Nearly all clouds form in the troposphere, the lowest 10 miles of our atmosphere. But noctilucent clouds develop in the mesosphere, about 50 miles up. Noctilucent means "glowing at night", and these clouds DO glow with a distinctive white or pearly-blue cast. They look a bit like streaks of cirrostratus or "mare's tails."
They're most common when the sun is about 5 to 15 degrees below the horizon, or about the point when street lights come on. Noctilucent clouds are most often seen in high-latitude places like Canada, Finland, and Russia. That's where the upper atmosphere is coldest and the summer twilight can last for hours.
Until last year, they'd never been recorded in the middle of the U.S. But on June 22, three observers in Colorado and Utah reported noctilucent clouds just after sunset. This beautiful sight might actually be an omen. It's possible that chemical changes in our atmosphere could make noctilucent clouds more prevalent and widespread in the years to come. Keep your eyes open.
Today's contributing writer is Bob Henson and for a picture of noctilucent clouds, visit our website at mountwashington.org. The Weather Notebook is underwritten by Subaru, with support provided by the National Science Foundation.