When I taught physics, I used to tell my students: your body is basically a heat engine that runs on burritos and stubborn optimism. On a hot day, the engine’s cooling system is sweat. The heat index is our attempt to put a number on how well that cooling system is going to work.
Two afternoons can share the exact same thermometer reading and feel wildly different. The difference is humidity, because humidity controls how easily sweat can evaporate off your skin. The heat index takes air temperature plus relative humidity and spits out an “apparent temperature” that better tracks heat stress than temperature alone.
What it is estimating
The heat index is not a direct measurement of your skin temperature, your core temperature, or the temperature of the air. It is an estimate of heat stress for a typical adult in the shade with light wind and light activity, based on how effectively sweat can cool the body under those conditions.
Here is the key physical idea: evaporation steals heat. When sweat evaporates, it takes energy with it, cooling your skin. If the air is already holding lots of water vapor, the vapor pressure gradient from your skin to the air shrinks and evaporation slows down. That means your body has to work harder to dump heat, so the same air temperature feels hotter.
So when someone says “It’s 95°F, but it feels like 110°F,” the “feels like” number is usually a heat-index estimate that assumes you are in the shade and your body is trying to cool primarily through sweat evaporation.
The inputs
Air temperature
This is the ordinary dry-bulb air temperature, the one a typical weather app shows. For the standard heat index equation used in U.S. forecasts, the temperature is in degrees Fahrenheit.
Relative humidity
Relative humidity (RH) is a percentage that compares how much water vapor is in the air to the maximum the air could hold at that temperature. Warm air can hold more water vapor than cool air, so 50% RH at 95°F means more actual water vapor than 50% RH at 75°F.
Humidity is the reason “It’s not the heat, it’s the humidity” is annoyingly correct.
The calculation
In the United States, the heat index shown in many forecasts is based on the National Weather Service “Rothfusz regression,” an empirical fit to earlier work on how heat and humidity affect the body. It is commonly presented like this (intended mainly for hot, humid conditions):
HI = -42.379
+ 2.04901523*T
+ 10.14333127*RH
- 0.22475541*T*RH
- 0.00683783*T*T
- 0.05481717*RH*RH
+ 0.00122874*T*T*RH
+ 0.00085282*T*RH*RH
- 0.00000199*T*T*RH*RHWhere:
- HI is the heat index in °F
- T is air temperature in °F
- RH is relative humidity in percent (0 to 100)
That is a mouthful, but you do not need to memorize it. What matters is the structure: the formula includes terms like T*RH and squared terms like T*T because the relationship is not linear. Humidity matters more as the temperature rises, and temperature matters more as humidity rises.
Also important: this equation is designed for the range where heat index is meant to be used, roughly T at or above 80°F and RH at or above about 40%. Outside that zone, many calculators apply NWS adjustments or switch methods because the regression is not intended to be a universal “feels like” tool for every season.
Worked examples
Let’s hold the thermometer steady and change only the humidity. Suppose the air temperature is 95°F.
Example A: 95°F with 20% RH
Dry air is good at accepting evaporated sweat. Plugging T = 95 and RH = 20 into the standard equation gives a heat index of about:
- Heat index ≈ 93°F
Yes, it can come out slightly lower than the air temperature in very dry air. That does not mean the day is safe or pleasant. It means evaporative cooling is relatively efficient compared with muggy air at the same temperature.
Example B: 95°F with 60% RH
Now the air is already loaded with water vapor. Sweat has a harder time evaporating, so your body’s cooling system gets throttled back. Using T = 95 and RH = 60 gives:
- Heat index ≈ 114°F
Same thermometer reading, but a dramatically different heat index because the body is losing one of its main heat-release pathways.
Example C: 85°F with 70% RH
You do not need triple-digit temperatures for humidity to make a day feel oppressive. With T = 85 and RH = 70, the equation gives:
- Heat index ≈ 95°F
This is why some “only 85°F” days can still feel like you are walking through warm soup.
Note: These example values are rounded. Some online calculators will differ by a degree or two because they apply additional NWS adjustments in specific low-humidity or high-humidity corners of the map.
