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High Wet Bulb Globe Temperature (WBGT) is forecast to hit the Southeastern United States over the next few days. The image below shows a forecast for June 29, 2023, 18 UTC with WBGT as high as 34°C (93°F) forecast for a location near Jackson, Mississippi, of WBGT up to 34°C (93°F).
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| [ click on images to enlarge ] |
WBGT is a measure used by weather.gov to warn about expected heat stress when in direct sunlight. It estimates the effect of temperature, relative humidity, wind speed, and solar radiation on humans using a combination of temperatures from three thermometers:
- A Wet bulb measures the temperature read by a thermometer covered in a wet cloth. As water evaporates from the cloth, evaporation cools the thermometer. This mirrors how the human body cools itself with sweat.
- A black globe is used to measure solar radiation. Solar radiation heats the globe and wind blowing across it cools the globe.
- A Dry bulb calculates the air temperature measured in the shade. It is the temperature you would see on your thermometer outside.
As the EPA animation on the right illustrates, a relatively small rise in average temperature can result in a lot more hot and extremely hot weather.
The three images underneath, from the IPCC, show the effect on extreme temperatures when (a) the mean temperature increases, (b) the variance increases, and (c) when both the mean and variance increase for a normal distribution of temperature.The thermodynamic wet-bulb temperature is determined by temperature, humidity and pressure (hPa), and it is the lowest temperature that can be achieved by evaporative cooling of a water-wetted ventilated surface.
As temperatures and humidity levels keep rising, there comes a point where the wind factor no longer matters, in the sense that wind can no longer provide cooling.
Once the wet-bulb temperature reaches 35°C, one can no longer lose heat by perspiration, even in strong wind, but instead one will start gaining heat from the air beyond a wet-bulb temperature of 35°C.
The 'Misery Index' is the perceived air temperature as a combination of wind chill and heat index (which combines air temperature and relative humidity, in shaded areas).
The image below shows high readings on the Misery Index for parts of Pakistan, the 'feels like' temperature at nullschool.net. On June 22, 2023, an air temperature of 45.4°C (113.7°F) and a relative humidity of 25% resulted in 'feels like' temperatures as high as 51°C (123.7°F) at the area marked by the green circle.
The image below shows high readings on the Misery Index for parts of Pakistan, the 'feels like' temperature at nullschool.net. On June 22, 2023, an air temperature of 45.4°C (113.7°F) and a relative humidity of 25% resulted in 'feels like' temperatures as high as 51°C (123.7°F) at the area marked by the green circle.
The above image also shows the Jet Stream (wind at 250 hPa). Distortion of the Jet Stream can lead to circular wind patterns that amplify heatwaves. As temperature rise, the temperature difference between the Equator and the Arctic narrows, distorting the Jet Stream resulting in more extreme weather.
Meanwhile, heatwave conditions have also been affecting China, Texas and Mexico recently, with all-time high temperature records broken in each of these places.
Links
• Wet Bulb Globe Temperature
https://digital.mdl.nws.noaa.gov
• National Weather Service - Wet Bulb Globe Temperature: How and when to use it
https://www.weather.gov/news/211009-WBGT
• Nullschool.net
https://earth.nullschool.net
• Weather tracker: China issues heatstroke alert amid historic heatwave
https://www.theguardian.com/environment/2023/jun/23/weather-tracker-china-issues-heatstroke-alert-amid-historic-heatwave
• Peaks matter
https://arctic-news.blogspot.com/2018/08/peaks-matter.html
• It could be unbearably hot in many places within a few years time
https://arctic-news.blogspot.com/2016/07/it-could-be-unbearably-hot-in-many-places-within-a-few-years-time.html
• The emergence of heat and humidity too severe for human tolerance - by Colin Raymons et al.
https://www.science.org/doi/10.1126/sciadv.aaw1838
• Brief periods of dangerous humid heat arrive decades early
https://www.climate.gov/news-features/featured-images/brief-periods-dangerous-humid-heat-arrive-decades-early
• Extinction
https://arctic-news.blogspot.com/p/extinction.html
• Climate Plan
https://arctic-news.blogspot.com/p/climateplan.html
• Climate Emergency Declaration
https://arctic-news.blogspot.com/p/climate-emergency-declaration.html
• Extinction
https://arctic-news.blogspot.com/p/extinction.html
• Climate Plan
https://arctic-news.blogspot.com/p/climateplan.html
• Climate Emergency Declaration
https://arctic-news.blogspot.com/p/climate-emergency-declaration.html
