Extending the heat index



Romps, David — Lawrence Berkeley National Laboratory

Area of research:

Atmospheric Thermodynamics and Vertical Structures

Journal Reference:

Lu Y and D Romps. 2022. "Extending the Heat Index." Journal of Applied Meteorology and Climatology, , 10.1175/JAMC-D-22-0021.1.


The Heat Index, introduced in 1979, is undefined for temperature-humidity combinations arriving soon with global warming. We fix that.

The existing Heat Index is well-defined for most combinations of high temperature and humidity experienced on Earth in the preindustrial climate, but global warming is increasingly generating conditions for which the Heat Index is undefined. Therefore, an extension of the original Heat Index is needed. Here, we extend the Heat Index using the same physiological model as in the original work of Steadman to ensure backwards compatibility.


Following Steadman, each value of the Heat Index is mapped onto a measurable physiological variable, which can be useful for assessing the health impacts of various combinations of temperature and humidity, especially for outdoor workers.


The correspondence between the Heat Index and the physiological state allows us to assess future habitability under different warming scenarios. Even for a very fit individual (approaching the optimal human assumed in the thermoregulation model), the rate at which a human can expel heat is still constrained by physical laws that govern, for example, heat conduction and the evaporation of water. In a sufficiently hot and humid environment, the human body runs out of tricks for regulating its core temperature, leading to sickness or death.