Heat Index Chart
The full National Weather Service heat index chart, computed live with the same Rothfusz regression the NWS prints: find your air temperature down the left side and the relative humidity across the top, and the cell where they meet is how hot it actually feels. Humidity is the multiplier that matters — at 96 °F the feels-like temperature climbs from about 101 °F at 40% humidity to roughly 126 °F at 70%, because sweat stops evaporating efficiently and the body loses its main cooling channel.
Heat Index Chart reference last updated · reference source
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Dated static reference; no live data is fetched. Verify current source values and assumptions before relying on results.
What this tool does
The full National Weather Service heat index chart, computed live with the same Rothfusz regression the NWS prints: find your air temperature down the left side and the relative humidity across the top, and the cell where they meet is how hot it actually feels. Humidity is the multiplier that matters — at 96 °F the feels-like temperature climbs from about 101 °F at 40% humidity to roughly 126 °F at 70%, because sweat stops evaporating efficiently and the body loses its main cooling channel.
How to use the Heat Index Chart
- Check Air temperature (°F): 90.
- Check Relative humidity (%): 70.
- Review the Heat Index Chart result and its exact-value comparison table.
- Verify assumptions before applying the result.
Formula
NWS Rothfusz regression: HI = −42.379 + 2.04901523·T + 10.14333127·R − 0.22475541·T·R − 0.00683783·T² − 0.05481717·R² + 0.00122874·T²·R + 0.00085282·T·R² − 0.00000199·T²·R² (T in °F, R in %), with the NWS low- and high-humidity adjustments; below 80 °F the simpler Steadman formula is used.
- temp
- Air temperature (°F)
- rh
- Relative humidity (%)
The Rothfusz regression is the US National Weather Service standard, fitted for shade with light wind; full sun can add up to 15 °F. It is defined for temperatures of 80 °F and above — below that a simple Steadman average is shown.
Worked example
Heat Index Chart: Air temperature (°F) = 90; Relative humidity (%) = 70. The full National Weather Service heat index chart, computed live with the same Rothfusz regression the NWS prints: find your air temperature down the left side and the relative humidity across the top, and the cell where they meet is how hot it actually feels. Humidity is the multiplier that matters — at 96 °F the feels-like temperature climbs from about 101 °F at 40% humidity to roughly 126 °F at 70%, because sweat stops evaporating efficiently and the body loses its main cooling channel.
Inputs
- Air temperature (°F)90
- Relative humidity (%)70
Result
- Heat index at your values (°F)105.9
- NWS risk categoryDanger — heat cramps and exhaustion likely, heat stroke possible
- Highest value on the chart (110 °F at 100% RH)278
- Chart grid16 temperatures × 7 humidity levels
NWS heat index risk bands
| Heat index (°F) | Category | Effect on the body |
|---|---|---|
| 80–90 | Caution | Fatigue possible with prolonged exposure and physical activity |
| 90–103 | Extreme caution | Heat cramps and heat exhaustion possible |
| 103–124 | Danger | Heat cramps and exhaustion likely; heat stroke possible |
| 125+ | Extreme danger | Heat stroke highly likely |
Results explained
- Heat index at your values (°F)
- The Rothfusz-regression heat index for the entered temperature and humidity — the single cell of the chart for your conditions.
- Highest value on the chart (110 °F at 100% RH)
- The extreme corner of the NWS grid, included to anchor how far the scale runs.
- Chart grid (table)
- The full NWS heat index chart: rows are air temperature, columns are relative humidity, cells are feels-like temperature in °F.
Frequently asked questions
Pick the row for the current (or forecast) air temperature and move across to the column for the current relative humidity. The number in that cell is the heat index. This page's table is the complete NWS grid from 80 to 110 °F and 40% to 100% humidity, so it works for any combination in that range without interpolating a printed chart.
Your body cools by evaporating sweat. At 40% humidity sweat evaporates freely, so 96 °F feels like roughly 101 °F; at 70% the air is already holding so much water vapour that evaporation stalls, sweat drips instead of cooling, and the same 96 °F feels like about 126 °F — a roughly 25-degree difference from humidity alone.
The grid is generated with the Rothfusz regression, the formula behind the National Weather Service's published chart (fitted to R. G. Steadman's apparent-temperature work in 1979), including the NWS adjustments for very low and very high humidity. The risk bands in the reference table are the NWS's own categories.
Sun and wind. The NWS values assume shade with light wind; full sun can add up to 15 °F to the heat index, and strong wind can lower it slightly. Clothing, acclimatisation, age and exertion level also shift personal risk at any given cell of the chart.
There is no single legal cutoff in the chart itself, but the bands guide most policies: extra water and rest breaks from 90 °F (extreme caution), major precautions and rescheduling in the 103–124 °F danger band, and suspension of strenuous outdoor activity as values approach 125 °F. OSHA heat guidance uses these same bands.
Below 80 °F the heat index barely differs from the air temperature, so the NWS chart does not print it (the calculator uses Steadman's simpler formula there). Below 40% humidity the index stays close to the air temperature even at 100 °F — dry heat is still dangerous, but through dehydration rather than failed evaporation, which is why desert warnings emphasise water over the index.