TO SEE THE BIRTH OF AN ATLANTIC HURRICANE, LOOK TO AFRICA
THE WINTER CRUSHED the East Coast of the US. So let us crush your dreams of spring with a gentle reminder: Hurricane season is right around the corner. And the hurricanes that will slam into the Atlantic seaboard in just a couple of months are already glints in the eyes of storms-yet-to-be-born in Africa.
The Atlantic hurricane season officially kicks off in June, but it starts over the Sahara. In the Sudanese highlands—the same place the Nile begins—sun-heated air bubbles upward and condenses into mushroom-shaped thunderheads thousands of miles high. At the same time, enormous waves of air in the upper atmosphere push those storms west, toward the Atlantic. Most of these tempests will die on the coast. But some get second lives—as tropical storms or hurricanes. That turns out to be an important connection if you're trying to predict hurricanes. New research shows that the temperature of clouds in those African storms could help meteorologists figure out which will mutate into coast-and-island-pummeling monsters on the other side of the ocean.
While these tall clouds are growing out of the high desert, a massive, oscillating atmospheric pattern called the Africa Easterly Wave is settling in thousands of feet above. It's a vast, sine wave-shaped air flow that carries weather across the Sahara, east to west. When those Sudanese thunderstorms rise, they get caught up in the flow, off to drench Africa's west coast. And there they stop; the Atlantic Ocean is cold, and thunderstorms thrive on heat.
But the Africa Easterly Wave persists, and sometimes so does the energy that used to be a thunderstorm. "And when the wave reaches the central part of the Atlantic basin, then some of the thunderstorms will start to develop again," says Robert Rogers, a meteorologist with NOAA's Atlantic Oceanographic and Meteorological Laboratory. Out in the mid-Atlantic, the water is warmer, the air more humid, and the wind stronger. And the Africa Easterly Wave's undulations spin all those conditions into a nice cyclonic spiral.
Only about one in 10 of the African storms re-emerge, though. "That's the million dollar question: What causes some of the storms in these Africa Easterly Waves to develop into a tropical cyclone?" says Jim Kossin, a climate researcher with the National Centers for Environmental Information. Even with a perfect accounting of environmental conditions in the mid-Atlantic, scientists still can't predict which storms will complete the transition.
The fate might be written in the clouds of the thunderstorms. In new research, meteorologists from Tel Aviv University used geostationary satellites to look at the thunderstorms right before they disappeared off the coast of Africa into the Atlantic. Specifically, the scientists were looking at the temperatures at the very tops of each storm's cumulonimbus clouds. Temperature is a proxy for height—clouds grow predictably colder as they rise. And the taller a cloud, the more energy it contains.
And surprise: These tall, cold clouds often prefigured later tropical storms. If the temperature of about 5 percent of the clouds in a thunderstorm drops to -58˚F mark—corresponding roughly to the maximum altitude for cumulonimbus clouds—the odds jump that it'll resurrect as a cyclone.
And when does that happen? Well, 2015 is an El Niño year, so this season could be another dud. "Then again, it only takes one hurricane to make for an awful year," says Kossin. Hurricane Andrew (the 1992 monster that flattened Miami) struck during an El Niño. No amount of prediction will ever stop a storm like Andrew (or any storm, for that matter), but right now meteorologists start tracking storms about a week before they look likely to make landfall. Looking at the cloud heights of African thunderstorms could double that lead time—and every added bit of foresight helps more people get out of the way.