Air & Space Smithsonian - - In The Museum -

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.

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