Nacreous clouds form in the lower stratosphere over polar regions when the Sun is just below the horizon. The clouds are illuminated from below and often glow in vivid colours.
Height of base: 68,500 - 100,000 ft
Shape: large thin discs, often reflecting vivid colours
Latin: no direct Latin translation, but the old English word “Nacre” means “mother of Pearl”
What are Nacreous clouds?
Nacreous clouds are rare and very high clouds, known mainly for the coloured light they reflect after sunset and before sunrise. The colours are reminiscent of the colours which reflect from a thin layer of oil on top of water, an effect known as iridescence.
How do Nacreous clouds form?
Nacreous clouds form in the lower stratosphere over polar regions when the Sun is just below the horizon. The ice particles that form nacreous clouds are much smaller than those that form more common clouds. These smaller particles scatter light in a different way, which is what creates the distinctive luminescent appearance.
Due to their high altitude and the curvature of the Earth’s surface, these clouds are lit up by sunlight from below the horizon and reflect it to the ground, shining brightly well before dawn and after dusk. They are most likely to be viewed when the Sun is between 1º and 6º below the horizon and in places with higher latitudes, such as Scandinavia and northern Canada. For this reason, they are sometimes known as polar stratospheric clouds. Nacreous clouds only form below -78 °C so are most likely to occur during the polar winter.
Because of the very low temperatures required, nacreous clouds are usually only visible from the UK when the cold air which circulates around polar regions in the stratosphere (known as the stratospheric polar vortex) is displaced and hovers temporarily over the UK.
What weather is associated with Nacreous clouds?
Because they form in a polar vortex, they are associated mostly with very cold and dry weather.
How do we categorise Nacreous clouds?
Nacreous clouds are classified into three types, Ia, Ib and II, according to their chemical composition, height and ambient temperature.