Heat index calculator

What the heat index actually measures

Your body has one serious way to shed heat: evaporating sweat off your skin. Every gram that evaporates carries away about 580 calories of heat, and that process is what keeps your core near 98.6 °F on a hot afternoon. It only works if the surrounding air can accept more water vapor.

That is where humidity comes in. When relative humidity climbs, the air is already close to holding all the moisture it can, so sweat stops evaporating and starts simply running off you. The cooling system stalls while your body keeps generating heat, and your core temperature begins to rise. The heat index puts a number on that failure: it answers the question "what dry-air temperature would stress my body as much as this humid air does?"

The practical consequence surprises people. At 90 °F with 30% humidity the heat index is about 88 — slightly cooler than the air, because dry air lets sweat work efficiently. Hold the temperature at 90 °F and raise humidity to 80% and the heat index jumps to roughly 113, deep into the range where heat stroke becomes a real possibility. Nothing about the temperature changed. Only your ability to cool yourself did.

The conditions the number assumes

The heat index was derived for a specific set of circumstances, and knowing them tells you when to distrust the figure. It assumes an adult of average build, walking at about 3.1 mph, in the shade, with a light breeze. Sunshine is explicitly excluded.

Full sun can add as much as 15 °F to the effective heat index. If you are working on an unshaded roof, standing on asphalt, or sitting in a car, the number this calculator gives you is optimistic — take the result, add roughly 10 to 15 degrees, and read the risk category for that instead.

Wind cuts the other way, but not always in your favor. A breeze helps while the air is cooler than your skin. Above about 95 °F, moving air begins to deliver heat to you faster than it removes it, which is why a hot desert wind feels like opening an oven rather than like relief.

Who is at risk before the numbers say so

The risk categories describe a healthy adult, and plenty of people are not the reference case. Adults over 65 sweat less and feel thirst less reliably. Young children have a higher surface-area-to-mass ratio and heat up faster. Anyone taking diuretics, beta blockers, antihistamines or certain psychiatric medications may have a blunted ability to sweat or regulate blood flow to the skin.

Acclimatization matters as much as any of that. It takes one to two weeks of regular exposure for the body to adapt — sweating earlier, sweating more, and losing less salt in the process. This is why the first hot week of the year causes more heat illness than hotter weeks in August, and why a new worker on an outdoor crew is at far higher risk than colleagues who have been out all summer. OSHA data consistently shows most heat deaths on the job happen in the first few days.

Know the progression. Heat cramps come first. Heat exhaustion follows: heavy sweating, cold and clammy skin, a fast weak pulse, nausea and dizziness — get to a cool place, loosen clothing, sip water. Heat stroke is the emergency, and the giveaway is often that sweating has stopped and the skin has gone hot and dry, with confusion or unconsciousness and a body temperature above 103 °F. Call 911 and start cooling the person immediately with cold water or ice; do not wait and do not give fluids to someone who is not fully alert.

The formula

Rothfusz regression, used by the National Weather Service
T = air temperature (°F), R = relative humidity (%)

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

Adjustment for very dry air (R < 13% and 80°F ≤ T ≤ 112°F):
  HI -= ((13 - R) / 4) · √((17 - |T - 95|) / 17)

Adjustment for very humid air (R > 85% and 80°F ≤ T ≤ 87°F):
  HI += ((R - 85) / 10) · ((87 - T) / 5)

Below about 80°F the NWS uses a simple average instead:
  HI = 0.5 · (T + 61 + ((T - 68) · 1.2) + (R · 0.094))

The regression is a curve fit to a much more complex model of human heat balance published by Robert Steadman in 1979, which itself accounted for body dimensions, clothing cover, activity level and blood flow. Rothfusz reduced it to something a forecaster could compute quickly, at the cost of about ±1.3 °F of accuracy. The two adjustments patch the corners where the plain fit drifts furthest.

Practical tips

• Drink before you are thirsty. Thirst lags dehydration, and by the time you feel it you have typically already lost 1 to 2% of your body weight in water.
• Plan heavy outdoor work for before 10 a.m. or after 6 p.m. Peak heat index usually lands between 3 and 5 p.m., a few hours after peak sun.
• Watch the dew point rather than the humidity percentage if you want one number. Above 65 °F it feels muggy; above 70 °F it feels oppressive; above 75 °F is genuinely dangerous for exertion.
• Never leave a child or pet in a parked car. The interior can gain 20 °F in ten minutes, and cracking a window barely changes that.
• A cool shower or a wet towel on the neck and wrists cools you faster than a fan alone once the air is above body temperature.

Frequently asked questions