EYES AND ARMS: THE STRUC­TURE OF A HUR­RI­CANE

Air & Space Smithsonian - - In The Museum -

AS THE AIR IN­SIDE A HUR­RI­CANE WARMS UP, much of the air at the sur­face rises long be­fore it reaches the core. The re­sult­ing clouds drop mas­sive amounts of rain. When the air even­tu­ally rises high enough to cool, it sinks back to the sur­face, where it re­joins the in­flow and starts the process all over again.

This is called the storm’s sec­ondary cir­cu­la­tion, and pro­duces rings of thun­der­storms ra­di­at­ing out from the cen­ter—rain bands. Though any­one who’s seen a satel­lite im­age of a trop­i­cal storm can tell you they don’t look like rings. They look like crazy, spi­ral­ing arms.

Wind speed isn’t the same through­out the storm; you’ll usu­ally find the high­est winds just shy of the cen­ter. The rea­son is that as the spi­ral tight­ens in­ward, the air moves faster (like a spin­ning fig­ure skater pulling in her arms, the smaller the radius of the spin, the faster the spin). The strange part is that the ac­tual cen­ter of­ten has no wind at all—only calm, blue sky.

The eye of a cy­clone is calm be­cause winds rush­ing to the cen­ter can’t con­verge on a point, so they rise, not just up but way up, shoot­ing out of the top of the storm like an in­vis­i­ble vol­cano. The place where all that sea-warmed air is forced to rise, where it’s wettest and where the air is mov­ing fastest, is called the eye­wall. It’s ba­si­cally a cylin­dri­cal, su­per-de­struc­tive wall of clouds, and it’s the part of the storm that does the most dam­age when it passes over land.

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