People are reporting sightings of the silver-blue clouds – called noctilucent or night shining clouds – that light up summer night skies. These clouds are typically seen at high latitudes – say, about 45 degrees north or south – from May through August in the Northern Hemisphere, and from November through February in the Southern Hemisphere.
Every year around this time, we hear from people who have begun spotting them again.
Over this past week, several people reported noctilucent clouds above northern Europe. The video below is from Mindaugas Gasparavirius in Lithuania, who caught these clouds on June 8, 2018:
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: between about 45 degrees and 60 degrees 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.
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 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 here.
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 high-latitude regions.