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:
|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.