The secrets of night-shining clouds

Noctilucent cloud season has returned to Earth’s high latitudes. These “night-shining” clouds are beautiful … photos here.

High, rippled, shining clouds above waterway in twilight, man standing on beach.

View at EarthSky Community Photos. | Noctilucent – or night-shining – clouds just after midnight on June 16, 2020. Nature photographer Ruslan Merzlyakov in Limfjord, Denmark, said these clouds were: ” … disturbed by the high-altitude turbulence.” Thank you, Ruslan!

Every year – from May through August in the Northern Hemisphere, and from November through February in the Southern Hemisphere – people at high latitudes report seeing noctilucent or night-shining clouds. We read at SpaceWeather.com this weekend that these beautiful clouds have now descended to their lowest latitude of the 2020 season so far: +44 degrees north in Bend, Oregon. Bend resident Roy Reynolds, who photographed the glowing clouds on June 18, 2020, told SpaceWeather:

I woke up at 3:30 a.m. to a very bright sky shining through the shades. I got up to take a look and was surprised to find noctilucent clouds. I live in Bend Oregon and since living here (18 years) have seen this only two other times. Beautiful.

Noctilucent clouds typically descend even lower after the summer solstice, according to Tony Phillips of SpaceWeather.com. If you’re at a northerly latitude, now is a good time to watch for them!

Close shot of swirling narrow light blue clouds against a dark blue sky.

View at EarthSky Community Photos. | Menno van der Haven reported from Waddinxveen, the Netherlands, on June 21, 2020: “Last night, the ‘evening view’ of noctilucent clouds in the northwest was quite small and weak. However, if you use a zoom lens and overexpose with a certain amount, you often see some pretty curls. One in particular resembled a corkscrew and also behaved like this …” Thank you, Meeno!

Diagram of light shining from the sun, bouncing off high clouds to a location over the horizon from the sun.

When the sun is below the ground horizon but visible from the high altitude of noctilucent clouds, sunlight illuminates these clouds, causing them to glow in the dark night sky. Illustration via NASA.

What are noctilucent clouds? Noctilucent clouds form in the highest reaches of the atmosphere – the mesosphere – as much as 50 miles (80 km) above the Earth’s surface. They’re thought to be made of ice crystals that form on fine dust particles from meteors. They can only form when temperatures are incredibly low and when there’s water available to form ice crystals.

Why do these clouds – which require such cold temperatures – form in the summer? It’s because of the dynamics of the atmosphere. You actually get the coldest temperatures of the year near the poles in summer at that height in the mesosphere.

Here’s how it works: during summer, air close to the ground gets heated and rises. Since atmospheric pressure decreases with altitude, the rising air expands. When the air expands, it also cools down. This, along with other processes in the upper atmosphere, drives the air even higher causing it to cool even more. As a result, temperatures in the mesosphere can plunge to as low as -210 degrees Fahrenheit (-134 degrees Celsius).

In the Northern Hemisphere, the mesosphere often reaches these temperatures by mid-May, in most years.

Since the clouds are so sensitive to the atmospheric temperatures, they can act as a proxy for information about the wind circulation that causes these temperatures. First of all, they can tell scientists that the circulation exists, and also tell us something about the strength of the circulation.

How can I see noctilucent clouds? If you want to see the clouds, what steps should you take? Remember, you have to be at a relatively high latitude on Earth to see them: typically between about 45 degrees and 60 degrees north or south latitude, although the clouds can sometimes be seen at lower latitudes, particularly following the solstices.

For best results, look for these clouds from about May through August in the Northern Hemisphere, and from November through February in the Southern Hemisphere.

Noctilucent clouds are primarily visible when the sun is just below the horizon, say, from about 90 minutes to about two hours after sunset or before sunrise. At such times, when the sun is below the ground horizon but visible from the high altitude of noctilucent clouds, sunlight illuminates these clouds, causing them to glow in the dark night sky.

Shimmery pale blue to white clouds in a night sky, with silhouetted trees in foreground.

View at EarthSky Community Photos. | Marek Nikodem captured this image at 11:59 p.m. on June 16, 2020, near Szubin, Poland. He wrote: “The noctilucent clouds were visible all night, from dusk to dawn.”

Scientists studying these clouds have included those from NASA’s Aeronomy of Ice in the Mesosphere (AIM) satellite. This satellite, launched in 2007, has observed noctilucent clouds using several onboard instruments to collect information such as temperature, atmospheric gases, ice crystal size and changes in the clouds, as well as the amount of meteoric space dust that enters the atmosphere. You can find out what they are learning at NASA’s AIM page.

Black space, shining ripply layer of clouds, dark orange narrow stripe above black silhouette of Earth.

Noctilucent clouds can be seen from space, too. Astronauts in the International Space Station (ISS) took this photo on January 5, 2013, when ISS was over the Pacific Ocean south of French Polynesia. Below the brightly-lit noctilucent clouds, across the center of the image, the pale orange band is the stratosphere. Image via NASA.

Bottom line: Noctilucent or night-shining clouds are seen during summer in Earth’s high-latitude regions. They form in the highest reaches of the atmosphere – the mesosphere – as much as 50 miles (80 km) above the surface.

Visit SpaceWeather’s RealTime Noctilucent Cloud Gallery

Visit the Facebook page Noctilucent Clouds Around the World

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