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Polar Mesopheric Clouds
When the sun is around 6 degrees below the observer's horizon, noctilucent clouds start to become perceptible. The theoretical and observed maximum illuminescence of the clouds is when the sun is 10 degrees below the observer's horizon and the clouds become imperceptible again when the sun is 16 degrees below the observer's horizon.
Noctilucent clouds are generally seen at latitudes between 55 and 65 degrees in the summer, and are mainly seen in the Northern Hemishpere due to topography and population density at these latitudes.