What iridescent clouds form in the polar stratosphere, known as mother-of-pearl clouds?

The iridescent clouds that form in the polar stratosphere, known as mother-of-pearl clouds, are nacreous clouds. The name comes from nacre, the lustrous inner layer of some shells, because these clouds can glow with pearly, shifting colors.

They develop in the winter polar stratosphere, usually about 15–25 kilometers above Earth. Temperatures must fall below roughly −78 °C for the necessary ice or supercooled chemical particles to form. Their high altitude allows sunlight to illuminate them after sunset or before sunrise, when ordinary lower clouds would already be dark.

Nacreous clouds are closely connected with polar atmospheric chemistry. Some polar stratospheric clouds contain nitric acid and water and provide surfaces for reactions that activate chlorine, contributing to ozone depletion. Pure-ice Type II clouds are the classic nacreous form. They are often confused with noctilucent clouds, but those form much higher, in the mesosphere, rather than the stratosphere.