Green Rim.. ..and More

Sunset Green Rim.. ..and More

Michiel de Boer (images plus) looked across the North Sea from near Ijmuiden in The Netherlands. He made these stills from 4K video.

A distinctive green rim tops the sun, distinctive that is to a camera but always too narrow to be seen by eye.

Mirages and temperature abnormalities are not needed although some are present here (see below). The air anyway thins with height and the gently slanting rays of sunset refract across layers of different density. Rays curve downwards and more steeply into the denser lower atmosphere making the sun appear raised from its true position. Normal refraction raises the sun by half a degree at sunset.

But refraction is frequency (colour) dependent. Blues and greens refract more than yellow and reds. The green and blue sun is raised highest creating the green rim. Why no blue rim? Sometimes, but rarely, there is one. Usually the weaker blues are absorbed or scattered during their long travel through the sunset air.

Green rims are too often invoked as explanations for the green flash. Flashes need more powerful stuff - mirage conditions. All images ©Michiel de Boer

Red Rim?

Earlier in the sunset Michiel recorded a red rim on the lower half of the solar disk. Miraging is playing a role as evidenced by the inverted sliver of sun rising up from the horizon. A classic inferior mirage sunset. More of this in the next OPOD.