Long-time listeners to Weather Break know that I’m a huge fan of clouds. When you tell someone that you’re a meteorologist, you get two questions right off the bat: what’s the weather going to be like tomorrow, and what’s you’re favorite cloud? The forecast for tomorrow’s weather may or may not be difficult, but it’s always a tough call to pick my favorite cloud, because so there are so many difficult types of clouds and they all have different characteristics that make them fascinating to atmospheric scientists.
When you think back over all the different kinds of clouds that you have seen in your lifetime, all of these clouds had one thing in common that you probably didn’t realize at the time—they were all located in the layer of the atmosphere called the troposphere. The troposphere is the lowest layer of the atmosphere, typically occupying the lowest 6 miles or so. Pretty much everything that you or I would call “weather” happens in the troposphere, including almost all clouds. Even the tops of the very tallest thunderstorms only barely reach out of the top of the troposphere and into the next layer of the atmosphere, the stratosphere.
That means that almost all clouds in the atmosphere occur at heights less than about 6 or 7 miles above the earth surface, with most of the clouds occurring at far lower heights than that. The puffy, cumulus clouds that you would see drifting by on a warm summer day? They might be no more than a few hundred to a few thousand FEET above the ground.
All of these clouds are practically on the ground compared to the very highest clouds in the atmosphere—the noctilucent clouds. Noctilucent clouds occur in a layer of the atmosphere knows as the mesosphere, meaning that these clouds are typically about 30 to 60 miles above the earth’s surface. But these clouds aren’t much like the regular clouds that you see closer to the surface of the earth, and they are pretty rare. At the extreme heights where noctilucent clouds occur, the air is very thin—at most, just a few percent as dense as it is near the surface. At these very low pressures, air can’t hold very much water vapor, so you might think that the clouds would be common; after all, down in the troposphere, clouds normally form when air can no longer hold as much water vapor as the air currently is holding. But actually the noctilucent clouds are very rare, mainly because it’s extremely difficult for water vapor to GET to the mesosphere. The source of water vapor for the atmosphere is evaporation off of bodies of water at the earth’s surface. In order to get into the mesosphere, that water vapor would somehow have to rise 30-60 miles without condensing out and becoming liquid water drops or ice crystals that would fall back to the surface as precipitation.
The name “noctilucent” tells us something important about these clouds; they glow at night, or at least in the evening. Noctilucent clouds are very thin; in the harsh light of day, they are too thin to be seen by the human eye. (The same is often true of cirrostratus clouds.) But shortly after sunset, the rays of light from the sun can still be hitting these extremely high clouds, even though the sun appears to be below the horizon to an observer at the surface. At these times, noctilucent clouds glow brightly for a period of minutes before they, too, pass into darkness.
The exact cause of noctilucent clouds is not known. There are several reasons to believe that they may be caused by human activity, even though, of course, they occur at altitudes where humans don’t spend much time. It’s possible that these clouds are somehow seeded by the exhaust from rockets or the space shuttle as they pass through the mesosphere on their way to orbit. Regardless of how they form, they are unquestionably the highest clouds known to form in the atmosphere.
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