Efficient and Accurate Shortcuts for Calculating the Extended Heat Index Journal of Applied Meteorology and Climatology

The heat index (HI), based on Steadman's model of thermoregulation, has found wide applicability for estimating human heat stress. It has proven useful in research endeavors aimed at estimating future changes associated with global climate change as well as operationally for the issuance of heat advisories by the US National Weather Service (NWS). The actual computation of the HI based on ambient temperature and humidity has been streamlined by the use of a polynomial fit by Rothfusz to Steadman's model output. Recently Steadman's model has been updated by Lu and Romps to provide applicability to temperature and humidity values that were "out-of-range" of Steadman's model. The authors of this Extended Heat Index (EHI) have provided computer code to enable application. However, because of the complexity of the model, execution of the code would be relatively costly for use in applications involving dense grids and multi-model ensembles covering decades such as those appropriate for assessing the range of possible changes in human heat stress. Here shortcuts are provided in the form of a "lookup table" and polynomials that provide computational savings of several orders of magnitude. Error analyses provide estimates of accuracy.