Heat Index Calculator
Feels-Like Temperature & Heat Safety Levels
Calculate the heat index (feels-like temperature) from air temperature and relative humidity using the official NWS Rothfusz regression equation. Shows heat safety category — Caution through Extreme Danger — with practical cooling recommendations and heat illness warning signs.
What Is the Heat Index?
The heat index — also called "feels like" temperature — combines air temperature and relative humidity to determine how hot the air feels to the human body. High humidity reduces the efficiency of sweating, the body's primary cooling mechanism, making the same air temperature feel significantly hotter.
Heat-Related Illness Progression
| Condition | Symptoms | Action |
|---|---|---|
| Heat Cramps | Painful muscle spasms, heavy sweating | Rest, cool fluids, move to shade |
| Heat Exhaustion | Heavy sweating, weakness, cold/pale skin, fast pulse | Cool environment, lay down, hydrate |
| Heat Stroke | High body temp 103°F+, no sweating, confusion | Call 911 immediately — life-threatening |
Staying Safe in Extreme Heat
Drink water before you feel thirsty — by the time you feel thirst, you may already be mildly dehydrated. Avoid strenuous outdoor activity between 10 AM and 4 PM. Check on elderly neighbors and never leave children or pets in parked vehicles. Heat stroke is a medical emergency requiring immediate cooling and 911 activation.
What Is the Heat Index Calculator?
The heat index estimates how hot it actually feels when relative humidity is combined with air temperature. Humid air slows the evaporation of sweat, which is the body's main cooling mechanism, so a humid 90°F day stresses the body far more than a dry one. This calculator uses the National Weather Service's Rothfusz regression, the same equation behind official US heat advisories, and maps the result to the NWS heat-safety categories from Caution through Extreme Danger.
Heat Index Calculator Example
At 94°F with 70% relative humidity, the Rothfusz regression gives a heat index of about 122°F, well into the NWS Danger zone where heat cramps and heat exhaustion are likely and heat stroke is possible with continued exposure.
The same 94°F with only 20% humidity produces a heat index of roughly 91°F, barely above the actual temperature. That 31-degree swing, from the same thermometer reading, is entirely due to how much evaporative cooling the air allows.
Understanding Heat Index
How the Rothfusz Regression Works
The NWS heat index is a multiple regression fitted by Lans Rothfusz in 1990 to Robert Steadman's 1979 model of human heat balance, which simulated a typical adult walking in the shade. The equation combines temperature and relative humidity through nine terms, with adjustment corrections applied at low humidity and at very high temperatures. Because it models human physiology rather than pure physics, it assumes average height, weight, clothing, and activity.
Who Is Most at Risk
Heat illness risk concentrates in older adults, infants, outdoor workers, athletes, and people on medications that impair sweating or fluid balance. Urban areas add a heat-island effect that keeps nights hot, preventing overnight recovery, and consecutive hot nights are strongly associated with heat-wave mortality.
Practical Heat Safety
Hydrate before you feel thirsty, schedule strenuous work for early morning, take shaded breaks at least hourly in Danger conditions, and never leave children or pets in vehicles, where interior temperatures can climb 20°F in ten minutes. Recognize the progression: heavy sweating and cramps, then pale clammy skin and nausea (heat exhaustion), then hot dry skin, confusion, or collapse (heat stroke), which requires emergency cooling immediately.
Frequently Asked Questions
What are the NWS heat index categories?
Caution runs from 80 to 90°F (fatigue possible with prolonged exposure), Extreme Caution from 90 to 103°F (heat cramps and heat exhaustion possible), Danger from 103 to 124°F (heat exhaustion likely, heat stroke possible), and Extreme Danger at 125°F or above (heat stroke highly likely). Advisories are typically issued when the index reaches local Danger thresholds.
Why does humidity make heat feel so much worse?
Your body sheds most excess heat by evaporating sweat, and evaporation slows as the air fills with moisture. At high humidity sweat drips off instead of evaporating, removing little heat. The heat index quantifies this loss of cooling capacity as an equivalent feels-like temperature.
Is the heat index measured in sun or shade?
In shade, with a light breeze assumed. The NWS notes that full sunshine can raise the effective heat index by 10 to 15°F, so a shaded reading of 100°F can behave like 110°F or more on an exposed field, road crew site, or stadium.
What is the difference between heat index and wet-bulb globe temperature (WBGT)?
Heat index uses only temperature and humidity, assuming shade. WBGT additionally accounts for direct solar radiation and wind, and is preferred for athletics and occupational safety. Heat index remains the standard for general public forecasts because it needs only standard weather-station data.
At what heat index should outdoor exercise stop?
Many athletic and occupational guidelines recommend rescheduling strenuous outdoor activity above roughly 103°F heat index, and treating anything above 115°F as unsafe for exertion. Heat stroke, marked by confusion, loss of coordination, or collapse, is a medical emergency requiring immediate cooling and a 911 call.
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