Pink sky

Photograph by Vivian Dullien.

Pink sky
Pink sky, February 20, 2021, approximately 7 a.m., Boulder, Colorado.

The photographer was facing west, a short time, possibly as much as an hour, after sunrise. The mountain is illuminated by direct sunlight, but you can see the shadow of the mountain on which Dr. Dullien is perched. The top of the mountain looks reddish, in part because the rocks are red and probably in part because the sun is reddish early in the morning. In the shadow, the mountain is bluish because, well, because shadows are bluish because they are illuminated by scattered light.

But why is the sky pink? The photograph of the mountain is sharp, so there is no aerosol to the west. I was stumped. For an answer, I checked in with Stephen Corfidi of the NOAA/NWS Storm Prediction Center in Norman, Oklahoma. I discovered Dr. Corfidi through his splendid post, The Colors of Sunset and Twilight.

Without going into detail, Dr. Corfidi thinks that the pink sky in the west is essentially a reflection of the red sky in the east, by the aerosols in the west. That is, the aerosols in the west are scattering (reflecting) some of the red light back eastward. The scattered light may be pink rather than red or red-orange if there are scatterers in the lower atmosphere, but that explanation gets a little messy. I am also going to guess that some blue sky resulting from direct sunlight peeks through and shifts the spectrum slightly to the blue, as well as possibly reducing the saturation of the back-scattered light.