Wet-Bulb Temperature and Snowfall

We hit 40° by lunchtime, but then it started snowing! Wondering why that is??? It has to deal with what’s called the wet-bulb temperature.Capture

The wet-bulb temperature is the air temperature at the surface that the air can cool to if the humidity reached 100% (meaning the air is fully saturated with water). It falls between the dew point (the green line) and the air temperature (the red line). A simple way to calculate an approximation to it is to look at the dewpoint depression (which is the difference between the temperature and the dewpoint), divide it by 3 and then subtract that number from the air temperature. 

So in our case today at noon, we saw an air temperature of around 40° and a dew point of around 18°. This gives a dew point depression of 22°, which divided by 3 is around 7°. Subtract this from the air temperature, and we would expect the air to cool to around 33° when fully saturated. This is cold enough to support wet snow to fall, but not to stick on the ground. We see after noontime, the temperature rapidly fell and the dew point rapidly rose, which is around the time the heavier precipitation bands were falling. This suggests that some of the falling snow was evaporating before these heavier bands moved in, as we only saw light snow flurries an hour before that. This cooled the air quickly towards the wet-bulb temperature and allowed for the heavier bands of precipitation to fall as snow as the atmosphere had been cooled by evaporation.

This is a typical case for the early and late winter precipitation events. Snow can happen up to 40-42°F sometimes and the best way to predict it happening is with the wet-bulb temperature.