Wet Bulb Temperature

 by Maximilian Bergmair

Wet Bulb Temperature is a measure of perceived atmospheric temperature. What makes this value special, and why is it significant to what H.E.A.T. is trying to achieve?

 

What separates Wet Bulb Temperature from other values derived from similar temperature measuring techniques is that it accounts for the cooling effect caused by evaporation. Wet Bulb Temperature is taken as a reading from a thermometer wrapped in water-soaked cloth with air being passed over it. It can be more formally defined as the temperature of a parcel of air cooled to saturation by the evaporation of water. 

 

The exact mechanism of evaporative cooling effects is touched on in the previous article discussing the Urban Heat Island Effect; that explanation follows: During evaporation, fast-moving liquid molecules break away from the surface of said liquid to enter a gaseous phase. Since only fast-moving, and thus more energetic, molecules can break away, the average energy of remaining molecules decreases as a result. Since temperature is a measure of average energy, a decrease thereof is represented on a larger scale by a cooling effect.

 

Why does H.E.A.T care about this number and what is it used for? Wet Bulb Temperature is significant because it is the most accurate measurement for representing the heat stress borne by a person. The evaporation of sweat is the most effective way the human body cools itself, and is the only viable cooling mechanism when ambient temperatures exceed those of the body. By extension, this number has significant ramifications on the human effects of heat stress within HK’s cage homes. 

 

In other words, Wet Bulb Temperature is the ideal metric of perceived heat. Other metrics of perceived heat, such as the Heat Index (which accounts for air temperature and relative humidity levels), are not as effective as the Wet Bulb Temperature, which also accounts for wind, direct sunlight, and cloud cover. Because of its inclusion of additional factors which are relevant to a person engaging in activities outdoors or managing their daily itinerary, the Wet Bulb Temperature is a more useful metric for developing policies and finding solutions to heat stress that are aimed at helping the people affected.

 

The Wet Bulb Temperature conveys vital information about health effects. Human bodies can only function within a certain temperature range due to the complex assortment of chemical and biological processes required to keep them functioning (more on this next week). For example, Wet Bulb Temperatures of 32℃ or higher put you at extreme risk of heat stroke and other heat-related dangers; prolonged exposure to Wet Bulb Temperatures of 35℃ or higher are not survivable.

 

An example chart of typical Wet Bulb Temperatures, along with sample guidelines for those temperatures, can be found here[1]. A more in depth analysis of the effect of heat on human physiology will follow next week. Until then, keep cool.

 

Maximilian Bergmair

 

  1. “WBGT Chart.” Ariel’s Checklist, 2017, arielschecklist.com/wbgt-chart.