Auroras (North/South Polar Lights; or aurorae, sing.: aurora) are natural colore light displays in the sky, usually observed at night, particularly in the polar zone. They typically occur in the ionosphere. Some scientists[who?] call them "polar auroras" (or "aurorae polares"). In northern latitudes, the effect is known as the aurora borealis, named after the Roman goddess of dawn, Aurora, and the Greek name for north wind, Boreas. It often appears as a greenish glow or sometimes a faint red, as if the sun was rising from an unusual direction. The aurora borealis is also called the northern polar lights, as it is only visible in the North sky from the Northern Hemisphere. The aurora borealis most often occurs from September to October and from March to April. The Cree call this phenomenon the Dance of the Spirits.