Wet Bulb Globe Temperature Explained
When training or racing in the heat, it’s important to predict how well we can cool ourselves. Reading a thermometer and humidity gauge is easy but doesn’t always tell us what we need to know. Finding the heat index (which combines air temperature and relative humidity) gives a better perspective. The Wet Bulb Globe Temperature (WBGT) provides an even more relevant measure of the air’s cooling potential.
This knowledge, coupled with a CORE sensor, can help athletes better cool themselves and race more strategically.
What Matters for Cooling?
During exercise, the body primarily cools itself by sending more blood to the skin, enlarging the blood vessels at the skin, and by sweating. This allows four types of heat transfer to occur.
When sweat evaporates, it cools the skin, which in turn cools the large volume of blood flowing just beneath the skin. This evaporation accounts for the vast majority of cooling during land-based exercise. If moisture does not evaporate from the skin, then it is very hard for an exercising body to cool itself. A high relative humidity inhibits evaporation and cooling. This is because the air is already saturated with moisture and cannot hold much more.
Convection is the loss of body heat via adjacent molecules of air. As those heated molecules move away from the body, they are replaced by cooler molecules, which in turn absorb more heat from the body. A strong wind (or fan when indoors) moves a lot of heated air away from the body, increasing the cooling. The high speeds of outdoor cycling generates the same convective cooling effect as a strong wind – the body is always surrounded by cool air, as the heated air is quickly blown away.
Radiation is the transfer of heat via infrared waves. It occurs between objects of two different temperatures. In cool temperatures (less than 20°C/68°F), the body readily radiates heat to the air. In direct sunlight, when the sun is high in the sky, the skin absorbs substantial heat.
Conduction is the transfer of heat between two objects in direct contact with each other (eg. touching a cold bike frame). It accounts for a small amount of heat loss during exercise in the air, but a large amount during water-based exercise (eg. swimming). This is because the dense water conducts a lot more heat than air.
Why Humidity is Important
Evaporation is the most important method of cooling for activities like cycling and running. But evaporation can happen only if the air around the body can absorb more water. When the air is already full of moisture, we say that the relative humidity is high. When the air contains very low moisture, relative humidity is low.
During low relative humidity, sweat and water evaporates very quickly from the skin, quickly cooling the core temperature. During high humidity, sweat and water evaporates from the skin very slowly, and very little cooling is achieved from evaporation/sweating.
We know from experience that “hot and dry” can feel much cooler than “warm and humid”. This is because the dry air allows a lot of sweat evaporation. CORE users can confirm this with objective data, as they know their core temp rises more slowly in those dry conditions.
The Heat Index
The well-known heat index combines air temperature with relative humidity to give a “feels like” temperature. The heat index describes the cooling ability of the air. For example, a temperature of 37.8°C/100°F and 40% relative humidity feels the same as 30°C/86°F and 95% relative humidity. They both have the same heat index (109) because both conditions have the same cooling ability – sweat will evaporate from the body at the same rate in each condition. Because of the influence of humidity on evaporative cooling, the heat index is an important number for athletes. Even though the air temp of 30°C/86°F doesn’t “seem” too hot, the 95% humidity limits its cooling ability.
Relative humidity is calculated by wrapping a wet cloth around a thermometer. This measures the lowest temperature at which air can be cooled by evaporation and is called “wet bulb” temperature. This is then compared to air temperature (“dry bulb” temperature). A mathematical formula then converts these numbers to the heat index.
Limitations of the Heat Index
But the heat index doesn’t always give the full picture of the ability for athletes to cool themselves. For example, the heat index is measured in the shade, meaning it does not account for radiative heat gain caused by sunlight. The heat index also assumes a light wind. However, if the air is stagnant, much less convective cooling is available. And the heat index fails to account for the cooling effect of a strong wind.
What is Wet Bulb Globe Temperature?
The Wet Bulb Globe Temperature (WBGT) addresses the heat index’s limitations. WBGT considers not just air temperature and humidity, but also cloud cover, sun angle, and wind speed. This gives a more accurate picture of the amount of radiative heating that athletes will receive and the amount of convective cooling available.
For instance, the angle-of-sun calculation means the WBGT temperature can increase dramatically between morning and mid-afternoon, even if the heat index stays relatively constant across that time. And because WBGT is measured in the sun, it accounts for microclimates, like the heat radiating from hot tarmac.
Likewise, a strong wind will cause WBGT to drop, and stagnant air will cause it to rise. These variable wind conditions do not affect the heat index.
Measuring WBGT
Many nations’ government weather services predict WBGT for different times of the day. These predictions are generalised across regions and cannot account for microclimates (like a wind-shielded stadium, or a steep slope with southern exposure). A number of consumer-grade WBGT thermometers are available for purchase. These allow an athlete or coach to measure real-time WBGT at a specific location, such as a running track. These devices may take considerable practice to operate precisely and meaningfully.
WBGT Danger Index
Anyone using WBGT to guide athletic training must understand that the WBGT is calibrated much differently than the heat index. For example, many guidelines suggest that the danger zone for strenuous exercise starts at a heat index of 40°C/104°F. However, this heat index “danger” value is equivalent to a WBGT of only 31°C/88°F Which means it is critically important to understand and clearly communicate the index and scale being used for assessing heat danger.
Limitations of WBGT for Athletes
WBGT considers wind speed and sun exposure in addition to temperature and humidity, which means it generally better estimates real-world cooling conditions.
However, WBGT may not provide a more precise “feels like” temperature in some settings. For example, in cycling, environmental wind speed is often not important for cooling, as cyclists can benefit from the self-generated convective cooling wind of their 35 km/hr travel speed. But on a different part of the same course, such as a long, steep climb on a sunny slope (where cyclist speeds are low), WBGT may give very valuable information.
Similarly, any cyclist who has ridden in strong winds on a hot sunny day knows the relevance of wind direction. For example, a 35 km/hr crosswind may be cooling. But as a tailwind, it can cause overheating as the cyclist rides in stagnant air and convective cooling is lost. Similar conditions exist for runners – even a 10 km/hr tailwind can cause a loss of convective cooling and a rise in the “feels like” temperature.
In other words, real-world cooling conditions are sometimes strongly influenced by microclimates. And while WBGT can often give a better idea of those microclimates than the heat index can, the information needs to be supplemented by the experience and judgement of coaches and athletes.
Coupling WBGT with CORE
One reasons athletes train with CORE is to practice their cooling strategies. These strategies can vary widely based on specific environmental conditions. Hot and dry conditions mean that misting and dousing the skin with water is very effective. Intense solar radiation can be countered by choices of clothing type and colours. High relative humidity can be mitigated with pre-race cooling.
Knowing the WBGT (and the factors influencing it) before every workout allows athletes to see how their core temp reacts in different situations. They can try a cooling strategy and get immediate feedback from their CORE sensor indicating if it is effective or not. They will learn how some strategies are more effective in some conditions than others.
Before competition, knowing the predicted WBGT allows athletes to plan their cooling strategies. Based on their training with CORE, they will know how quickly their core temp will rise in various conditions. If temperatures are so hot that cooling is not effective, their CORE device will inform them when their core temp is approaching the performance degradation zone. And then they can make strategic decisions, which may even involve slowing the pace to stay at a sustainable core temperature.
References
- NCHSAA Heat Guidelines. https://www.lcsnc.org/cms/lib/NC01911169/Centricity/Domain/1695/NCHSAA%20Heat%20Guidelines.pdf