300px|thumb|right|Whiteout in [[Saskatchewan (March 2007)]]

thumb|Whiteout [[snow squall northwest of Waterloo, Ontario, Canada (February 27, 2014)]]

thumb|Background shows large, stationary whiteout on [[Mount Lyell (Canada)|Lyell Icefield that lasted 4 hours]]

Whiteout or white-out is a weather condition in which visibility and contrast are severely reduced by snow, fog, or sand. The horizon disappears from view while the sky and landscape appear featureless, leaving no points of visual reference by which to navigate.

A whiteout may be due simply to extremely heavy snowfall rates as seen in lake effect conditions, or to other factors such as diffuse lighting from overcast clouds, mist or fog, or a background of snow. A person traveling in a true whiteout is at significant risk of becoming completely disoriented and losing their way, even in familiar surroundings. Motorists typically have to stop their cars where they are, as the road is impossible to see. Normal snowfalls and blizzards, where snow is falling at /h), or where the relief visibility is not clear yet having a clear field of view for over , are often incorrectly called whiteouts.

Types

There are three different forms of a whiteout:

  1. In blizzard conditions, snow already on the ground can become windblown, reducing visibility to near zero.
  2. In snowfall conditions, the volume of snow falling may obscure objects reducing visibility to near zero. An example of this is during lake-effect snow or mountain-effect snow, where the volume of snow can be many times greater than normal snows or blizzards.
  3. Where ground-level thick fog exists in a snow-covered environment, especially on open areas devoid of features.

Variations

A whiteout should not be confused with flat-light. Whilst there are similarities, both the causes and effects are different.

A whiteout is a reduction and scattering of sunlight.

: Cause: Sunlight is blocked, reduced and scattered by ice crystals in falling snow, wind-blown spin-drift, water droplets in low-lying clouds or localised fog, etc. The remaining scattered light is merged and blended.

: Result: Due to a reduction in reflected light, visual references e.g. the horizon, terrain features, slope aspect, etc. are significantly reduced or completely blocked. This leads to an inability to position yourself relative to the surroundings. In severe conditions an individual may experience a loss of kinesthesia (ability to discern position and movement), confusion, loss of balance, and an overall reduction in the ability to operate.

thumb|Flat-light on Ekström Shelf Ice, [[Antarctica]]

Flat-light is a diffusion of sunlight.