Heat can be dangerous on the human body. One way we measure how hot it feels is by the heat index. Another is the wet bulb globe temperature (WBGT), and that is what Missouri and Illinois schools use to determine if practice are games can be held.
What You Need To Know
- The heat index, or "feels like" temperature, is commonly used, but it has limitations
- The wet bulb globe temperature uses a wider range of factors
- It's useful for those who work or exercise in hot conditions
Developed in the late 1970s by NOAA, the heat index, or "feels like" temperature, accounts for what the temperature and humidity feel like to our bodies. Most notably, higher humidity impedes our ability to cool off through evaporative cooling by sweating.
But, we calculate the heat index in the shade and do not take other significant factors into account.
The wet bulb globe temperature (WBGT) is a measure of the heat stress in direct sunlight, which uses factors such as temperature, humidity, wind speed, sun angle and cloud cover. We use the WBGT as a metric for assessing the safety of athletic practices and contests.
It’s also used by the U.S. Occupational Safety and Health Administration (OHSA) and military agencies.
Scientists developed the WBGT as an environmental monitoring measure during exercise in the heat in the early 1950s in response to the number of heat casualties occurring in the U.S. armed services in the prior decade. From 1942 to 1944, 198 soldiers died from heat illness during military training.
We calculate the wet bulb globe temperature using three different thermometers. The first is a standard dry bulb thermometer.
The second is a standard wet-bulb thermometer, whose bulb is wrapped in a wet cotton sleeve. This simulates sweat cooling the body through evaporation off the skin. Its reading is known as the wet-bulb temperature.
The third is a standard black globe thermometer, whose bulb is inserted into a large black ball to measure the effects of sunshine and other radiant heat. This is known as the black globe temperature.
WBGT is used by many high schools to keep athletes safe during practices and games.
The MSHSAA Board of Directors approved the WBGT as the recommended measurement practice and device for measuring acceptable heat/humidity levels for practices and contests. While it is mostly used during summer and early fall, the recommended use of WBGT is throughout the entire year when temperatures are above 80 degrees.
Here's a tool to calculate the WBGT in the St. Louis area. Based on this information, the WBGT will be at its highest on Tuesday and Wednesday afternoon, in the high or red flag zone.
But why is this measurement not more widely used? The answer probably has more to do with public perception than scientific value.
The general public better understands the "feels like" temperature. When we say the high will be 95, but it will feel like 110 degrees, most people know innately what that means.
Calculations of the WBGT are expressed in degrees, with the low end at 80 degrees or lower. The high end is at 90 degrees or above.
For most, numbers in the WBGT may not sound ominous enough to take note of. With critical values "only" in the upper 80s to lower 90s, it just doesn’t sound all that significant.
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