Wet bulb temperature: The lowest temperature that can be obtained by evaporating water into the air at constant pressure. The name comes from putting a wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb (actual temperature) at the same time and place. The wet bulb temperature is always lower than the dry bulb temperature in the same surroundings, because of evaporative cooling.

The Mid-Atlantic WX.com thermometer and hydrometer that calculates our wet bulb temperature is located 2 meters above the surface, so, our wet bulb temperature is approximately six feet above the ground. It certainly is possible to realize a greater (or lesser) wet bulb temperature higher in the atmosphere therefore this is not a perfect indicator of when rain may change to frozen precipitation or what type.

How does this help you? You may see precipitation on Radar but it isn't reaching the surface. Watch our wet bulb thermometer and you'll see the wet and dry bulb temperatures eventually get closer...the atmosphere near the surface is moistening up. When the wet and dry bulb temps are about the same the air at the surface is nearly saturated and precipitation should shortly begin.

Knowing the wet bulb temperature is especially helpful in winter. Remembering the wet bulb temperature will always be lower that the dry bulb (actual) temperature, it can assist in determining what type of precipitation may fall...and when. If the wet bulb indicates 32°, precipitation likely will be rain. Wet bulb readings below 32° indicate the temperature near the surface can support frozen precipitation.

What type of precipitation?  Experience indicates as a general guideline that in the Lexington, VA area, precipitation will fall as rain when our wet bulb is at or above 32.0°. Typically, freezing rain or ice pellets are observed when our wet bulb is between 30.5° and 32.0°. Precipitation will be snow or a wintry mix when both the wet and dry bulb is below 30.5°.

Keep an eye on our wet and dry bulb temperatures, especially when both temperatures hover 32° and you'll have a better idea of when and what type of precipitation is likely to fall at the weather station.