20 October 2015
I listened to an excellent talk today at the 40th National Weather Association Conference here in Oklahoma City, and It was about a better way of alerting the public to dangerous heat. Yes, you’ve heard of the Heat Index, and it does indeed have value, since the amount of moisture in the air plays a big role on how efficiently your body can cool itself on a hot day. There are other important factors that the heat index does NOT measure, and they can make a big difference.
The heat index does NOT factor in whether or not it is cloudy, and it does NOT factor in the wind, or the sun angle, and the related amount of solar radiation that is reaching the surface. The Wet Bulb Globe Temperature does, but it’s not been as easy to measure. That may be about to change though and I suspect this index will become much more popular in the coming decade.
The formula for the Wet Bulb Globe Temperature (WBGT) is based on the wet bulb temperature, the globe temperature and the air temperature. The wet bulb temperature is the way (especially in the past) that we measured how much moisture is in the air. Take a thermometer and put a moist cloth on the bulb and spin it. The drier the air, the cooler the temperature you will get. This is the wet bulb. More sophisticated ways of measuring moisture are used now, but if you have wet bulb, dew point or relative humidity, you can easily figure out the other two.
You probably have heard of wet bulb temperature, but not globe temperature. Globe temp. is measured by putting a thermometer in a closed black ball. This is a dull black ball, and it will absorb the sun’s radiation and get quite warm in direct sunlight. Think of the inside of a dark metal shed on a sunny day. If it is cloudy, or if the wind is blowing, it will be cooler, and outside air temperature plays a role, as well as the sun angle.
What is interesting is that the actual air temperature plays a rather small direct role in calculating the index. 70% is based on the wet bulb, with 20% based on the globe and only 10% on the actual air (dry bulb) temperature. By using the globe thermometer, you make allowance for the intensity of the sun and this means your latitude and even altitude make a difference.
The talk today by Bradley Illston at OU, mentioned that forecasters at the NWS in Tulsa had been working to come up with a way to estimate the globe temperature using observations we already make, and they appear to have succeeded. They derived a 4th order polynomial that estimates the globe temperature, and when added to the other data in the formula, it gives an answer within a degree of the old method.
Once we start to transition to the WBGT method, there will be a real learning curve for the public, because the new scale does NOT match the heat index. Bradley Illston suggested that the descriptors of the index level be emphasized versus the actual index temperature. The image below has more info on this, and I linked the image to a very good page at the Tulsa NWS with more info.
The coming decades are going to get hotter and hotter, and we need a modern index that will tell us when the heat and humidity is reaching dangerous levels. NOAA is looking hard at this already, and hopefully, college and high school athletic teams will adopt this soon.
Some colleges already have!