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SNOW AND BLOWING SNOW HAZARDS

Hazards to flight

Snow and blowing snow can greatly affect aircraft on the ground, as well as during ascent and descent. When visibility is significantly reduced, as a result of snow activity, aircraft acceptance rates are often reduced. This causes operational delays, and at times aircraft may need to divert to alternate airfields. Significant snow accumulation can impact airport operations, and runways may close for plowing. If the rate of snowfall is such that plowing and treatment are not able to keep runways clear, a particular airfield may be forced to close.

4. Snow accumulation

Snow is designated by intensity, based on rate of accumulation and visibility restrictions. Heavy snow is indicated by "+," moderate snow has no designation, and light snow is indicated by "-." As snow is often accompanied by fog, using visibility alone as an intensity descriptor will not necessarily be accurate. Snowfall rates can be calculated, but if snow is melting as it falls or while on the ground, projected snow depth may not be accurate. Pilots need to be aware of these differences.

5. Visibility impact

Snow can greatly affect visibility and has potential to reduce visibility rapidly. This is particularly true in situations in which the intensity of snowfall is increasing. When snow already on the ground is blown about by strong winds, this often leads to rapid reduction in visibility. The following definitions use visibility in determining snow fall intensity. Use these with caution as other factors – such as liquid water content, temperature, and daylight/darkness – also impact visibility.
Below you will see the visibility based on the snow intensity:

• Light Snow: Snow conditions in which visibility is greater than 0.5 mile
• Snow: Snow conditions in which visibility is in a range from 0.25 to 0.5 mile
• Heavy Snow: Snow conditions in which visibility is less than .25 mile

6. Flat-light and white-out conditions

The FAA Safety Library provides a good reference how to avoid white-out situations.

Flat light

An optical illusion also known as "sector or partial white out." It’s not as severe as a "white out," but this condition causes pilots to lose depth-of-field and contrast in vision. Flat-light conditions are usually accompanied by overcast skies, inhibiting any good visual clues. These conditions can occur anywhere in the world but primarily occur in snow-covered areas. Flat light can completely obscure features of terrain, creating inability to distinguish distances and closure rates. As a result of reflected light, this condition can give pilots the illusion of ascending or descending when they’re actually flying level. With good judgment and proper training/planning, however, it’s possible to safely operate aircraft in flat-light conditions.

White out

As defined in meteorological terms, is a condition where a person becomes engulfed in a uniformly white glow. This glow may be a result of being surrounded by blowing snow, dust, sand, mud, or water. In a white-out condition, there are no shadows, no horizon, and no clouds, and all depth-of-field/orientation are lost. White-out situations can be severe in that there are not any visual references to depend on. Flying is not recommended during any white-out situation. Be aware that flat-light conditions can lead to a white-out environment quite rapidly. Both atmospheric conditions are insidious. They sneak up on you as your visual references slowly begin to disappear. White outs, in snow-covered areas, have been the cause of several aviation accidents over recent years.

Self-induced white out

Typically occurs when a helicopter takes off/lands on a snow-covered area. Rotor down-wash picks up particles and re-circulates them through the rotor system. The self-induced white-out effect can vary in intensity, depending upon the amount of light on the surface. This phenomenon can occur even on the sunniest, brightest day with good light contrast. When it happens, there can be a complete loss of visual clues. If the pilot has not prepared for this immediate loss of visibility, the results can be disastrous

View original post: http://www.universalweather.com/blog/2015/01/snow-its-effects-on-aircraft-runways-part-2-dealing-with-snow/#ixzz3PeLWu8di
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