Efficient and Accurate Shortcuts for Calculating the Extended Heat Index

Abstract 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 U.S. 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 HI (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 multimodel 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. Significance Statement Over 40 years ago, an index was developed to estimate the heat stress on the human body due to the combined effects of temperature and humidity. It has been used widely both to issue public heat advisories and in studies aimed at assessing possible future changes in heat stress due to climate change. Recently, that index has been updated and extended to handle higher temperatures that are becoming more common due to climate change. This work provides efficient and accurate shortcuts to compute the new index which require much less computing time and can be beneficial for more demanding tasks such as estimating the range of possibilities in the future.