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| Earth on Jun 02, 2015

The secrets of night-shining clouds

People at high latitudes are seeing glowing clouds in a dark night sky. They’re called noctilucent or “night-shining” clouds.

Glowing silver-blue clouds – called noctilucent or night shining clouds – sometimes light up summer night skies. They’re seen at high latitudes – say, about 45 degrees N. or S. – from May through August in the Northern Hemisphere and from November through February in the Southern Hemisphere. In 2015, the northern noctilucent cloud season has already begun. We’ve received some photos (below), and NASA’s AIM spacecraft has also detected these clouds drifting across the Arctic Circle. NASA said the first clouds appeared on May 19, 2015, a bit earlier than researchers expected. Follow the links below to learn more about this beautiful, seasonal phenomenon.

What are notilucent clouds?

What does the science say?

How can I see notilucent clouds?

Sandor Botor in Sweden captured notilucent clouds on June 1, 2015.  Thank you, Sandor!

EarthSky Facebook friend Sandor Botor in Sweden captured notilucent clouds on June 1, 2015. Thank you, Sandor!

Jörgen Norrland Andersson saw the same display from Sweden on June 1.?

Jörgen Norrland Andersson in Sweden posted this photo at EarthSky Facebook, also from June 1, 2015.

View larger. |

View larger. | Andy Stables on the Isle of Skye, Scotland captured these noctilucent clouds on May 28, 2015. Visit Andy’s Facebook page to see his photos of auroras, noctilucent clouds, comets, meteors, the Milky Way and more in the night sky from the Isle of Skye.

Noctilucent clouds streaming across the sky in Utrecht, The Netherlands on June 16, 2009. Credit: Robert Wielinga (via NASA).

What are notilucent 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 are 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°F (-134°C).

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. They can tell scientists that the circulation exists first of all, and tell us something about the strength of the circulation.

A composite image, taken by AIM, of noctilucent clouds above the Southern Pole on December 31, 2009. Image via NASA/HU/VT/CU LASP

What does the science say? Scientists studying these clouds have included those from NASA’s AIM (Aeronomy of Ice in the Mesosphere) 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, changes in the clouds, as well as the amount of meteoric space dust that enters the atmosphere.

Scientists have used data from this satellite to study the details of how noctilucent are formed and why they change over time. In 2014, they announced they had discovered unexpected teleconnections in noctilucent clouds. For example, Cora Randall, AIM science team member, said in April 2014:

… we have found that the winter air temperature in Indianapolis, Indiana, is well correlated with the frequency of noctilucent clouds over Antarctica.

The video below has more about the teleconnections in noctilucent clouds.

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.   Illustration via NASA.

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.

How can I see notilucent 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: between about 45° and 60° North or South latitude.

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.

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

Noctilucent clouds over the Southern Hemisphere on January 30, 2010, taken by an astronaut aboard the International Space Station. Image credit: NASA.

Noctilucent clouds captured from Soomaa National Park, Estonia, in 2009. Image via Martin Koitmäe via Wikimedia Commons.

Bottom line: Noctilucent or night-shining 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 seen during summer in polar regions.

Electric-blue clouds appear over Antarctica

Video: Noctilucent or night shining clouds in motion