It’s a Dry Heat!
May 18th, 2011
Summer is approaching, and if you live in the Southwest, it is already here - May has already seen triple digit temperatures in Phoenix, Arizona. But 100 degrees in the Southwest desert is not the same as even 90 degrees in the Midwest, South or East Coast (which is what makes the heat in this region of the United States so unique). So today class, we will discuss the heat, how it effects you and why the statement "It's a dry heat" actually means more than just some silly cliche to justify living in Arizona. Also, it is this dry heat that makes outdoor activities bearable if you understand the climate, fluid loss and how to stay cool on the hottest of days.Summers in the Arizona deserts can be grueling and can reach temperatures of 122 degrees F - even in the shade. Add 10-20 degrees if you are in the sun. However, it is the humidity that makes the heat exponentially more dangerous when participating in outdoor activities. This is why you may have read or heard about the "feels like" temperatures - or the Heat Index - versus the actual temperatures. Here are a few examples:
A typical June in Arizona is the hottest, but also driest, month of the year. At 110 degrees F and a relative humidity of 20%, the heat index is 112 degrees. Yes, that sounds hot, but let's look at temperatures in a humid state such as Illinois. Springfield, IL can experience humidity of 80% in June, with average temperatures of 85 degrees F. The heat index is 97 degrees F, a 12 degree increase. If the temperature climbs to 90 with 80% humidity, the heat index will soar to 113 degrees F.
You might remember the 2007 Chicago marathon that was canceled mid-way through the race due to excessive heat - 88 degrees and 86% humidity (heat index of about 116 degrees F). One person died, 50 were hospitalized and another 300 were treated for heat related illness. Many races are held in Phoenix with temperatures in the 80s, in fact, the Tempe Ironman used to be held in April (average temperatures of 85 degrees and about 30-40% humidity - a heat index of 84-86 degrees F). The Ironman is now in November, but April is a month full of organized races and outdoor activities in Phoenix.
The dry heat also means your body can actually cool itself off, which is not much of a possibility regions of high humidity. Temperature regulation represents the balance between heat produced (heat-in) and heat removed (heat-out). When the body's temperature regulation system is working correctly, heat-in and heat-out are in perfect balance. Radiant heat from the sun and heat created from burning fuel (carbohydrates, protein and fat) contribute to an increase body temperature. Therefore, the body must find a way to dissipate the same amount of heat that has been added to maintain a constant body temperature. This is done is two ways; 1) moving more blood to the skin to allow for heat dissipation through radiation and 2) increasing the rate of sweat production. These two systems account for 85% of heat removal, while during exercise, virtually all heat loss occurs via evaporation (sweat). However, in order for evaporation to occur, the air must be dry enough to allow moisture into the atmosphere (think of a very small atmosphere around your body). If the atmosphere is already saturated from high humidity, then the sweat produced by your body will not evaporate and thus will not regulate your body temperature. In dry climates, evaporation does occur, thus, regulating body temperatures. See our blog about wearing cotton when exercising in the desert to further explain this process.
In order for these processes to work, adequate blood volume must also be maintained. As blood volume decreases, so does movement of blood to the skin and sweat production. With low blood volume, one or both of these systems fail, resulting in diminished performance, dehydration, and if allowed to continue to decrease, heat stroke and death. So how do we keep an adequate blood volume? Fluid and electrolyte replacement, which is another topic entirely.
So, long story short, the "dry heat" in the desert is not such a bad thing and quite easy to deal with if you understand just a little bit of the science behind it.
Sources:
Dan Benardot, Advanced Sports Nutrition, 2006.
"Record Heat Halts Chicago Marathon," Washingtonpost.com, October 8, 2007.
