Noctilucent clouds

Noctilucent clouds are extremely rare very high clouds seen in the night sky, usually on clear, summer nights. They become visible about the same time as the brightest stars and are usually bluish or silvery.

Noctilucent clouds form in the highest reaches of the atmosphere – the mesosphere – as much as 50 miles 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.
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)..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.

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