Wet Bulb Globe Temperature (WBGT) Tool
What is WBGT?
What is WBGT?
The Wet-Bulb Globe Temperature (WBGT) is a way to measure the heat stress on a body. WBGT takes into account:
1. temperature
2. humidity
3. wind speed
4. the intensity of the sunlight (i.e. sun angle and degree of cloud cover)
How does WBGT differ from Heat Index?
Like the heat index, the WBGT takes into account the temperature and humidity of the air. It also considers the effects of solar radiation (e.g. heat stress is greater in the sun) and the wind speed (e.g. heat stress much greater when the winds are not blowing).
Wet-Bulb Globe Temperature is used by industrial hygienists, athletes, and the military as a guide to managing workload and level of activity.
Did you know?
In the 1950s, the US military developed the Wet-Bulb Globe Temperature index. In response to a large number of heat illness cases, the military made changes to their existing policy, which used temperature and humidity in a single value (heat index) to evaluate the risk to soldiers working and training in high heat situations. To better represent risk, wind speed, sun angle, and cloud cover were incorporated, forming the Wet-Bulb Globe Temperature. After implementing new policies based on the WBGT measures, there was a drastic reduction in heat illness during basic training.
WBGT has 3 components:
WBGT has three components that incorporate many factors into one measure of risk.
1. dry-bulb temperature
2. natural wet-bulb temperature
3. black globe temperature.
The dry-bulb temperature is also known simply as air temperature.
The natural wet-bulb temperature, is best described as an old-fashioned thermometer, wrapped in a wet cloth. As water evaporates from that wet cloth, evaporation cools the thermometer, which is identical to how the human body cools itself with sweat. Ultimately, the natural-wet bulb temperature incorporates the effect of air temperature, humidity, and solar radiation into one individual temperature.
Lastly, the black globe temperature is the temperature measured by a thermometer placed inside a black globe that is about 6 inches (150 millimeters) in diameter. Solar radiation from the sun heats the outside of the black globe, and the wind blowing across the globe cools the globe, and the thermometer inside measures these changes. In this way, the black globe thermometer reacts in the same way as human skin, including the effects of solar radiation and wind speed on body temperature.
These three temperature components are added together with the NWB given highest weighting:
WBGT = 0.1 * DB + 0.7 * NWB + 0.2 * GT
Click HERE to download a flyer about WBGT!
How to measure WBGT
Tips for measuring WBGT:
1. If the WBGT device has been inside, allow at least 15 minutes for it to adjust to conditions when beginning measurement.
2. WBGT can change quickly in response to momentary fluctuations in wind and cloud cover; therefore, it is best to average readings over at least a 10 to 15 minute period.
3. If there is a marked change in weather/sky conditions during practice (e.g. skies clear, wind stops blowing), new WBGT measurements should be taken. Note that WBGT often increases rapidly during the mid-morning hours (as the sun rises in the sky) and decreases rapidly during the early evening hours (as the sun falls in the sky).
4. WBGT can vary greatly over different surfaces (e.g. grass field, artificial turf, tennis courts), so take your measurements at the same place as the outdoor activity.
5. WBGT should be measured 5 feet above the ground.
Understanding the Tool
Background:
1. Because this is a forecasting tool, the WBGT values should be interpreted like that from any other weather forecast. Over the first few days, the tool has much skill in forecasting WBGT. However this skill declines towards the end of the five-day forecast period. Consequently, its output should be considered as a rough guide of the expected WBGT.
2. The WBGT forecast is updated every day around 6:30 am and 12:30 pm.
Interpreting the Forecast:
1. The undulating thick line is the predicted wet bulb globe temperature (WBGT).
2. The shaded area around the WBGT line represents the possible range of values from full sun exposure (top of shaded area in the plot) to full shade (bottom of shaded area in the plot) for WBGT. Note that shady conditions will also be observed if there is a thick overcast sky.
3. Hover your mouse over the WBGT line to see the forecasted WBGT and the range of values for varying amounts of sunlight.
4. The color bands represent warning levels as outlined by the NCHSAA. Scroll down to see recommendations for action for each warning level.
5. It is important to note that WBGT can fluctuate on scales smaller than what the forecast can resolve. Variations in solar radiation from passing clouds, differences in tree cover, and changes in surface type (e.g. grass, asphalt, concrete), wind, and humidity can occur within short distances, such as from one side of a street or field to the other, and over short time periods, such as from one minute to the next. Users should interpret the forecast as a general indication of expected conditions for their location, keeping in mind that actual conditions at a specific point and time may differ, potentially shifting the forecast by multiple flag levels (e.g. green flag on a shaded side of the street and yellow or red flag on the exposed side).
Click HERE to download a PDF version of the instructions.
FAQ
Why did my forecast change?
The wet bulb globe temperature (WBGT) forecasts are calculated from National Weather Service (NWS) forecasts of temperature, humidity, sky cover, and wind speed. These forecasts often change slightly in response to new information produced by the latest weather models.
Why do I get different measurements at different locations?
WBGT accounts for temperature, humidity, sunlight, and wind speed, each of which can change from one location to another; consequently, the WBGT measurement can change as well. For example, the humidity of the air near a lake or water source is usually higher than that over a dry open field, thus WBGT will be higher. Because of these local variations, it is important to take WBGT measurements at the actual location of planned activities.
Where is the best location to take my WBGT measurement?
The best location to take the WBGT measurement is the location(s) in which the activity will occur. Since WBGT accounts for four meteorological variables (e.g. see above), different surfaces (e.g. grass field versus a tennis court) often have different WBGT values. Therefore, it is important to realize a WBGT value is specific to the place you are taking the measurement.
How does WBGT differ from Heat Index?
Like the heat index, the WBGT takes into account the temperature and humidity of the air. It also considers the effects of solar radiation (e.g. heat stress is greater in the sun) and the wind speed (e.g. heat stress much greater when the winds are not blowing).
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