3 Degree Kelvin Split for Landing Performance.

[safetypee]

“The ‘3-Kelvin-spread-rule’: Moisture in combination with contaminated runways plays a more significant role in relation to ‘slipperiness’ than previously understood. In most occurrences the difference between the air temperature and dew point (at 2 m height above the runway surface - METAR values) was ≤ 3 Kelvin. This is referred to as the ‘3-Kelvin-spreadrule’ and indicates that the humidity is 80 % or more.”

http://www.aibn.no/Aviation/Reports/...-File&attach=1 Page 3

IIRC this was published as a recommendation (main report page 10, Winter Operations, Friction Mesurements and Conditions for Friction Predictions | aibn) / advisory information and circulated by ICAO/EASA/UK CAA; I don’t have the references immediately to hand.