How to Freeze in the Hot Utah Desert

Photo Credit: Gigi Thorsen

I was on a river trip the past couple of days and it was a great lesson in the power of evaporative cooling.  Temperatures in Moab reached the high 80s yesterday, and floating on a raft with a personal floatation device on was hot, if you were dry.  If you were wet, however, it was absolutely frigid and any breeze or gust of wind provided a great lesson in the power of evaporative cooling.

The temperature commonly cited in weather reports is called the dry-bulb temperature by meteorologists.  It is the temperature measured by a thermometer that is not exposed to the sun or moisture.

If wet, however, the evaporation of water can affect the temperature of an object.  The lowest temperature that an object can reach through the evaporation of water is called the wet-bulb temperature.  The wet-bulb temperature is a function of the dry-bulb temperature and the relative humidity.  In a dry airmass, the wet-bulb temperature can be much lower than dry-bulb temperature.  In a humid airmass, however, they may not differ by much (in fact, they are the same when the relative humidity is 100%).

Back in the day, the wet-bulb temperature (as well as dewpoint and relative humidity) was measured using something called a sling psychrometer.  The sling psychrometer was a thermometer with piece of fabric on it that you would wet.  You'd then "sling" the psychrometer around and the evaporation of water from the fabric would lower the temperature measured by the thermometer to the wet-bulb temperature.  This is a common experiment done in Earth science classes.

Contrary to conventional wisdom, meteorologists don't measure wet-bulb temperature (or dewpoint and relative humidity) manually anymore.  Meteorological observations are fully automated.

Nevertheless, I was the human wet-bulb thermometer the past couple of days on the Colorado River, and the observations from the USGS/National Park Service Building in Moab pretty much show what I observed.  Yesterday afternoon (29 May), for example, the dry-bulb temperature was in the high 80s yesterday (red line), but the wet-bulb temperature was in the low 50s (green line).

This difference of more than 40F reflects the remarkably dry airmass resident over the upper Colorado basin yesterday.  The dewpoint was below 20F and the relative humidity less than 10%.

Thus, there was tremendous potential for evaporative cooling.  When you were dry, it was hot in the sun, but when you were wet, your liver would quiver, especially when the wind blew hard and the energy loss to evaporation increased.

This is a common situation in Utah during the warm season and a reason why evaporative (a.k.a. swamp) coolers are quite effective for home cooling.  Of course, that effectiveness goes down quickly during monsoon surges when the dewpoint, relative humidity, and wet-bulb temperature are higher.