Precipitation, Icing
1. General
Precipitation can severely reduce Visibility (VIS) and can give a false optical illusion during the landing phase. A flooded, contaminated RWY may deteriorate the braking coefficient towards zero (aquaplaning).
2. Categories of Precipitation
- • Vapor – Consists of very small drops of water which form haze, mist, and fog. This form of precipitation reduces the VIS drastically.
- • Liquid – Forms of precipitation are drizzle and rain.
- • Frozen – Forms are snow, snow grains, snow pellets, ice pellets, and hail.
3. Classification of Precipitation Intensity
With regard to precipitation, detection thresholds expressed in Millimeters per Hour (mm/h) are given for some sensors. The World Meteorological Organization (WMO) reporting thresholds for light, moderate and heavy precipitation are shown in the table below:
| Intensity | Drizzle | Rain | Snow |
|---|---|---|---|
| Light | <0.1 mm/h | <2.5 mm/h | <1.0 mm/h |
| Moderate | 0.1 and <0.5 mm/h | 2.5 and <10 mm/h | 1.0 and <5 mm/h |
| Heavy | 0.5 mm/h | 10 mm/h | 5 mm/h |
4. Airframe Categories of Icing
- • Rime – Thin milky film of frozen vapor.
- • Frost – Rough, milky, and opaque ice formed by the instantaneous freezing of small water droplets or snow flurries.
- • Clear ice – Glossy, clear, or translucent ice formed by slow freezing of water droplets.
Source: ICAO
See also:
Precipitation, Icing
Anti-Icing Codes
Winter Operation Definitions