Here is a related question that noone has ever answered for me.
Dew point is calculated by comparing the temperatures of a dry-bulb thermometer and a wet-bulb thermometer. The idea being that evaporation cools the wet bulb thermometer by an amount related to relative humidity.
I've never been quite sure if dew point requires some calculation or whether it is a direct reading of the wet bulb temperature. However , the question is - how does that work when (as often happens) the temperatures are below freezing?