The
polar night is the night lasting more than 24 hours, usually inside the
polar circles. The opposite phenomenon, when the sun stays above the horizon for a long time is called the
polar day, or
midnight sun.
A common misconception is that at each point inside the polar circle, or that at each place where midnight sun occurs, the shortest day is totally dark. Because of twilight, this is not the case. In places very close to the poles this is true, but in areas very close to the Arctic and Antarctic Circle, midnight sun is experienced, but polar night is never experienced. In fact, polar regions typically get more twilight throughout the year than regions located closer to the equator.
In regions inside the polar circles, the length of the time when the sun is below the horizon varies from 20 hours at the Arctic Circle and Antarctic Circle to 179 days at the Poles. However not all this time is classified as polar night, since there may be plenty of sunlight because of refraction. Also, one might notice that the time when the sun is above the horizon at the poles is said to be 186 days. The asymmetry in numbers is because the time when the sun is partially above the horizon is counted towards the "daytime".
Various kinds of polar night exist. This is because polar night is the period during which no twilight occurs; but there are various kinds of twilight. These latitude definitions are based on relative clear skies. If there are thick clouds, it gets darker.
