Atmospheric
Optics
Sunrise Green Flash
~ Chih Hsiung Chen captured this dramatic green flash sequence looking eastwards over the ocean from Taiwan.
Sunrise flashes are hard to capture. This one also gives the lie to beliefs that a clear sky is needed.
All images ©Chih Hsiung Chen, shown with permission
“Almost all green flash photos are taken in sunset.
I live in eastern Taiwan in the western Pacific Ocean. The only way to observe a green flash is at sunrise. A sunrise green flash is more difficult due to the unknown position where the sun will first appear and its precise timing.
I used solar (crepuscular) rays converging towards the hidden sun several minutes before sunrise to predict where a flash might appear. Local sun rise/set predictions offered the sunrise time to minute level. I finally took the sunrise green flashes by continuous shooting.”
Green flashes are mirages where vertical magnification separates otherwise only slightly dispersed colours. The refraction by a normal atmosphere merely produces a green rim on the horizon sun that is invisible to the unaided eye.
This green flash is of the classical 'inferior mirage' variety and is produced by warm air beneath cooler air.
Rays passing through the cool/warm air boundary are refracted back upwards. The eye sees an inverted solar image beneath the 'true' one. The latter is produced by rays passing only through cooler air.
Each image is vertically magnified when they overlap just above the horizon. The two slivers of sun would show bright blue but for strong scattering of blue light by the air. We usually see green instead.
As the sun climbs the two mirage images separate to give an 'omega' or 'Etruscan vase' sunrise.
When the sun is setting, a mirage like this is a good sign that a classical green flash might follow.
There is no such warning at sunrise!
The flash is slightly above the horizon.