What the number means
Think of the heat index as a rough indicator of how hard your body must work to shed heat.
- When the heat index is close to the air temperature, evaporation is doing its job reasonably well.
- When the heat index shoots far above the air temperature, evaporation is being choked off, so your core temperature can rise faster, especially during exertion.
Under the hood, the main risks come from a few connected effects: sweating more (dehydration risk), higher heart strain as your body redirects blood flow toward the skin, and less effective cooling as humidity climbs.
One practical way to read the heat index is as a “shade, light breeze, light activity, average adult” estimate of stress. If you are exercising, wearing heavy gear, very young, older, pregnant, on certain medications, or managing cardiovascular conditions, your risk can rise at lower apparent temperatures.
Risk levels
The National Weather Service also ties heat-index ranges to general risk categories: Caution, Extreme Caution, Danger, and Extreme Danger. The exact cutoffs vary a bit by chart, but the basic idea is consistent: as heat index climbs into the upper 90s and beyond, heat illnesses become more likely, especially with time, exertion, and inadequate hydration or cooling breaks.
When it misleads
This is the part I wish every weather app made easier to see: the heat index is useful, but it comes with assumptions. Break those assumptions and the number can mislead you.
Full sun
The classic heat index assumes shade. In direct sunlight, your body absorbs extra radiant energy from the Sun and surrounding surfaces. That can make conditions feel substantially hotter than the calculated heat index, especially on bright days when pavement and dark surfaces are soaking up and re-radiating heat.
If you need a metric that handles sun and exertion better, you will often see workplaces and athletic programs use wet-bulb globe temperature (WBGT) instead.
Strong wind or still air
Wind changes the boundary layer of air next to your skin. A steady breeze can help by moving heat away and helping sweat evaporate, while still air can make you feel hotter than expected. Heat index tables typically assume light wind, so a windy day or a dead-calm day can shift your real experience away from the number.
Very dry air and hard work
In very dry air, sweat evaporates quickly, which can keep your skin cooler and keep the heat index closer to air temperature. But that same fast evaporation can hide how much fluid you are losing. You might feel “fine” right up until dehydration and overheating sneak up on you, particularly during exercise or outdoor work.
Indoors and microclimates
The heat index is meant for outdoor ambient air. Indoors, you are dealing with different airflow, radiant heat sources, and sometimes higher humidity from poor ventilation. Outdoors, microclimates matter too: a shaded tree line, a breezy hill, and a windless alley between buildings can all behave differently within a short walk.
Cooler weather
Below the warm and humid range the formula was built for, the heat index is not the right tool. It is not designed to describe chilly damp days or mild temperatures where sweat evaporation is not the limiting factor for comfort.
Build intuition
If you want a mental model you can use in real time, try this:
- Start with the air temperature.
- Add “bonus heat” as humidity rises. The bonus grows quickly once you are already hot.
- Assume shade. If you are in sun, treat the heat index as an underestimate.
- Assume light wind. If wind is strong, you might get relief. If air is still, you might feel worse.
The exact number needs a calculator. The direction of change does not: at the same temperature, higher RH pushes the heat index up because it slows evaporation.
Heat index FAQ
Is heat index the same as “feels like”?
Often, but not always. “Feels like” in many apps can be a blended metric that uses wind chill in cold weather and heat index or other apparent-temperature models in hot weather. When it is hot and humid, the “feels like” value is frequently the heat index.
Why use relative humidity instead of dew point?
Relative humidity is widely measured and easy to express as a percentage, so it is a convenient input for public-facing guidance. Dew point is a great measure of how much water vapor is actually in the air, but the traditional heat index regression is built around RH.
Can heat index be lower than the air temperature?
Yes, in very dry conditions, the regression can produce a value a bit below the air temperature. Interpret that as “evaporation is relatively effective,” not as “heat is harmless.” High air temperatures can still be dangerous, especially with exertion and dehydration risk.
What is the main takeaway?
Heat index is a humidity-adjusted estimate of heat stress in shaded, light-wind conditions. It is most useful on hot, muggy days, and it becomes less trustworthy in full sun, unusual wind conditions, and situations that differ from the assumptions of the model.
