HeatGuard field guide
Heat index chart: ranges, meaning and examples
A heat index chart combines air temperature and relative humidity to estimate how hot conditions feel to the human body. It is a screening tool for planning—not a guarantee that a particular person is safe.
01
How to read the chart
Find air temperature on one axis and relative humidity on the other. Their intersection is the estimated heat index. The result rises quickly when warm air and high moisture limit evaporation.
The National Weather Service uses the Rothfusz regression in hot conditions and a simpler approximation at lower values. Calculators can therefore show a feels-like value close to—or occasionally below—the air temperature in milder weather.
02
Understand the ranges
HeatGuard groups heat index below 80°F as low, 80–89°F as moderate, 90–102°F as high, 103–124°F as very high and 125°F or higher as extreme. These are planning categories, not diagnoses.
Illness can occur below a boundary during hard or prolonged work. Direct sunlight, heavy clothing, medications, limited acclimatization, age and health conditions can increase risk.
- Use thresholds as prompts to change the plan.
- Check the hottest hour and coolest workable window.
- Respond to symptoms regardless of category.
03
Why sun changes the decision
Standard heat index describes shaded, light-wind conditions. Full sunshine and hot pavement, roofs or turf can add local exposure that a distant station does not capture.
For outdoor activity, pair heat index with UV and—when a protocol requires it—an on-site WBGT reading. Shade reduces radiant load but does not remove humidity.
Primary sources
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This guide provides environmental planning information. It is not medical advice, an emergency service, or a substitute for an applicable workplace, school, league or governing-body protocol.
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