Nacreous

Pearl clouds (mother-of-pearl clouds or polar stratospheric clouds) resemble Cirrus or Altocumulus in the form of lenses. These clouds have very visible iris. The colors of the irises are at their brightest when the sun is a few degrees below the horizon. They need temperatures below -80ºC to form.

The physical constitution of the nacreous clouds is still unknown. These clouds are thought to be made up of tiny water droplets or spherical ice particles. They were mainly observed in Scotland, Scandinavia, France and Alaska. Measurements made in the pearly clouds observed in southern Norway showed that they were at altitudes between 21 and 30 kilometers.

Pearl clouds are similar to noctilucent clouds. Although rare, these clouds have been observed more frequently lately, attributing this fact to the larger number of aircraft expelling condensation cores and vapors at high altitudes.

Warming the planet, as a result of what is apparently paradoxical, causes the stratosphere to cool down. Pearl clouds form precisely when the temperature drops most in the stratospheric region. Over the past two decades, the stratospheric temperature has been in sharp decline, at a speed greater than the rise in the surface temperature of the planet.

Other scientists claim that the appearance of these clouds is favored by volcanic eruptions, and recently several volcanoes have been erupting around the world.