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Precipitation, Icing

Wikis > Meteorology > Precipitation, Icing
Precipitation, Icing

1. General
Precipitation can severely reduce Visibility (VIS) and can give a false optical illusion during 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 Millimeter 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

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