Environmental Factors Affecting Heat Loss
The air temperature and wind speed determine the amount of heat that can be lost from the skin by convection, which is the heating of the surrounding air by the skin.
High facing-wind speeds cause a large volume of unwarmed air to cross the skin in unit time and therefore allow for greater heat loss by convection. Running itself produces an effective wind speed that aids convective heat loss but may not be sufficient to increase heat loss adequately in severe environmental conditions. Obviously, a wind coming from behind the runner at the same speed that the runner is moving forward will cause him or her to run in a totally windless environment, which will prevent convective heat loss. In contrast, the wind speed developed by cyclists appears to be sufficient to compensate even for severe conditions, which explains why heat is not as great a problem for endurance cyclists as it is for runners.
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At rest, the body skin temperature is about 33 °C. If you exercise in environmental temperatures greater than 33 °C, heat cannot be lost by convection, because the air temperature is higher than that of the body surface. In this case the direction of heat transfer is reversed, and the superficial tissues gain heat from the environment. In these conditions, the only avenue for heat loss is by sweating. Sweating removes 1,092 to 2,520 kJ of heat per liter of sweat evaporated (see Exercises 4.1), depending on whether all the sweat evaporates or, as usually happens, a large percentage drips from the body without evaporating. As the air humidity increases, the body’s ability to lose heat by this mechanism decreases.
The body can absorb additional heat from the environment, in particular from the sun. The body temperature is cooler than that of the sun, thus, the body will absorb radiant energy from the sun. Obviously, the amount of radiant energy to
Sweat rate (Uhr) which the athlete is exposed is greatest when there is no cloud cover and is least when cloud cover is absolute.
These three environmental factors that determine the athlete’s ability to lose heatwind speed, the humidity and temperature of the air, and the radiant energy loadare measured in the wet bulb globe temperature (WBGT) index.
The WBGT index integrates the measurement of radiant energy (as the temperature of a black globethe globe temperature) with the wet bulb temperature (measured by a thermometer covered by a wick permeated with water). The difference between the wet bulb temperature and the prevailing air (dry bulb) temperature is a measure of the humidity of the air and therefore is also a measure of the ease with which sweat will evaporate from the athlete. Furthermore, wind blowing over the wet wick of the wet bulb thermometer will increase the rate of evaporative cooling and will therefore lower the wet bulb temperature. In this way, the prevailing wind speed also influences the WBGT index.
post 9 and Exercises 9.4 describe the use of the WBGT index to determine precautions that should be taken to prevent heat injury during marathon races. Recently, the corrected environmental temperature (CET) has been proposed as an even better predictor of heat stress during running. This is discussed in greater detail in post 9.