Polar Stratospheric Clouds, also known as a Nacreous Clouds are at altitudes of 15-30 km above the ground, in the winter polar stratosphere. These clouds can be seen after sunset, when the sun is just below the horizon for the observer (one to six degrees).
Polar Stratospheric Clouds form at very low temperatures, at -80°C or below. There are two forms of the cloud – the dense form, which causes iridescence, and a diffuse form which doesn’t show any colouration and appears somewhat like a stratocumulus.
In the picture the diffuse form of the Polar Stratospheric Cloud is seen just over the west horizon, with the sliver of moon visible in the sky above it.