Summer Quest for Noctilucent Clouds

By: Bob King June 19, 2014 7

Clear nights are precious, so what self-respecting skywatcher would waste one chasing clouds? Unless they're noctilucent clouds, that is. These electric blue billows at the boundary between Earth and space incite the imagination and inspire us to keep watch at dusk for their arrival.

Noctilucent (night-shining) clouds return every summer between about 45° and 60° latitude, from May through August in the Northern Hemisphere or November through February in the Southern Hemisphere. Infrequent appearances and a habit for hugging the horizon make spotting them a quest akin to finding the Holy Grail.

Noctilucent clouds in atmosphere layers — View of noctilucent clouds from 250 miles up in the International Space Station. They're close to the edge of space and require extremely cold temperatures to form. Credit: NASA

These oddballs of the cloud world are unbelievably high, floating 47 to 53 miles (76 to 85 km) overhead in a layer of Earth’s atmosphere called the mesosphere. Typical high clouds, such as the feathery cirrus we see on a summer’s day, crystallize 4 to 8 miles high, about the same altitude as a transcontinental jet flight.

U.S. map showing the 45 degree latitude lower limit for typical noctilucent cloud sightings. Take it with a grain of salt - clouds have been seen farther south.

Both are composed of ice crystals, but they grow in different ways. Dust from windstorms and salt from ocean spray are some of the materials that supply condensation nuclei for water droplets and ice crystals in the lower clouds. Noctilucent clouds (NLCs) rely on dust left by flaring meteors — interplanetary soot, as it were. Particulates from volcanic eruptions may also play a role.

"We've detected bits of 'meteor smoke' embedded in noctilucent clouds," reports James Russell of Hampton University, principal investigator of NASA's AIM mission to study the phenomenon. "This discovery supports the theory that meteor dust is the nucleating agent around which NLCs form."

As Earth travels around the sun, it sweeps up some 40,000 tons of interplanetary dust a year, plenty enough to get the job done. As for the water vapor, some may find its way to the mesosphere from summer storms, but methane gas released from both human activities and nature provides a key source. Methane rises high into the stratosphere where it reacts with hydroxyl radicals (-OH) derived from ozone to create the water vapor that freezes onto meteor smoke.

Striking noctilucent cloud display on July 31, 2008. Look for blue-colored swirls, eddies, and ripples very low in the northern sky. Credit: Bob King — Striking noctilucent cloud display on July 31, 2008. Look for blue-colored swirls, eddies and ripples very low in the northern sky. The clouds are best visible when the first stars come out in mid-late twilight. Credit: Bob King

Indeed, it's thought that an increase in the extent and frequency of noctilucent clouds in recent years may be yet another bellwether of global warming. Methane released from intensive agriculture, as well as a drop in mesospheric temperatures caused by a build-up of greenhouse gases such as carbon dioxide, may be behind the "cloud-creep." Once restricted to the boreal north, they've now been spotted as far south as Italy and Utah.

Noctilucent clouds are only visible in deep twilight, from about 75 minutes to 2½ hours after sunset or before sunrise. At latitude 46° north, I live at their fringe of visibility and catch a display about once a summer.

We see noctilucent clouds well after sunset, when other clouds have gone dark, because they're much higher up and can still catch sunlight and reflect it back to Earth. Credit: NASA — We see noctilucent clouds long after sunset, when other clouds have gone dark, because their much higher altitude allows them to still catch sunlight and reflect it back to Earth. Credit: NASA

Because of their great height, NLCs reflect light long after sunset, when other clouds have faded to gray. Their color is imparted by the ozone layer, which absorbs the yellows and oranges of reflected sunlight, tinting the clouds a striking silvery-blue.

While they have been seen near the zenith from the higher end of their latitude range, in the northern U.S. they rarely extend above 10° altitude.

In June and early July, displays reach about as high as the bright star Capella in Auriga (6°-8° altitude) for locations in the northern U.S. That means that your cloud-spotting site needs a prime view to the north. Bring binoculars to help you better appreciate noctilucent clouds' supple shapes.

NLCs linger from dusk through early nightfall across the north and northwestern sky and then return around the first light of dawn in the northeast. Not only are they beautiful to look at, but you can contribute to science by noting their form and appearance and reporting your observations to the Noctilucent Cloud Observing Network (NLCNET).

Comments

David

June 19, 2014 at 11:15 am

Interesting article. But instead of citing a narrow band of the northern USA, why not point out that almost all of populated Canada and much of Alaska are well within the specified limits for viewing noctilucent clouds? When I lived in Saskatoon, Saskatchewan and Edmonton, Alberta, these beautiful clouds were a common sight in the early weeks of summer. I'm sure they still are. Indeed, I wrote about one such observation in the April 1965 issue of Sky & Telescope.