Nacreous clouds

Photo: María H. Tryggvadóttir

This morning we got these beautiful clouds on the morning sky. 
Nacreous clouds, or polar stratospheric clouds (PSCs), are also colloquially known as mother-of-pearl clouds because of their striking iridescent colors. These clouds are rare and form in extremely cold conditions in the stratosphere.

They typically appear in the winter months when temperatures in the stratosphere drop below -78°C (-108°F). They form at altitudes of 15 to 25 kilometers (9 to 15 miles) in polar regions, primarily during twilight when the sun is just below the horizon. 

The clouds are visible when the stratosphere remains illuminated by the sun while the ground is in shadow, giving them their characteristic shimmering, iridescent colors.