Air & Space Smithsonian - - In The Museum -

AS THE SUN HEATS THE OCEAN, wa­ter mol­e­cules at the sur­face jump around in ran­dom di­rec­tions. Some jump up, and if they’re ex­cited enough, they stay up, trans­fer­ring heat en­ergy from the wa­ter to the air. Voilà, evap­o­ra­tion. Be­cause of small vari­a­tions across the wa­ter’s sur­face, the process is un­even—more evap­o­rates in some places than in oth­ers. At each of those spots, there’s a par­cel of air that has a higher per­cent­age of wa­ter va­por (which is lighter than air) than the air around it. This causes the air to rise in a process called con­vec­tion.

Some ver­sion of this process is hap­pen­ing ev­ery­where, all the time. It’s what makes clouds. The warm, moist air re­sult­ing from evap­o­ra­tion rises into lower-pres­sure air and ex­pands. That costs en­ergy, and the air cools (what’s called adi­a­batic cool­ing). The wa­ter va­por mol­e­cules, now more lethar­gic, get stuck to tiny dust par­ti­cles called con­den­sa­tion nu­clei, in the process re­turn­ing to a liq­uid state, and the re­sult is a cloud. The air within the cloud con­tin­ues to rise, en­coun­ter­ing colder air. More con­den­sa­tion oc­curs, the con­den­sa­tion nu­clei start stick­ing to­gether and co­a­lesc­ing into droplets, and you get rain. As they fall, the rain­drops par­tially evap­o­rate, which cools the air within the cloud the same way evap­o­rat­ing sweat cools your body.

In a typ­i­cal thun­der­storm, that cool air sinks all the way to the sur­face and spreads out, what me­te­o­rol­o­gists call a down­burst. Re­plac­ing warm sur­face air with cool air ef­fec­tively shuts off the con­vec­tion that cre­ated the storm in the first place. The sky clears and it’s over.

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