Recently, a novel environmental stress index (ESI) which is composed from commonly used meteorological variables: ambient temperature (T_a), relative humidity (RH), and solar radiation (SR) was suggested as follows: ESI=0.63T_a-0.03RH+0.002SR+0.0054(T_a·RH)-0.073(0.1+SR)^-1; (°C) The purpose of the present study was to evaluate and validate the ESI for hot dry and hot wet climatic conditions. The ESI was applied to large meteorological databases from 2 different locations resembling hot/wet and hot/dry climates. Data analysis revealed high correlation between ESI and the wet bulb globe temperature (WBGT) index for each of the two databases: P<0.05, R²=0.985 and 0.982, for the hot/dry and hot/wet conditions, respectively. Therefore, it is concluded that ESI, which is constructed from fast response and commonly used weather variables (T_a, RH, SR), and also found in a microsensor format is validated for hot/dry and hot/wet zones and as a potential index to serve as an alternative to the WBGT for heat category assessment.