Cloud en­coun­ters of the third kind

Popular Science - - HEAD TRIP -

THE DISC-SHAPED FOR­MA­TIONS TO THE LEFT look a lot like UFOs. Thou­sands of baf­fled on­look­ers even re­port them as such each year. But this alien at­mo­spheric phe­nom­e­non starts right here on Earth.

An iso­lated moun­tain is the first in­gre­di­ent. The lone peak forces flow­ing air up­ward as grav­ity pushes it down. This trig­gers a con­tin­u­ous up-and-down at­mo­spheric wave above the moun­tain. Nor­mally, you can’t see this wave, but if the tem­per­a­ture in its peaks drops to the dew point (when wa­ter con­denses) clouds will be­gin to de­velop.

For the clouds to mold into saucers, a sec­ond in­gre­di­ent is needed: a sym­met­ri­cal and cone-shaped hill­top. This form means that air rises at sim­i­lar heights along the moun­tain’s sur­face, end­ing in a lens-like con­fig­u­ra­tion. With enough wa­ter va­por and air­flow, more than one layer could stack up.

Be­cause these wispy discs—called lentic­u­lar clouds— come from strong ris­ing and sink­ing air­flow, they in­di­cate un­safe tur­bu­lence for planes. Thrill-seek­ing glider pilots, of course, chase them, at­tempt­ing to catch and ride the ris­ing air. One gutsy air­man hit a record high of 50,000 feet.

By Cici Zhang / pho­to­graph by Kyle Mi­jlof

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