What Is a Heat Index?

By Mark Debson · June 17, 2026 · 8 min read · Everyday Science

Why 90F can feel like 105F, how the National Weather Service calculates apparent temperature, and the humidity threshold where your sweat stops cooling you.

Quick Answer

The heat index, sometimes called the apparent temperature, is how hot the air actually feels to a human body when relative humidity is combined with the ambient air temperature.

It exists because your body cools itself through sweat evaporation. When the air is already loaded with water vapor, sweat cannot evaporate efficiently, so heat stays trapped against your skin and your core temperature climbs faster than the thermometer suggests.

The National Weather Service heat index chart starts at 80F (27C) and crosses into the Danger category around 103F (39C). Add direct sunlight and you can pile another 15F (8C) on top of the published number.

The science: why humidity multiplies heat

Human core temperature stays near 98.6F (37C). When you get hot, your sweat glands release water onto your skin. Each gram of sweat that evaporates pulls roughly 580 calories of heat out of your body. That is the engine that keeps you alive in summer.

Relative humidity measures how saturated the air is with water vapor as a percentage of its maximum capacity at that temperature. When relative humidity is high, the air is already close to full. Your sweat has nowhere to go. It beads up, drips off, and accomplishes almost no cooling.

At 95F (35C) and 70 percent humidity, the heat index is about 124F (51C). At the same 95F but only 30 percent humidity, it feels like roughly 96F. Same thermometer reading, drastically different physiological strain.

How the heat index is calculated

The published heat index is based on the Rothfusz regression, an empirical formula adopted by the National Weather Service in 1990. It was built to approximate the work of Robert Steadman, who modeled apparent temperature based on assumptions about a typical adult walking in the shade with a light breeze.

You do not need the equation to use the chart. What matters is knowing the conditions baked into it:

• Adult of average size, lightly clothed.
• Standing or walking, not running.
• In the shade with a light wind.
• Body weight around 147 pounds (67 kg), 5 feet 7 inches tall.

Step outside any of those assumptions (direct sun, no wind, heavy exertion, body armor, larger body mass) and your real heat stress is higher than the number says.

The NWS heat index categories

The full sun adjustment

This is the single most underappreciated point about the heat index. The chart is calculated for shaded conditions. Standing in direct, unshaded sun can add up to 15F (8C) to the apparent temperature.

That means an 88F (31C) day at 65 percent humidity, which sits comfortably in the Caution band at a shaded heat index of around 96F, climbs to about 111F in full sun, which is solidly in the Danger band. The difference between the umbrella and the open lawn is not aesthetic, it is medical.

Heat index vs wet bulb temperature

The two metrics measure related things but are not interchangeable.

• Heat index is a calculated value based on dry bulb temperature and relative humidity, designed for public weather forecasts.
• Wet bulb temperature is measured directly by wrapping a wet cloth around a thermometer and reading it. It represents the lowest temperature your body can theoretically cool itself to through evaporation.

Wet bulb is the metric athletic trainers, the military, and industrial safety teams use because it sets the survivability ceiling. A wet bulb temperature of 95F (35C) is considered the theoretical limit for human survival even with shade, water, and rest, because at that point sweat evaporation can no longer remove enough heat to keep your core stable. Wet bulb readings above 88F (31C) already cause heat related deaths in fit, acclimatized adults.

Symptoms to watch for

Knowing the heat index is only useful if you also know what heat illness looks like.

• Heat cramps: painful muscle spasms, usually in the legs or abdomen, from sodium loss in sweat. Treat with rest, shade, and an electrolyte drink.
• Heat exhaustion: heavy sweating, cool clammy skin, weak rapid pulse, nausea, headache, dizziness. Move the person to a cool place, loosen clothing, sip water, apply cold compresses. If symptoms worsen or persist beyond an hour, seek medical help.
• Heatstroke (medical emergency): body temperature 103F (39.4C) or higher, hot dry or damp skin, confusion, slurred speech, possible loss of consciousness. Call 911 immediately. Cool the person aggressively with cold water, ice packs to the neck, armpits, and groin while you wait.

Who is most at risk

The CDC consistently lists the same higher risk groups during heat waves:

• Adults 65 and older, whose ability to regulate temperature declines with age.
• Children under 4, whose smaller bodies heat up faster.
• People with chronic conditions (heart disease, diabetes, kidney disease).
• Outdoor workers and athletes.
• People taking certain medications (diuretics, antihistamines, some psychiatric drugs) that interfere with sweating or hydration.

FAQs

Why does it feel hotter at 85F in Miami than at 95F in Phoenix?

Miami's humidity routinely sits above 70 percent, which sharply limits sweat evaporation. Phoenix often sits at 10 to 20 percent humidity, where sweat evaporates almost instantly and provides strong cooling. The dry heat is still dangerous (and Phoenix sees more heat fatalities per year), but the moment to moment perception is very different.

Does wind lower the heat index?

A light breeze helps because it speeds evaporation. But hot wind above body temperature (around 95F+) can actually add heat, the same way a convection oven cooks food. That is why the calmest, hottest desert afternoons feel survivable while a hot dry wind storm does not.

Why are some heat index charts in Celsius missing high values?

The Rothfusz regression is empirical and was fit to a specific range. The official NWS chart tops out around 137F (58C) because beyond that the math becomes unreliable and the conditions are functionally fatal regardless.

Is the heat index the same as humidex?

No, but they measure the same idea. Humidex is the Canadian equivalent, developed by Environment Canada and calculated from temperature and dew point instead of relative humidity. The two numbers come out close but not identical.

The takeaway

The thermometer alone does not tell you how hot it is. Combine it with humidity and you get the heat index, which is how your body actually experiences the day. When the index crosses 103F (39C), or any time you are in direct sun on a humid afternoon, scale back exertion, hydrate, and pay attention to early symptoms. Heatstroke is one of the few weather hazards you can almost always avoid if you respect the number.