I'm a big believer in knowledge, having been acquired, is worthless without sharing it. So thanks to @rycuda I looked up the Stoll Equation, which is an empirical formula (https://journals.ametsoc.org/view/journals/apme/50/11/jamc-d-11-0143.1.xml Roland Stull 2011) that can be used to estimate the wet bulb temperature from dry bulb temperature and humidity without faffing around with putting a wet sock on a conventional thermometer.
Why is that important? Well it's an estimate of what temperature you can achieve by evaporative cooling alone, and next week is going to be in the 30s across all of Europe, with France promising to be hitting 40 °C in large areas. If you can't cool yourself then you're going to have a bad time, and the wet bulb temperature this seems to happen at for humans is about 31 °C.
What can you do about this? If you have Home Assistant and a sensor that's feeding in temperature and humidity, you can use this to make a template helper:
{{ ((states('sensor.temperature')|float) * atan(0.151977 * ((states('sensor.humidity')|float) + 8.313659) ** 0.5) + atan((states('sensor.temperature')|float) + states('sensor.humidity')|float) - atan((states('sensor.humidity')|float) - 1.676331) + (0.00391838 * ((states('sensor.humidity')|float) ** 1.5) * atan(0.023101 * (states('sensor.humidity')|float) )) - 4.686035)| round(1) }}
Just remember to replace sensor.temperature and sensor.humidity with the appropriately named sensors, and you should be good to go. I've added rounding to 1 decimal place, largely for convenience, because otherwise it produces enormously long floating point outputs that cannot possibly retain any sort of accuracy.
If you come up with any enhancements of your own, please put them here in the replies. (Edit: I just realised this only really applies to Celsius so if you come up with something that works for Fahranheit feel free to add it back)
Wet-Bulb Temperature from Relative Humidity and Air Temperature
Abstract An equation is presented for wet-bulb temperature as a function of air temperature and relative humidity at standard sea level pressure. It was found as an empirical fit using gene-expression programming. This equation is valid for relative humidities between 5% and 99% and for air temperatures between −20° and 50°C, except for situations having both low humidity and cold temperature. Over the valid range, errors in wet-bulb temperature range from −1° to +0.65°C, with mean absolute error of less than 0.3°C.









