Green Flashes of Two Types
Two types of green flash seen by Jim Grant during the sunset of 8th December at San Diego, California.
©Jim Grant, shown with permission.

Atmospheric
Optics

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"Many people do not believe the green flash can actually occur. Tonight at Beacons Beach 15 or 20 people saw a fantastic series of them.....as the sun set several were seen and the people who were watching saw them very very clearly....here are the two best!"


A green flash needs a mirage, ordinary atmospheric dispersion is too weak to separate colours sufficiently.  A mirage needs layers of air at different temperature and therefore density.  Sunlight passing through the density gradients is refracted to produce the extreme colour separation required to see a flash.

The flash over the partially set sun is a Mock-Mirage type. A temperature inversion layer, abnormally warmer air above cooler, produces it.  It is hard to see visually but it is the one photographed most often.

The intense flash near the horizon is an Inferior-Mirage type. This is the classical flash seen as the last sliver of sun slips below the sea. It needs a shallow layer of warm air next to the sea with cooler air above.

On 8th December the atmospheric temperature profile was complicated. Warm air hugged the sea with several temperature inversions stacked above. The inversions are visible at right where they break the sun into 4-5 slices. The glitter path is a bonus.

The California coast is famous for frequent green flashes of both types.