Have you ever seen Noctilucent Clouds? If you are like me with most things you probably have heard of them but can’t remember what they are. They are the clouds that shine at night and that is why they are called Noctilucent. Sometimes we get the chance to see these this time of the year at the end of the Gunflint Trail.
Jörgen Norrland Andersson
According to Earth Sky, “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.”
The challenge with viewing these clouds is you have to stay up late. They don’t appear until the sun is below the horizon so 1-2 hours after sunset or before sunrise. You can recognize the clouds by not only the time of the day but also where they are located. If you see a clear sky except for cirrus like clouds low in the north then you can probably assume you are seeing night shining clouds. You’ll want to start looking now because they usually only appear from May through August.
Diagram from Astro Bob Blog