Feature Article

Don't Be a Dim Bulb. Use a Wet Bulb!

Charles Gruenwald

Traditionally, when a meteorologist wanted to see how uncomfortably muggy the weather was, she would use a sling psychrometer:

This classic instrument housed two thermometers in an apparatus she whirled around her head. One thermometer wore a little cotton sock over its bulb. The sock was soaked with water; the other bulb was kept dry. Evaporation cooled the sock and the bulb inside it.

The sock-clad thermometer reading was the wet-bulb temperature. The other reading was the dry-bulb temperature, but most people would just call it temperature.  These two readings were used to calculate the relative humidity and dewpoint, derived values commonly announced on the radio or available on various weather websites.

When you cycle enough to perspire, you're like the wet bulb!  If you knew the wet-bulb temperature, you'd have some idea how well your high-tech, water-wicking club jersey will keep you cool during your ride.

Unfortunately, the wet-bulb temperature isn't shown on wunderground.com, and unless you buy or make your own psychrometer, it'll take some fiddling to get it.

1. An oft-used rule of thumb is that the wet-bulb temperature is roughly a third of the way from the actual (dry-bulb) temperature down toward the dewpoint. For example, if the current temperature is 88°F, and the dewpoint is 70°F, the wet-bulb temperature is about 88-18/3 or 82°F.
2. The wet-bulb temperature is important in agriculture, so it is served up by specialized websites (e.g., browse to http://fawn.ifas.ufl.edu, click on 'Wet Bulb Temp', and mouse over Alachua, Bronson or Citra to display the value at those stations.)  
3. You can find web-based calculators and smartphone apps to calculate wet-bulb temperature from the relative humidity (or dewpoint), temperature, barametric pressure, and your height above sea level.  But, entering these data can be tedious, confusing and sometimes error prone.
4. Easiest is just to bookmark this page on the club website.  You'll see dewpoint, wet-bulb and dry-bulb temperatures.  These data are calculated from GNV airport measurements (referred to as METAR) and updated near the top of each hour.
5. The data are also displayed under the weather data on the GCC website main page.

More reading about wet-bulb temperature here and here.