Have you ever seen snow coming down when the temperature is 40 degrees or higher? How is this possible? Hint: it’s the wet bulb.
Wet bulb? What is that? The wet bulb temperature is the lowest temperature to which air can be cooled by evaporation under constant air pressure. Yes, evaporation has a cooling effect on the air, just the same as it does on our body.
The wet bulb temperature is easily measured with a sling psychrometer.
A sling psychrometer has two thermometers. One is wrapped with a wet cloth, while the other remains dry. Then, you take the psycrhometer and ‘sling’ it around in the air.
After doing this, you will see that the wet thermometer often shows a cooler temperature than the dry thermometer. Sometimes, it’s significantly cooler.
If the air is dry and cool enough immediately around a falling snowflake, then it can reach the surface even though the air temperature is well above freezing. It’s because the wet bulb temperature is below freezing.