What is heat index?
Heat index is the "feels like" temperature that describes how hot it feels when humidity is factored in. High humidity reduces the evaporation rate of sweat — your body's primary cooling mechanism — making you feel significantly hotter than the actual air temperature.
For example, an air temperature of 90°F with 70% relative humidity produces a heat index of approximately 105°F, meaning it feels 15 degrees hotter than the thermometer reads.
How is heat index calculated?
The heat index formula used by the National Weather Service (NWS) is the Rothfusz regression equation:
HI = -42.379 + 2.0490T + 10.1433R - 0.2248TR - 0.00684T² - 0.0548R² + 0.00123T²R + 0.000853TR² - 0.00000199T²R²
Where:
- T = air temperature in degrees Fahrenheit (°F)
- R = relative humidity (%)
The formula is valid for:
- Air temperatures at or above 80°F (27°C)
- Relative humidity of 40% or more
Adjustments are applied when humidity is very low (below 13%) or very high (above 85%).
Step by step: how to calculate heat index
- Measure the air temperature in °F (or convert from °C)
- Measure the relative humidity as a percentage (0-100%)
- Apply the Rothfusz regression formula with T and R values
- Apply adjustments if humidity is below 13% or above 85%
- Round the result to the nearest whole number
- Look up the danger level in the heat effects chart
Example calculation
Air temperature: 95°F, Relative humidity: 60%
Using the simplified approximation:
Heat Index ≈ -42.379 + 2.0490 × 95 + 10.1433 × 60 - 0.2248 × 95 × 60 - ...
Heat Index ≈ 111°F (Extreme Caution)
Heat index chart
| Temp (°F) | 40% | 45% | 50% | 55% | 60% | 65% | 70% | 75% | 80% | 85% | 90% | 95% |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 80 | 80 | 80 | 81 | 81 | 82 | 82 | 83 | 84 | 84 | 85 | 86 | 86 |
| 85 | 85 | 86 | 87 | 88 | 88 | 89 | 90 | 91 | 93 | 95 | 97 | 99 |
| 90 | 91 | 93 | 95 | 97 | 99 | 102 | 105 | 108 | 112 | 116 | 121 | 127 |
| 95 | 98 | 101 | 104 | 107 | 110 | 114 | 118 | 123 | 128 | 134 | 141 | 148 |
| 100 | 105 | 109 | 113 | 117 | 122 | 127 | 132 | 138 | 144 | 151 | — | — |
| 105 | 113 | 118 | 123 | 128 | 133 | 139 | 145 | 152 | — | — | — | — |
| 110 | 122 | 127 | 133 | 138 | 144 | 151 | — | — | — | — | — | — |
Values above 130°F indicate Extreme Danger. Dashes (—) indicate off-the-chart values.
Heat danger levels
| Heat Index | Danger Level | Health Effects |
|---|---|---|
| 80-90°F | Caution | Fatigue possible with prolonged exposure and activity |
| 90-103°F | Extreme Caution | Heat cramps and heat exhaustion possible |
| 103-124°F | Danger | Heat cramps and exhaustion likely; heat stroke probable |
| Above 125°F | Extreme Danger | Heat stroke imminent — seek medical attention |
Sun exposure adjustment
The standard heat index assumes shady, light-wind conditions. Exposure to direct sunshine can increase heat index values by up to 14°F (8°C). If you are planning outdoor activities in full sun, add approximately 14°F to the calculated heat index for a more accurate picture.
How humidity affects your body
Your body cools itself through perspiration — heat is removed from the body as sweat evaporates from your skin. When humidity is high:
- The air already contains a lot of moisture
- Sweat evaporates more slowly (or barely at all)
- Your body's natural cooling system becomes less effective
- Body temperature rises, increasing the risk of heat-related illness
This is why a 90°F day in dry Arizona feels much more tolerable than a 90°F day in humid Florida.
Heat-related illness warning signs
Heat exhaustion:
- Heavy sweating
- Cold, pale, clammy skin
- Fast, weak pulse
- Nausea or vomiting
- Muscle cramps
- Tiredness, weakness, dizziness
Heat stroke (emergency — call 911):
- High body temperature (103°F or higher)
- Hot, red, dry or damp skin
- Fast, strong pulse
- Confusion or loss of consciousness
- Headache and dizziness
Heat index vs wind chill
Heat index and wind chill are complementary concepts. Heat index measures how hot it feels in summer when humidity slows heat dissipation, while wind chill measures how cold it feels in winter when wind accelerates heat loss. Both provide a "feels like" temperature more useful than the actual air temperature for assessing comfort and safety.
FAQ
How does the heat index affect outdoor exercise?
High heat index values significantly increase the risk of heat-related illness during exercise. When the heat index exceeds 90°F, reduce exercise intensity, take frequent breaks, and drink water every 15-20 minutes. Above 105°F, outdoor exercise should be avoided entirely.
Is heat index the same worldwide?
Different countries may use slightly different formulas. The US NWS uses the Rothfusz regression equation. The UK Met Office uses a "feels like" temperature that also accounts for wind speed. Results may vary slightly between methods, but the underlying principle is the same.
Can heat index be lower than the actual temperature?
At very low humidity levels, the heat index can be slightly lower than the actual temperature because dry air allows efficient sweat evaporation. However, the NWS heat index is only defined for temperatures above 80°F with humidity above 40%, so this is rarely seen in the standard calculation.
Why is heat index only valid above 80°F?
Below 80°F, the body's heat dissipation through convection and radiation is sufficient to maintain normal temperature regardless of humidity. The heat index formula was specifically designed for hot conditions where humidity impairs the body's evaporative cooling system.