Discomfort Heat Index Intensity in Jerusalem 1940–2025

Abstract

This publication introduces Discomfort Heat Index Intensity (DCI) to measure cumulative yearly heat stress. It uses a baseline "Comfort Heat Index" (CHI) of 26.6°C (79.9°F), above which cooling is required. The Heat Index (HI) is the temperature equivalent of discomfort caused by air temperature and humidity. DCI is calculated as the positive difference between the Heat Index and CHI multiplied by the cumulative time during which the Heat Index exceeds the comfort threshold, and is expressed in °C•hr. Four annual parameters were analyzed for Jerusalem during 1940–2025: the annual Discomfort Heat Index (DC), annual Maximum Discomfort Heat Index (maxDC), annual Discomfort Heat Index Duration (DDC), and annual Discomfort Heat Index Intensity (DCI). A 31-year moving average of DC identified a change in the long-term trend around 1982, separating the record into Period1 (1940–1982) and Period2 (1983–2025). For each parameter, annual values were fitted by parabolic trendlines. The annual rate of change (velocity) was obtained from year-to-year changes and represented by a linear trendline, while the slope of this trendline was defined as the acceleration. During Period2, DC increased from 2.02°C to 2.97°C (+47%), maxDC increased from 8.2°C to 12.0°C (+47%), DDC increased from 884 to 1,230 hours (+39%), and DCI increased from 1,797 to 3,693 °C•hr (+106%). Exponential growth rates increased substantially after 1982, with DCI showing the largest increase, from +0.31%/year in Period1 to +1.73%/year in Period2. The analysis of velocity and acceleration further shows that the rates of change of the different parameters evolved differently over time, revealing changes in the dynamics of heat discomfort.