Formation
The stratosphere is very dry; unlike the troposphere, it rarely allows clouds to form. In the extreme cold of the polar winter, however, stratospheric clouds of different types may form, which are classified according to their physical state and chemical composition.
Due to their high altitude and the curvature of the surface of the Earth, these clouds will receive sunlight from below the horizon and reflect it to the ground, shining brightly well before dawn or after dusk. Because of this, an observer, unfamiliar with this type of cloud may think one to be a UFO.
PSCs form at very low temperatures, below −78 °C (−108 °F). These temperatures can occur in the lower stratosphere in polar winter. In the Antarctic, temperatures below −88 °C (−126 °F) frequently cause type II PSCs. Such low temperatures are rarer in the Arctic. In the Northern hemisphere, the generation of lee waves by mountains may locally cool the lower stratosphere and lead to the formation of PSCs.
Read more about this topic: Polar Stratospheric Cloud
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