Discomfort Heat Index Intensity in Miami, FL (1940–2025)

This work focuses on the practical impacts of climate change by analyzing changes in the Heat Index (HI) that affect thermal comfort. The term “Comfort Heat Index” (CHI = 26.6°C (79.9°F)) is defined as a Heat Index that is sufficiently comfortable and does not require cooling. A Heat Index above CHI is considered as a “discomfort” Heat Index (DC). The calculations are for various levels of “discomfort”, from 0°C to 8°C, each 2°C, which is 0°C to 8°C above the CHI. The publication introduces the “Summer Discomfort Heat Index Intensity” (DCI) which quantifies the cumulative annual Discomfort Heat Index during the summer. It is calculated as the difference between the actual mean Heat Index and the Comfort Heat Index (CHI = 26.6°C) multiplied by the cumulative time the Heat Index remains above the threshold. The analyzed parameters are: Summer Discomfort Heat Index - DC Duration of Summer Discomfort Heat Index - DDC Summer Discomfort Heat Index Intensity - DCI The annual mean Discomfort Heat Index has increased significantly between 1940 and 2025. The average Heat Index increased from 30.1°C to 30.9°C (86.1°F to 87.7°F), from 3.5°C to 4.3°C above CHI, and its duration increased from 2,893 to 3,621 hours per year. The Discomfort Heat Index Intensity has increased by 60% in the last 86 years, from 9,883 to 15,763 degree-hours/year. The duration of the Discomfort Heat Index above the CHI on non-summer days increased much more than on summer days, from 87 hours in 1940 to 227 hours in 2025, an increase of 161%. The Summer Discomfort Heat Index increased between 1940 and 2025 from 30.1°C to 31.1°C and its duration increased from 2,806 to 3,394 hours per summer, reaching 64% of all summer hours. The increase in the Summer Discomfort Heat Index duration analyzed with the parabolic trendline for the highest threshold (equivalent to a Heat Index of 34.6°C) was 235%, from 155 hr/summer to 520 hr/summer, while the actual increase was from 71 hr/summer in 1940 to 545 hr/summer in 2025, 668%. The increase in the Summer Discomfort Heat Index Intensity analyzed with the parabolic trendline for the highest threshold (8°C, equivalent to a Heat Index of 34.6°C) was 243%, from 1,430 to 4,900 degree-hours/summer, while the actual increase was from 614 degree-hours/summer in 1940 to 5,161 in 2025, +741%.