Heat Index Chart Explained — How to Read the Danger Levels

How the Chart Works

The NOAA heat index chart has temperature on the vertical axis and relative humidity on the horizontal axis. Find your current temperature on the left, your humidity percentage across the top, and the intersection is the heat index. The cells are color-coded from yellow to dark purple based on risk level.

Chart Zones and Actions

Surprising Combinations

Some combinations are deceptively dangerous. 90°F with 60% humidity gives a heat index of 100°F. 85°F with 80% humidity gives a heat index of 97°F. A seemingly mild 85°F day can be dangerously hot if humidity is extreme. This is why the heat index is important — you cannot judge risk by temperature alone.

Quick Lookup Table (NOAA Values, °F)

This condensed version of the NOAA chart covers the most common conditions. Find your air temperature row, then read across to your relative humidity column.

Notice how steeply the values climb: at 90°F, going from 40% to 80% humidity adds 22 degrees of felt heat. At 95°F and 70% humidity you are already in the danger zone at 124°F — conditions that occur regularly in the Gulf states every summer. “Off chart” combinations fall into extreme danger where heat stroke is imminent.

The Formula Behind the Chart

The chart is generated by the Rothfusz regression, a 9-term equation NOAA fitted in 1990 to Robert Steadman’s 1979 “sultriness” model: HI = -42.379 + 2.049T + 10.143R - 0.2248TR - 0.00684T² - 0.0548R² + 0.00123T²R + 0.000853TR² - 0.00000199T²R² (T in °F, R in percent). Steadman’s model assumes a 5′7″, 147 lb adult walking in the shade with a light breeze — the chart describes that reference person, not necessarily you.

The Chart Assumes Shade — Add Up to 15°F in Direct Sun

The single most misunderstood fact about the heat index: every value on the chart assumes you are in the shade. The National Weather Service notes that exposure to direct sunlight can increase the heat index by up to 15°F. A 95°F reading at 50% humidity is 105°F in the shade — but close to 120°F working in an open field. This is why outdoor-worker guidelines (like OSHA’s heat rules) use WBGT, which directly measures solar load, for job-site decisions.

When the Chart Doesn’t Apply

• Below 80°F air temperature: the heat index is undefined; humidity barely changes felt temperature.
• Below 40% relative humidity: the regression loses accuracy; in very dry heat the heat index can actually read below the air temperature.
• Strong wind, which improves evaporative cooling at moderate humidity but adds oven-like convective heat above ~99°F air temperature.
• Individual factors: age, medications (diuretics, beta-blockers, antihistamines), body size, and acclimatization shift personal risk well away from the reference person’s.

Why Humidity Hits So Hard

Your body sheds roughly 70% of its excess heat through sweat evaporation. Evaporation depends on the vapor-pressure gap between your skin and the air — and high humidity closes that gap. At 90°F and 85%+ humidity, sweat drips instead of evaporating, removing almost no heat. Your core temperature then rises regardless of how much you drink, which is why high-humidity heat waves (like the 1995 Chicago event, with heat indices above 119°F) are far deadlier than hotter but drier days.