By Bruce McDuffee
Relative humidity is the ratio of two pressures: %RH = 100 x p/ps where p is the actual partial pressure of the water vapor present in the ambient and ps the saturation pressure of water at the temperature of the ambient.
Relative humidity sensors are usually calibrated at normal room temperature (above freezing). Consequently, it generally accepted that this type of sensor indicates relative humidity with respect to water at all temperatures (including below freezing).
Ice produces a lower vapor pressure than liquid water. Therefore, when ice is present, saturation occurs at a relative humidity of less than 100 %. For instance, a humidity reading of 75 %RH at a temperature of -30°C corresponds to saturation above ice.