The Weather Notebook: A Polar Cloud Heads South

A Polar Cloud Heads South
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Because of their high altitude, in a very dry part of the atmosphere, noctilucent clouds are rather an enigma. Photo by Pekka Parvianien
If you're a skywatcher in the U.S., you have something new to look for in this new century. A rare and mysterious cloud type has been sighted for the first time in the heart of the nation. It's called a noctilucent cloud, and it's a breed apart. Hi, I'm Dave Thurlow from the Mount Washington Observatory and this is The Weather Notebook.

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."

Noctilucent Clouds (NLC) or Polar Mesospheric Clouds (PMC) are the highest on Earth. Photo by Pekka Parvianien, Turku, Finland, 21/22 July 1989, 02.15 GMT
Nobody knows exactly how noctilucent clouds form, but we know they take shape in the coldest part of the whole atmosphere, where temperatures plummet below minus 200 Fahrenheit. We also know that they're visible only during twilight, when the sun lights them up from below.

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.