Nacreous clouds, also known as “mother-of-pearl clouds” or “polar stratospheric clouds”, are a rare type of cloud best known for their vivid and sometimes luminous colours. Almost all clouds form in the troposphere, which typically extends 10-15km (6-9 miles) in altitude. However, nacreous clouds form higher up in the atmosphere, between 15 and 25km, in the lower part of the stratosphere, the layer of atmosphere above the troposphere in which we live.
The stratosphere is very dry, so very cold conditions are needed to form the ice crystals that are needed to develop nacreous clouds. The clouds require temperatures lower than -78C. Temperatures as low as this are generally only found in the stratosphere during polar winter, and nacreous clouds can form only in lower-than-average stratospheric temperatures. As a result, nacreous clouds are generally visible only at high latitudes and rarely observed in the UK.
However, if conditions are favourable, the best time to try to observe nacreous clouds is shortly after sunset or shortly before dawn in a period known as “nautical twilight”. When the sun is just a few degrees below the horizon, it can result in beautiful and colourful clouds as a result of interference of light waves through the Earth’s atmosphere.
