Whiteout is a weather condition in which visibility is reduced by snow and diffuse lighting from overcast clouds.

There are two different forms of a whiteout:

1. In blizzard conditions, the windblown snow in the air may simply make it too difficult to see very far.
2. In clear air conditions, when there is no snow falling, diffuse lighting from overcast cloud may cause all surface definition to disappear. It becomes impossible to tell how far away the snowy surface is. In polar regions this optical illusion can make whole snow-covered mountains invisible against the background white cloud, and the horizon cannot be identified, slopes cannot be judged for steepness, and snow surfaces cannot be seen. It is only when a dark or coloured object is placed on a snowy surface that the surface can be seen at all.