Belonging to the group of special clouds, the nacreous clouds or polar stratospheric clouds (PSC), are very rare and form in the winter polar stratosphere with very low temperatures. Their altitude is usually between 21-30 km.

The stratosphere is very dry and rarely shows clouds. But in the extreme cold of polar winter, stratospheric clouds of different types may form which are classified according their physical state and chemical composition.

In general there are two types of PSC:

Type I: Nitric Acid Trihydrate Clouds (NAT) are a mixture of water and nitric acid and form with temperatures of -78°C or less. The appearance of this type is described as very delicate (similar to noctilucent clouds) and they are widely spread over a large area resembling to a thin Cirrostratus layer. When they occur, the sky has a very pronounced red/orange glow long time before/after sunrise/set.

Type II: Mother-of-pearl clouds consist of water ice only and form with temperatures between -90°C and -95°C or less. They have mostly a lenticular shape and appear only in a small area, showing often very marked irisation.

Taken 30 minutes after sunset, the presence of PSC type I gives way to very prominent red glow, the Cirrus underneath appears as dark brown.