How can there be a relative humidity reading when the air temperature is well below freezing?
Water, especially in its energetic gaseous form known as water vapor, is always on the move. Even at air temperatures well below freezing above a frozen lake, there is water vapor in the air. Since relative humidity is the ratio of the amount of water vapor in the air compared to the maximum amount of water vapor possible in the air at a given temperature and pressure, there will always be a relative humidity reading. There’s plenty more about the phases of water on this USA TODAY resource page.