The period after sunset or before sunrise when all or part of the sky is visibly bright because of sunlight scattered by clouds or the clear sky. Twilight also refers to the sky's appearance during this period. By convention, there are three sequential stages of twilight: civil twilight, nautical twilight, and astronomical twilight. (Some definitions of twilight set its upper limit as high as sun elevations h₀ = 5°–10°. ) Regardless of cloud cover, illuminance at the earth's surface decreases steadily during evening twilight and increases during morning twilight. However, under partly cloudy or overcast skies, some minor brightness fluctuations can occur as twilight progresses. The color and luminance patterns of clear skies change in complex ways throughout twilight (see bright segment, dark segment, purple light), while overcast skies usually grow bluer during evening civil twilight. The length of nautical, astronomical, and civil twilight varies greatly with latitude and time of year. The annual range of the duration of twilight increases with latitude. For example, at polar latitudes twilight may last as long as 24 hours or may not occur at all. Compare dawn, dusk.