With a stretch of VERY hot weather ahead this week it is a good idea to look at how we measure the heat stress on your body. The National Weather Service uses two different measurements of heat stress. One is currently experimental and the other is already in widespread use.
The heat index is the measurement of heat most commonly known to the public. The wet-bulb globe temperature is still in its experimental phase, however, arguments have been made that it is a better measure of the true heat-related stress on your body rather than the heat index. We will explore why in this episode of Weather 101.
What is the Heat Index?
The heat index gives us an idea of the apparent temperature felt outside. It calculates this index using a combination of the current air temperature and relative humidity. When the relative humidity is higher less evaporative cooling is allowed to occur. This of course makes the temperature “feel” hotter.
For more information on the heat index check out this Weather 101 below.
Remember that relative humidity is NOT an accurate measurement of the moisture in the atmosphere. This is because the relative humidity changes based on the temperature. The dew point is a much more accurate measurement of moisture since it DOES NOT change based on the temperature. It can stand alone. For more information on relative humidity vs. dew point check our Weather 101 on that topic.
What is the Wet-Bulb Globe Temperature?
The wet-bulb globe temperature (WBGT) measures heat stress on the human body in direct sunlight using measurements such as air temperature, wind speed, humidity, sun angle, and cloud cover.
Heat stress is the stress exerted on the body due to extreme heat which according to the CDC, “can result in heatstroke, heat exhaustion, heat cramps, or heat rashes.” Let’s break down the measurements used in calculating the wet-bulb globe temperature a little further.
Air temperature- This one is pretty obvious. The higher the mercury rises, the greater the heat stress is on the body.
Wind speed- This one also makes sense if you think about it. I am sure you have noticed that when the wind blows, it feels less hot. This is ultimately due to the atmospheric mixing of the drier air aloft with the more humid air at the surface. This makes the humidity back off just a smidge alleviating the heat stress if only for a moment. Unfortunately, it takes some strong gusty winds to make any real difference.
Humidity- This one is also self-explanatory for the most part. The higher the relative humidity and the dew point, the greater the heat stress on your body will be.
Sun angle- This one might be a little strange to you. The higher the sun angle the more direct the solar radiation is. During the summer, the sun angle is very high and so the radiation impact is more direct. During the winter when you have a lower sun angle, the sun’s radiation is more diffuse. The diffuse sun DOES NOT heat the atmosphere as much resulting in cooler temperatures.
Cloud cover- This one should once again be self-explanatory. The more cloud cover in the sky, the cooler the temperatures and as a result, the heat stress is much lower on your body.
The National Weather Service Office in Tulsa, OK has created a tool that will help you calculate what the wet-bulb globe temperature is for your area anywhere across the country. The link below will take you to the calculator.
Which Measurement is Better at Keeping You Safe?
Both of these methods of measuring the potential of heat-related stresses on your body are good to use. There is one major difference though. The heat index is calculated for shady areas and the wet-bulb globe temperature is calculated for areas in direct sunlight.
You can see that both are useful depending on where you plan to spend your time outside. If you are primarily in the shade, then use the heat index. If you plan to be primarily in the sun, then use the wet-bulb globe temperature.
Another difference you will notice is the scale. The heat index scale is higher than the actual temperature.
The WBGT uses a much lower scale. While it may seem like the temperature is cooler, it is not using the same temperature scale as the heat index. This means the two scales CANNOT be compared! Wet-bulb globe temperatures in the 90s can be deadly if the proper actions are not taken. Look at the previous post below from the Tulsa National Weather Service concerning WBGT.
The WBGT is often used alongside the metabolic rate to assess the likelihood of seeing impactful heat stress on the human body. This is especially true in occupational and athletic settings.
Proper Hydration and the Wet-Bulb Globe Temperature
Based on the WBGT, you can calculate how much water you need to intake to stay properly hydrated on extremely hot days. Check out these Western Kentucky University calculations for you. Follow these guidelines to stay hydrated in this blistering heat.