Mesopherics
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Hi, I'm Bryan Yeaton and this is the Weather Notebook.

When Luke Howard, the father of modern-day cloud classification, assigned Latin names to various cloud types early in the 19th Century, there was one set of lofty clouds that escaped his observation. Mesospheric clouds.

Mesopheric clouds are also known as noctilucent clouds. They form at high altitudes some 50 miles above the Earth's surface. That means they are the highest clouds hovering over our planet. Another well known set of lofty clouds-- the ominous thunderstorms stalking the Plains fall-- far short of the noctilucents because their tops check in at a mere 10 or so miles overhead. Noctilucent clouds can only be found at high latitudes where temperatures approach 200 degrees below freezing, Fahrenheit.

That explains why so many scientists and weather enthusiasts journey to Canada, Alaska, and Northern Europe to hunt the twilight skies for these cloud which, though first observed in the mid 1800s, still remain a bit of a mystery today.

The summer months are the best for sighting them. That's when the sun lies some 6 to 16 degrees below the horizon, just the right angle to light up these illusive clouds. They look for complex, interwoven streaks or knots of clouds with a generally white or a distinctive pearly-blue tone, similar to cirrostratus clouds we might see during the day.

According to researchers, the frequency, brightness, and area covered by these high, diffuse clouds have increased over the last 40 years. Scientists believe that this may be linked to higher concentrations of water molecules at high altitudes due to the effects of global warming.

Thanks today to meteorologist, Mish Michaels of the Weather Channel. The Weather Notebook is a production of the Mount Washington Observatory and is underwritten by the National Science Foundation. For more on mesospheric clouds and the mountain with the world's worst weather go to our website, mountwashington.org