equatorial speed. Observers on the ground would perceive this as a wind blowing toward the east, or veering to the right of its northward course. This wind, Hadley argued, produced the steady breeze north of the Horse Latitudes that blew the explorers back home.
A century later, a French mathematician, Gaspard Coriolis, worked out the mathematics of all this, to prove that any object moving over the northern hemisphere would seem to veer to the right, while any object moving over the southern hemisphere would appear to veer left. Isaac, in his 1891 talk to the Galveston YMCA, gave a cruelly detailed explanation of the Coriolis effect. The crowd listened with iron concentration. “… At latitude 30 degrees the velocity of the earth eastward is 897 miles per hour, and at 45 degrees it is 732 miles per hour, or