Whiteout
Whiteout: polar optical phenomenon. Pilot engulfed in uniform white glow: no shadows, horizon, or clouds. Depth perception and orientation lost; only dark, nearby objects visible.
- Occurs: over unbroken snow cover beneath uniformly overcast sky, when sky light and snow-surface light are about equal (TC AIM AIR 2.12.7).
- Physics: overcast over snow/ice diffuses sunlight. Light strikes surface from many angles, reflects between snow and cloud. Shadows, depth perception, horizon vanish; buildings and dark objects appear to float (TP 1102 White-out; TC AIM AIR 2.12.7).
Three causes:
| Type | Mechanism | Visibility |
|---|---|---|
| Water-fog | Thin clouds of supercooled water droplets against cold snow surface | Can drop to minimal or nil |
| Blowing-snow | Fine snow plucked from surface by wind of 20 kt or more | Nil |
| Precipitation | Small wind-driven snow crystals falling from low cloud, sun shining above | Visibility and depth perception nil |
The real hazard: pilot does not suspect it: aircraft is in clear air.
- Pilots flew into snow-covered surfaces unaware of descending, confident they could "see" the ground.
Response (on meeting whiteout, or even suspicion of it):
- If at low level: climb immediately.
- Or: level off and turn toward area with sharp terrain features.
- Do not continue unless equipped with suitable instruments and experienced in low-speed, minimum-rate-of-descent landing technique.
Trap: treating clear air as proof it is safe to continue visually; whiteout removes horizon and depth cues while air stays clear.
References
- TC AIM AIR 2.12.7
- TP 1102 (Flight Training Manual) Ch.6 White-out