Over the past few days, conditions in the upper parts of the atmosphere have allowed us to be treated to a rare glimpse of Nacreous Clouds.
What are they?
Found in the lower stratosphere, these clouds are mainly seen over polar regions in winter, where very cold air – minus 80˚C and lower – condenses the small amount of water vapour present into tenuous clouds. These clouds are normally found at altitudes of around 20 km.
Compared to the water droplets in clouds we see every day in the troposphere, the water droplets which make up Nacreous Clouds are much smaller. Their smaller size means that the way in which they scatter light is different to regular clouds. This gives them their characteristic pearly luminescence, and has led to them sometimes being known as ‘mother of pearl’ clouds.
Nacreous clouds are best seen in the twilight hours, just after sunset and just before sunrise, when the clouds are illuminated by sunlight from below the horizon.
— Anita Nicholson (@AnitaNicholson) February 2, 2016
How rare are they?
As these clouds are more usually seen over the polar regions in winter, it is quite rare to see them as far south as the UK. We usually get to see them for short periods of several days every few years.
— Danny Spring (@strangequark77) February 2, 2016
Why are we seeing them now?
Currently, we are able to catch sight of them because cold air which usually circulates around polar regions in the stratosphere (the stratospheric polar vortex) has been displaced from its usual position over the north pole to be over the UK.
— Kildare Weather (@LiveNaasWeather) February 2, 2016
How much longer will we be able to see them?
Our weather forecast models indicate the cold polar vortex will remain nearby for the next few days, so we should be able to see Nacreous Clouds when the skies are clear. The position of the vortex shifts towards the end of the week taking the coldest air, and the Nacreous Clouds, away from above the UK.
These clouds are not to be confused with Noctilucent Clouds, which occur much higher up in the mesosphere – near altitudes of 85 km – and in the summertime.