Guide

Look closerWeather

Why shadows on snow look blue

December to March

On a clear day, shadows on snow can look faintly blue or violet next to the sunlit snow. The shaded snow gets no direct sunlight, so it is lit mostly by the blue sky, and the bright sunlit snow beside it makes that blue stand out.

Why this time of year

In a typical winter, snow falls in eastern Massachusetts from December through March (Boston Logan 1991-2020 normals: 9.0 inches in December, 14.3 in January, 14.4 in February, 9.0 in March). After a snowfall, a clear sunny day can turn the shadows of trees and houses blue, and late on a clear winter day, when the sun is low, they can look violet-blue.

How to notice it

  1. On a clear, sunny day after a snowfall, find a sharp shadow on the snow, like a tree trunk or a fence post.
  2. Look right at the edge where the shadow meets sunlit snow and compare the two colors side by side.
  3. Come back late in the afternoon when the sun is low. The long tree shadows can look violet-blue against the golden sunlit snow.
  4. Keep watching as the sun sets. Once the sunlight turns redder, the shadows may even look a little green.

Try this with kids: Stand in your own yard on a sunny snow day and make your own shadow on the snow. Is it gray, or is it a little bit blue?

Word to know: “Skylight” means the light that comes from the whole blue sky, not straight from the sun.

Safety

  • Look at shadows in your own yard or a park lawn, away from roads, plow piles, and roof edges where snow and ice can slide off.
  • Look at the shadows on the ground, never up at the sun.

Sources

  • theglobeandmail.comGlobe and Mail 2011, quoting University of Montreal physicist Sjoerd Roorda: a shadow blocks direct light so the snow scatters only the blue skylight, 'why a shadow appears bluish'.
  • opg.optica.orgChurma 1994, Applied Optics (Penn State Meteorology), abstract: outdoor shadows look blue mainly because blue skylight lights them; simultaneous contrast strengthens it; color constancy often makes them look colorless.
  • optica-opn.orgOptics & Photonics News 1992 (Houde-Walter): low sun at end of a clear winter day, long tree shadows on snow look violet-blue against gold sunlight; as sunset reddens, shadows may look green.
  • ncei.noaa.govNOAA NCEI Boston Logan normals: snowfall Nov 0.7, Dec 9.0, Jan 14.3, Feb 14.4, Mar 9.0, Apr 1.6 in.

Checked against these sources on November 3, 2026.

In the guide for December, January, February, March

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