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Precipitation, Icing

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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:

Light<0.1 mm/h<2.5 mm/h<1.0 mm/h
Moderate0.1 and <0.5 mm/h2.5 and <10 mm/h1.0 and <5 mm/h
Heavy0.5 mm/h10 mm/h5 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

The presented material is for training purpose only!