Whiteout or white-out is a

weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmo ...

condition in which visibility and contrast are severely reduced by

snow Snow consists of individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water througho ...

, fog, or

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

. The

horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...

disappears from view while the

sky The sky is an unobstructed view upward from the planetary surface, surface of the Earth. It includes the atmosphere of Earth, atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from ...

and

landscape A landscape is the visible features of an area of land, its landforms, and how they integrate with natural or human-made features, often considered in terms of their aesthetic appeal.''New Oxford American Dictionary''. A landscape includes th ...

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

cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles, suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...

mist Mist is a phenomenon caused by small droplets of water suspended in the cold air, usually by condensation. Physically, it is an example of a Dispersion (chemistry), dispersion. It is most commonly seen where water vapor in warm, moist air meets ...

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

snowfall Snow consists of individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water througho ...

s and

blizzard A blizzard is a severe Winter storm, snowstorm characterized by strong sustained winds and low visibility, lasting for a prolonged period of time—typically at least three or four hours. A ground blizzard is a weather condition where snow th ...

s, 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: # In

conditions, snow already on the ground can become windblown, reducing visibility to near zero. # In

conditions, the volume of snow falling may obscure objects reducing visibility to near zero. An example of this is during

lake-effect snow Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer lake water. The lower layer of air, heated by the lake water, picks up water vapor from the lake and rises through colde ...

or mountain-effect snow, where the volume of snow can be many times greater than normal snows or blizzards. # 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. Flat-light is a diffusion of sunlight. : Cause: Sunlight is both scattered and diffused by atmospheric particles (e.g. water molecules, ice crystals) and by snow lying on the ground; this causes light to be received from multiple directions. Commonly, the effect is increased during a whiteout and/or later in the day when the sun drops towards the horizon, due to sunlight passing through the atmosphere for a greater distance. : Result: Light is received from multiple directions with each light source producing overlapping shadows which cancel-out each other. This dulls the area and removes indicators such as tones and contrast, making it difficult to discern similarly coloured slope features. The loss of visual indicators of shape and edge detail results in objects and features seeming to blend into each other, producing a flat featureless vista. An effect of visual blending may be a loss of depth of field resulting in disorientation.

Hazards

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. One forward motorist may come to a complete stop when they cannot see the road, while the motorist behind is still moving. Local, short-duration whiteout conditions can be created artificially in the vicinity of airports and helipads due to aircraft operations. Snow on the ground can be stirred up by helicopter rotor down-wash or airplane jet blast, presenting hazards to both aircraft and bystanders on the ground.

References

{{reflist Severe weather and convection Weather hazards Snow or ice weather phenomena Fog Road hazards Weather hazards to aircraft Articles containing video clips

Types

Variations

Hazards

See also

References