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

There are three different forms of a whiteout:

1. In blizzard conditions, the windblown snow in the air may make it too difficult to see very far.
2. In snowfall conditions, the volume of snow falling may obscure objects reducing visibility.
3. 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 (and in eg the Scottish Hills in winter) 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. This effect is exacerbated by a smooth surface of fresh snow. It is only when a contrasting object is placed on a snowy surface that the surface can be detected. In less extreme cases, it may suffice to break the snow surface by throwing a snowball ahead. This form is also known as flat light.

Whiteout conditions pose threats to mountain climbers, skiers, aviation, and mobile ground traffic. Motorists, especially those on large high speed routes are also at risk. There have been many major multiple-vehicle collisions associated with whiteout conditions.