The Highest Clouds

Some Things You Probably Didn’t Know: Chances are that if some nine year-old asks you just how high up can clouds be, you’d probably think about those wispy cirrus clouds or towering cumulonimbus. But did you know that there are actually clouds that can be found much higher up still? How much higher? Would you believe fifty miles? Read on…

A Brief Primer; the Layers of the Atmosphere: First as you know, the earth’s atmosphere has several layers. We live in the bottom one — the troposphere. It extends to between 20,000 to 30,000 feet at the poles all the way up to between 50,000 to 60,000 feet at the equator. Unless you’re flying jets in the Air National Guard, or maybe a Citation, this is all you’ll ever see from a cockpit. From the top of the troposphere to about 175,000 feet (about 33 statute miles) lies the stratosphere. Here, the temperature slowly climbs to a maximum of about 50 degrees Fahrenheit. This layer is home to the jet stream in its lower regions and the ozone layer in its upper half. If we continued upward, we’d come to the rarefied realm of the mesosphere. This layer extends from about 175,000 feet up to about 250,000 feet (47 statute miles). In this layer, the temperature drops, much like in our own. In fact the top of the mesosphere, the mesopause, is where the atmosphere’s lowest temperature can be found (minus 160 degrees F). The mesosphere is rather clean, too–remarkably free of particulate matter — almost. Up at the top of the mesosphere, there is particulate matter; cosmic dust and meteoric dust particles like to hang out there. This is how the highest of high clouds, noctilucent clouds, probably originate; the dust provides condensation nuclei for ice crystals. (Ice? Up there? … a-yup.)

Where to Find Them: Why haven’t you heard about these exotic clouds before? Probably because not much is known about them, and also because they’re pretty good at hiding. They can usually be seen only during twilight and from high latitudes during late summer. Good places to spot them would be Alaska, Canada, and Northern Europe. This isn’t to say that they aren’t there during the day, or at lower latitudes; they are. But we’re talking fairly thin stuff: bright stars shine through them. The main reason they’re seen mostly from up North is simply statistical: They can only be seen during twilight (why in a minute) and the further north you go, the longer twilight lasts–hours, sometimes!

So what do they look like? First, they resemble thin cirrus clouds, but they’re bluish-white, with a ‘wavy’ appearance. These waves are quite large: dozens of miles from crest to crest, with amplitudes of several miles. The wind, if it can be called that, moves them along rather rapidly–about 400 miles an hour!

How to Spot Them: They are best seen when the sun is between 5 and 12 degrees below the horizon. The place to look is between the twilight arch at the horizon and the darkening vault of sky overhead–about 10 to 20 degrees above the horizon. (Obviously, you’ll need an otherwise cloudless night, out in the boonies, away from city lights.) Although you’ll be in darkness, because of their great height, the clouds themselves are still in sunshine and can be observed against the background of dark sky (somewhat like seeing those beautifully reddened bottoms of high stratus or cirrus clouds above the horizon at sunset.)

That’s about it for theoretical one-upmanship in the study of clouds. Now have I ever actually seen one?


Editor’s note: Jeff may not have seen them, but he does know where you can find them on the web