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Light reflected off fresh snow can outshine the moon

Artificial light that bounces off snow can be twice as bright as the full moon at its most radiant, a new study reveals. And that could mean bad news for cities trying to reduce their light pollution.

Researchers studied differences in sky brightness in three areas: a village near the Arctic Circle and the suburbs of Germany, both with recently fallen snow, and a remote, snowless Latvian beach. A blanket of new snow could increase the amount of light in the sky above a suburban area by as much as 33% on a clear night, the team reports this month in the Journal of Imaging. When the sky was cloudy, the brightness rose almost 200%, more than twice as bright as the full moon.

Excess light around suburban areas blocks dim stars, making the sky a much less interesting place, write the authors, who call their results “rather alarming.” More importantly, it also affects living things, changing daily rhythms, migration patterns, and even reproductive cycles.

Dark sky lovers and conservationists have urged cities to shift to light fixtures with shielded bulbs in order to reduce the amount of light brightening the night sky. But the new study suggests this approach may not always work, as that light can reflect back up to the sky, especially if snow has just fallen. Using adaptive smart outdoor lighting that could automatically dim lamps after a snowfall could help diminish this “snowglow,” the researchers suggest.