Reference chart
Heat Index Chart
By &Pixels, the studio that builds MyCalculator.to. Figures are the NOAA heat index, which assumes shade and light wind.
Temperature down the side, humidity across the top, and what each resulting band means for being outside in it.
What is the heat index?
The heat index is what a temperature feels like once humidity is taken into account. Your body cools itself by evaporating sweat, humid air slows evaporation down, and the index expresses the result as the dry-air temperature that would feel equivalent.
It is not a thermometer reading and it moves much further than people expect. At 90F, the gap between 40 percent humidity and 90 percent is thirty-one degrees of apparent heat: 91F against 122F, from the same number on the thermometer.
Heat index by temperature and humidity
| Temp | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|
| 80F | 80 | 81 | 82 | 85 | 86 | 88 |
| 84F | 83 | 85 | 88 | 90 | 94 | 98 |
| 88F | 88 | 91 | 95 | 100 | 106 | 113 |
| 90F | 91 | 95 | 100 | 105 | 113 | 122 |
| 92F | 94 | 99 | 105 | 112 | 121 | 131 |
| 94F | 97 | 103 | 111 | 120 | 131 | · |
| 96F | 101 | 108 | 116 | 128 | · | · |
| 98F | 105 | 113 | 123 | 136 | · | · |
| 100F | 109 | 118 | 129 | · | · | · |
| 104F | 119 | 131 | · | · | · | |
| 108F | 130 | · | · | · |
The marked cells are not gaps in the data. Air can only hold so much water vapour at a given temperature, so 100F at 80 percent relative humidity is not a condition that occurs at the surface.
The calculators behind this
What each band means
| Heat index | Band | What it means |
|---|---|---|
| 80 to 90F | Caution | Fatigue possible with prolonged exposure or activity. |
| 90 to 103F | Extreme caution | Heat cramps and heat exhaustion possible. Take breaks in shade. |
| 103 to 124F | Danger | Heat cramps and exhaustion likely, heat stroke possible. Limit exertion. |
| 125F and above | Extreme danger | Heat stroke highly likely. Strenuous activity outdoors is not safe. |
How to read it
- Read the air temperature. The figure on a thermometer in the shade, not in the sun.
- Find the relative humidity. Any weather app gives it. It is the other half of the number and the half people skip.
- Cross-reference on the table. Row for temperature, column for humidity. The cell is the apparent temperature.
- Add up to 15 degrees if you are in full sun. The table assumes shade. Direct sunlight pushes the apparent figure considerably higher.
- Use the band, not the number. What matters is which risk band you are in, because that is what changes the decision.
For one figure rather than the table, the heat index calculator takes a temperature and humidity and names the band. The dew point calculator is the other way to read the same air, and many people find dew point the more intuitive of the two once they are used to it: above about 65F it starts to feel muggy regardless of what the thermometer says.
These bands assume a healthy adult in shade. Age, medication, illness and how acclimatised you are all move the line, and none of that is in a table.
Formula & Methodology
Did you know? The heat index comes from a 1979 model of human thermoregulation by Robert Steadman, which assumed a 5 foot 7 person weighing 147 pounds, walking at about 3 mph, in the shade, wearing long trousers and a short-sleeved shirt. Every figure in the table inherits those assumptions.
Sources
- National Weather Service, NOAA: Heat Index chart and heat safety guidance
- Steadman 1979, Journal of Applied Meteorology: The Assessment of Sultriness
- Centers for Disease Control and Prevention, heat stress guidance for outdoor workers