Nacreous clouds blaze brightly with metallic iridescent colours. The colours slowly change and the clouds stretch and twist in the stratospheric winds. They are 15 - 25 km high and composed of small ice crystals. They need abnormally low temperatures of -85°C to form from the dry atmosphere. They are often embedded in much less iridescent TypeI Polar Stratospheric Clouds of water and nitric acids - there are hints of them in this image.

From now onwards is the time to look for them. High latitudes are required and they are sometimes in the lee of mountains or seen during very windy conditions. Look before dawn or after sunset.