The elu­sive green flash

Popular Science - - HEAD TRIP -

THE FI­NALE OF EV­ERY SUN­SET IS GREEN. TRY to catch it, if you can. The story of the so-called green flash be­gins by an­swer­ing this ques­tion: Why is the sky blue?

Sun­light con­tains ev­ery vis­i­ble color, each with its own wave­length. Light trav­els as rip­ples, with crests and val­leys; wave­length is the dis­tance be­tween the crests. The shorter the wave­length, the steeper the rip­ples. Of the three pri­mary vis­ual col­ors—blue, red, and green—blue has the short­est wave­length, and red the long­est. As sun­light hits Earth, blue light’s steep waves cause par­ti­cles in the air to scat­ter it al­most to­tally, turn­ing our skies cerulean. The left­overs com­bine to cre­ate the sun’s yel­low glow.

At sun­set, col­ors fade at vary­ing rates. Just be­fore the sun van­ishes, the red light’s shal­lower rip­ple causes it to shoot over­head and miss your eyes. Green, with its steeper wave­length, re­mains the sole color sur­vivor, if only for a sec­ond.

The green flash may hap­pen ev­ery evening, but it’s hard to spot. At­mo­spheric conditions, like mois­ture and pol­lu­tion, can warp and de­flect the ver­dant tone be­fore it reaches our eyes. A clear coastal night is typ­i­cally your best chance.

By Mark D. Kauf­man / pho­to­graph Nigella Hill­garth

Newspapers in English

Newspapers from USA

© PressReader. All rights reserved.