Cloud Color Evolution at Sunset

This photo was taken about an hour before sunset. Those puffy white clouds are probably 10-20 miles beyond the ridge. 30 minutes later, as the Sun dipped lower in the sky, those same clouds, slightly morphed in shape, took on some interesting colors. I shot another photo:

Although the two clouds appear to have roughly the same altitude, that’s an illusion caused by perspective. The cloud on the right has already experienced sunset, with only its top-most edge showing a slight amount of reddened light reflection. Its top must be around the same altitude as the base of the cloud on the left, which also shows reddening. The rest of the right cloud is “blue” because it’s basically reflecting diffused light from the sky. Snow on the ground can appear blue for the same reason.

The cloud on the left must be much higher in altitude because it’s still basking in the golden rays of early sunset. That reddening around its base is working its way upward. The dramatic changes in color on both clouds evolved over some 30 minutes. 10 minutes later both clouds were totally “blue.”

All the colors seen in these photos are a result of Rayleigh scattering. Of course, the original “white” (which isn’t technically a color) is the result of diffuse scattering of sunlight by tiny drops of liquid water in the cloud. And sunlight is “white” because it contains (literally) all the colors of the rainbow.

In fact, clouds can be pretty much any color depending on conditions. Green clouds are sometimes seen during severe weather, and can also be caused by smog or other air pollution. And violet and indigo clouds can appear at sunset when sunlight is refracted at just the right angle through the atmosphere.

If you only glance skyward occasionally you’ll miss a lot of fascinating changes — some elapse over a span of minutes, like an approaching shelf cloud. Others, like the buildup of a cumulonimbus, are more gradual and require longer dedicated observations (or time lapse) to be visible. It only takes minutes to enjoy a sunset or moonrise or rainbow, but there’s more wonders to see if you make the time for extended skygazing.

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