Noctilucent clouds look like thin, high
cirrus clouds that are bluish or silver in color, but are much higher in the
atmosphere. They are about 50 miles high and shine because of reflected sunlight.
Noctilucent clouds are seen only when the sun is 6° to 16° below the horizon and only at latitudes above 45°; for North America that’s pretty much Canada and Alaska.
There are two different views about what Noctilucent clouds are made of. The first is that they are made of ice particles and the second is that they are made of dust.
The first observations were in 1885, 2 years after the eruption of Mt. Krakatau in Indonesia. There were many Noctilucent cloud observations after the eruption, but then the clouds disappeared for decades. The observation of noctilucent clouds has picked up since the 1920’s, becoming more and more frequent, this is believed to be due to increasing industrialization and pollution.