Table 1.

Heat metrics or thermal indices often used within climate model projections in recent studies.

Metric	Description, application, and notes on use
Psychometric wet bulb temperature (Tpwb)	•Temperature of a parcel of air that is cooled to saturation by the evaporation of water into the air, with the latent heat for evaporation supplied by the parcel20 •Also called the thermodynamic wet bulb temperature •When Tpwb approaches skin temperature, all heat loss avenues are eliminated, thus net heat dissipation is zero •Assumes the human body becomes an adiabatic system
Wet bulb globe temperature (WBGT)	•Indicator of heat stress on the active/working body in direct sunlight •WBGT = 0.7 Tnwb + 0.2 Tg + 0.1 Tdb, where Tnwb is non-aspirated, “natural” wet bulb, Tg is black globe temperature, Tdb is shaded dry bulb (air) temperature •Intended for use in active populations outdoors; developed for military from studies in hot, humid environments •Studies, particularly climate projections, often neglect the Tg value, which is not its intended use
sWBGT (simplified WBGT)	•sWBGT = 0.567Tdb + 0.393e + 3.94; e = vapor pressure •Approximation does not account for variations in the intensity of radiation or wind speed, yet assumes a moderately high radiation level in light wind conditions21 •May lead to overestimates of thermal stress in windy and cloudy conditions or underestimates of thermal stress in dry, sunny, hot conditions when required sweat rates are high due to activity levels
Apparent temperature (AT)	•An adjustment to the ambient temperature based on the level of humidity for a typical human, which sometimes incorporates solar radiation •Derived from human heat balance principles •vVarious formulas exist to approximate AT, many of which ignore radiation
Heat index	•A simple hot weather version of the AT to describe a ‘feels like’ temperature •Uses multiple regression of temperature and relative humidity based on original AT (above) •Over 21 approximations exist