Like scientific mysteries? Then consider the electric-blue clouds that are becoming more common and appearing in unexpected parts of the world, the San Francisco Chronicle reports. Called "noctilucent clouds" or "night-shining clouds," they were first reported above polar regions in 1885 and have appeared every summer since, noted Space.com last year. They could be seen in northerly regions like Russia, Scandinavia, and Britain, but have recently appeared as far south as Virginia, Utah, and Colorado, prompting media outlets to revisit an old question of whether noctilucent clouds are caused by global warming. What's the connection? Well, noctilucent clouds—the "highest clouds of all," per the Guardian—form some 50 miles above ground in the Earth's upper atmosphere, or mesosphere, where conditions aren't always suitable to cloud formation.
Enter greenhouse gases, which scientists say warm the atmosphere but actually cool the mesosphere and therefore make noctilucent clouds more likely, the Guardian reports. That's because clouds are formed partly by ice crystals, and "extreme cold is required to form ice in a dry environment like the mesosphere," a professor told NASA years ago. Tony Phillips, who tracks clouds for SpaceWeather.com, says "the jury is still out" on the global-warming connection, but notes that the greenhouse gas methane "favor[s] the formation of NLCs at very high altitudes" by producing water that helps form the clouds. An interesting side note: Clouds' water molecules need something to stick to, like dust from Earth's sand storms, but NLCs are so high up that they may be gathering dust from meteorites. (Meanwhile, last month was the hottest recorded in 135 years.)