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Geo-Joint: Noctilucent clouds

Posted on May 31 2016

The sky is always out there. We get so used to it, whether it’s endless blueness in Southern California or persistent grey in London. Sometimes it catches our eye because of a dramatic showing, like a fiery sunset or a super-brilliant display of stars in the desert. Other phenomena are more subtle, but it’s worth keeping an eye out in order to catch their rare appearance. It helps that some of them are seasonal or more likely at certain latitudes, so you know when and where to be on the lookout.

After sunset, however crimson or ho-hum it might have been, in the months of late May through August in northerly parts of the world, water vapor particles attach themselves to dust from meteors. Sounds kind of magical, huh? They do this in a very lofty part of the atmosphere, well above the stratosphere, near where the mesosphere meets the thermosphere, a transitional area called the mesopause. The dust, sometimes called “meteor smoke,” is concentrated just below the mesopause, around 50 miles above the surface of the Earth. The comingling of water vapor and dust in this very cold realm forms tiny ice crystals, creating visible shapes called noctilucent clouds. These form in far southerly latitudes during their summertime, as well.

That name, noctilucent, comes from the fact that their extreme elevation allows them to reflect sunlight after the sky has gone into twilight (nocti = night, lucent = glowing). Against the darkened sky the clouds seem to luminesce from within. They frequently ripple in strands in the strong winds found so high in the atmosphere, but hardly seem to move across the sky because they are so far away. Unlike the ubiquitous white cumulonimbus clouds, these have a silvery blue appearance due to their icy composition. They form more readily in spring through summer because that is when winds tend to carry more moisture aloft, and also when the high atmosphere is, counterintuitively, at its coldest.

This may also sound odd, but these clouds were not observed before 1885, and at that time they were only visible from high northerly latitudes. The sightings followed the massive volcanic eruption of Krakatau in Indonesia and were thought to be related to the dust sent high into the atmosphere by that explosion. But the clouds persisted after the dust cleared. In the 20th century, they became visible down to between 40 and 50 degrees latitude, which means they might be seen anywhere north of Boulder, Colorado or Philadelphia, Pennsylvania. Why is this happening now? Climatologists note that methane, a potent greenhouse gas, can convert through some complicated chemistry to water vapor at high altitude. Water vapor from the lower atmosphere does not easily reach the mesopause. It is possible that the increased concentration of methane over time has made more water vapor available for the formation of noctiluminous clouds, and that the spread of these clouds is tied to climate change. Scientists from NASA are paying closer attention to them for this very reason.

It should be noted that these clouds, though they occur in similar regions and have similar otherworldly, glowing qualities, are not to be confused with the aurora borealis or aurora australis. Those northern and southern lights are caused by an entirely different process and mix of ingredients, grist for another Geo-Joint.

If you live too far south to experience noctilucent clouds (unless or until they start happening everywhere), a similar phenomenon can be observed after early evening or pre-dawn space launches. Here in Santa Barbara, we are sometimes treated to a light show in the dark sky after missile-testing flights sent from Vandenberg Air Force Base. High elevation winds turn the rocket vapor trails into twisty, snaky cloud streams that sometimes glow with bits of rainbow highlights. No doubt the same thing occurs in the skies above Cape Canaveral if the launch time is right. It’s totally artificial, but quite a striking effect.

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