Radioactive cloud in Murmansk

At high altitudes of about 20-30 km in the sky sometimes formed t. N. nacreous clouds consisting apparently of ice crystals or supercooled water droplets. These clouds are thin, translucent. It's a beautiful sight seen relatively rarely, usually at latitudes 55-60 °, directly after sunset or before sunrise. Day on the background of bright ambient light, they become invisible.
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