A SELF-SUSTAINING HEAT ENGINE
SCIENTISTS BELIEVE THAT CYCLOGENESIS has something to do with a disturbance in the area of a nascent storm, something external that acts as a catalyst. Kerry Emanuel of MIT likens a run-of-the-mill batch of tropical thunderstorms to a ball balanced on a hilltop. Little ridges surrounding the ball (pressure gradients that help shut down a normal storm) prevent it from rolling down the slope. But if something gives the ball a sufficient kick to get it over the ridges, away it goes.
The question is what provides the kick. The general idea is that some large weather feature enters the stormy area and forces the moist air inside it to rise en masse. Up until that point, air entering the storm at middle altitudes was drier, so rain falling through the clouds evaporated and cooled the air, creating downdrafts. Now, with the storm region becoming more humid, the air is saturated with water vapor and the raindrops have a harder time evaporating as they fall. The air stays warm, and there are no resulting downdrafts. That in turn leaves nothing to counteract the warm, rising air at the surface. The cycle that normally causes thunderstorms to dissipate has been shut off, and the ball is now rolling downhill with a vengeance.
All that air rising from the surface leaves an enormous vacancy beneath the cluster of storms, which at this stage meteorologists start calling a tropical depression. The vacancy is an area of low pressure near the surface, and the surrounding air tries to even things up and make the pressures equal. If you could stand in many places at once under the developing storm, you would feel a lot of wind now rushing in toward the storm’s core, and you’d get a visceral understanding of the heat engine idea. As the wind blows in toward the center, it removes moist air from directly above any given patch of ocean, increasing evaporation. More evaporation transfers more heat to the system, so you get more rising air and lower pressure at the core. Those changes fuel even stronger winds moving toward the center, which further accelerates evaporation. And so the storm grows.
A storm off Australia, viewed from space. In the southern hemisphere, the rotation is clockwise.