Photographs of noctilucent clouds appearing in the night sky over Britain on Jul. 15, 2009.
Jamie Cooper—Getty Images
August 25, 2015 12:25 PM EDT

If you’ve noticed some strange blue clouds in the night sky recently, you’re not alone. Uncharacteristically blue nighttime clouds, usually seen over polar regions, have been visible as far south as Colorado and Northern California in recent years.

The clouds, known as “noctilucent clouds” or NLCs, glow blue at night because tiny ice crystals 50 miles (80 km) above the earth are reflecting sunlight from the other side of the planet, according to SFGate. And some scientists say the glowing blue clouds may be yet another effect of climate change.

The vast majority of scientists agree that climate change is real, but NLCs are a good example of how sometimes the secondary effects of climate change may not yet be completely understood. It’s not clear exactly what the glowing clouds have to do with a changing climate, though there are some theories being discussed. One is that methane emissions can create water droplets at high altitudes, which can lead to NLCs,’s Tony Phillips told SFGate. Another idea is that as the Earth’s surface is heating up, the higher layers of the atmosphere (like the mesosphere, where NLCs form) are actually getting colder, allowing the tiny ice crystals for form, University of Colorado Professor Gary Thomas told NASA in 2003.

So if you see blue clouds glowing at night, it may be yet another effect of climate change.

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