Spotted strange clouds in the sky? Jon Mitchell explains what they are.

Nacreous clouds, sometimes known as mother of pearl clouds for their distinct appearance laced with vivid iridescent light have been evident in the sky over the Calendar region for the last 24 hours or so and many of you have been kind enough to send me pictures of them.

Nacreous clouds form in the lower stratosphere at an altitude of 70 000 to 100 000 feet. They form below minus 78 °C temperatures and so are most likely to occur during the polar winter. They are composed of tiny ice crystals, no more than a millimetre in length.

The clouds are very delicate in structure, and show brilliant irisation at angular distances up to about 40° from the sun. Irisation reaches its maximum brilliance when the sun is several degrees below the horizon and this, combined with the darkening skies after sunset can make for a spectacular sight.

Because of their high altitudes Nacreous clouds show little or no movement relative to the earth.

Nacreous clouds are extremely rare. The last time I remember seeing them over Yorkshire was on 16th February 1996, but once seen, never forgotten.

JON MITCHELL