Cattle Heat Stress (THI)
We're not experts in this field, but we have done some homework as we've had a few people ask if our meters will measure for this... THI is mostly used for cattle heat stress. The Heat Index that Kestrel provides is used for human exposure.
It is based on the National Weather Service table (and subsequent equation) found here:
It appears she is using the THI (Temperature Humidity Index) to predict cattle heat stress according to this equation:
THI = Tdb – [0.55 – (0.55 x RH/100)] x (Tdb – 58)
This formula uses dry bulb temperature (Tdb, ºF) and the relative humidity (RH). The RH is divided by 100 to express the percentage in decimals.
You can see a comparison between how the two charts (Heat Index vs Temperature Humidity Index) here:
Essentially THI may one day be added to Kestrel, but in the mean time, you can get all the data you need to calculate THI in Excel or could possibly use the comparison chart to quickly use the HI value to predict the THI value.