Water on Runways

Code	Surface description
6	Dry
5	Frost Wet (The runway surface is covered by any visible dampness or water up to and including 3mm depth) Up to and including 3mm depth: Slush Dry snow Wet snow
4	-15°C and lower outside air temperature: Compacted snow
3	Wet ('slippery wet' runway) Dry snow or wet snow (any depth) on top of compacted snow More than 3mm depth: Dry snow Wet snow Higher than -15°C outside air temperature¹: Compacted snow
2	More than 3mm depth of water or slush: Standing water Slush
1	Ice
0	Wet ice Water on top of compacted snow Dry snow or wet snow on top of ice

A runway is considered contaminated whenever the extent of the coverage is more than a quarter (25%) of the surface of at least one-third of the runway. The threshold for the effect of depth of fluid contaminants on aeroplane performance is 3mm.