I used to casually watch for noctilucent clouds each summer and rarely saw them. Then last June, after missing a bright display, I decided to check regularly and discovered they were more common than I'd imagined. After seeing the clouds once every other year or two, I observed six displays last summer alone.
So why would I or any self-respecting stargazer deliberately look for clouds on a clear night? In a word, they're extraordinary. Noctilucent clouds (NLCs) only appear in twilight from about 45 to 90 minutes after sunset or before sunrise low in the northern sky. They have cool tendril and wave shapes, glow blue unlike any other clouds, and they're made of meteor smoke. As far as I'm concerned that last quality makes them part extraterrestrial. Besides, you gotta love a cloud that has the consideration to disappear as soon as the sky gets dark.
Noctilucent literally means "night-shining". They form about 50 miles (80-85 km) up in the mesosphere, an extremely cold atmospheric layer just above the stratosphere where temperatures hover around -200° F (-130° C). We see them shining even after lower clouds have gone dark because they still catch the sun's rays even as the first stars begin to show.
While NLCs resemble their lower cousins, their color and wispy, undulating nature make them unique. They also have a short viewing season. In the Northern Hemisphere they typically first appear in May, peak around the summer solstice and then depart by mid-August. In the Southern Hemisphere the season runs from November to February.
Where to see NLCs
Noctilucent clouds are primarily visible between latitudes 50° and 70° north and south, but in recent years, displays have been observed as far south as Kansas, Colorado and even Los Angeles. Despite living south of the main viewing zone here in Duluth, Minn. (latitude 47° N), the clouds make regular if infrequent appearances. Farther north in Canada and the Scandinavian countries, NLCs occasionally cover much of the sky.
Last year's first impressive display occurred on June 8th, and it just got better from there with great shows on June 21, 23 and 27. That's why I'm getting the word out early. The sooner you start looking the more opportunities you'll have.. From the northern U.S. the clouds appear very low in the northern sky, from 3° to about 20° (two fists) high.
I've found that they're most obvious about an hour to 75 minutes after sunset or before dawn. You'll want to observe from a location with an unobstructed view to the north, similar to where you'd go to watch the aurora.
NLCs slowly emerge as dusk deepens and have a blue and luminous quality compared to other clouds that might be about. While distinctive, they're sometimes confused with wispy cirrus clouds. If you spot a suspect and aren't sure if it's an NLC, watch it for a while. Cirrus clouds will gradually darken and typically drift from west to east. Noctilucent clouds stay nearly put or slowly recede northward with the coming of night. They also also have sharper outlines when magnified in binoculars, a good tool to have at hand.
Clouds form when water vapor condenses on particles of dust, soot, salt or human-made pollutants. All the ingredients for cloud formation are abundant in the lower 5-9 miles of the atmosphere called the troposphere. Higher up, the atmosphere is much drier and contains fewer particulates. But during the early summer, water finagles its way up there just the same. Air flowing over mountains and updrafts from powerful thunderstorms can spawn atmospheric gravity waves that can loft water vapor to great altitudes.
“It’s like dropping a rock in a pond,” says climate researcher Julio Bacmeister of the National Center for Atmospheric Research in Boulder. “When a parcel hits the stratosphere, it generates oscillations that propagate in all directions.”
If you've ever seen a meteor streak with wiggles in it, that's a gravity wave doing its work. Propelled into the mesosphere, the water condenses as minute ice crystals on the sooty debris left behind by passing meteoroids, the ones we see flare as meteors.
Oddly, conditions are best for NLC formation around the summer solstice because the mesosphere experiences its coldest temperatures at that time. The little water vapor that climbs to that altitude saturates the rarified air, making it an ideal environment for cloud formation.
That makes noctilucent clouds as much a part of the summer season a fireflies, watermelon and days at the beach. I hope you're able to include them on your list of fun things to do. The key is finding a good spot (I park on an east-west road with an open view to the north) and routinely checking the sky on clear nights within a few weeks of either side of the June 20th solstice. It does entail a bunch of fruitless, short drives, but all that falls away when you hit the jackpot.
Bring a camera. As long as it's mounted on a tripod, NLCs are easy to photograph. Set your ISO to 400 (800 if it's getting dark) and expose at f/4.5 or f/5.6 for a few seconds. Check the camera's back screen and adjust the exposure as needed. As long as you can securely hold your mobile phone, it will also do the job.
Clear skies! Or maybe I should say "partly clouds skies!"
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.