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ACP Humid Heat Stroke Index Peak Wet-Bulb Temperature Characteristics of the hottest part of the day
I 74°F to 80°F Uncomfortable. Typical of much of summer in the Southeast.
II 80°F to 86°F Dangerous. Typical of the most humid parts of Texas and Louisiana in the hottest summer month, and the most humid summer days in Washington and Chicago.
III 86°F to 96°F Extremely dangerous. Comparable to the Midwest during peak days of the 1995 heat wave.
IV > 92°F Extraordinarily dangerous. Exceeds all US historical records. Heat stroke likely for fit individuals undertaking less than one hour of moderate activity in the shade.

Recent Wet Bulb Temperatures

Calculations for wet bulb temperatures from NOAA.