Glowing silver-blue clouds that sometimes light up summer night skies at high latitudes, after sunset and before sunrise, are called noctilucent clouds. Also known as night shining clouds, they 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 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.
Noctilucent clouds 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. Jump below the photos below to find out how it works.
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.
Scientists studying these clouds have included those from NASA’s AIM (Aeronomy of Ice in the Mesosphere) satellite. This satellite, launched in 2007, is in a polar orbit, 373 miles in altitude. Its mission has been to observe 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 are using the data 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.
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 only 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.
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.