Heat Index vs. Air Temperature: What Your Body Feels

Air temperature measures the heat of the air. Heat index estimates what that heat does to a human body once humidity is accounted for, because sweating only cools you if the sweat can evaporate. At 90°F and 40% relative humidity the heat index is about 91°F — essentially the same. At 90°F and 80% humidity it is about 113°F. The air has not changed temperature, but your ability to shed heat has collapsed, and the physiological load is closer to a 113° day. That gap is the entire point of the number.

If you track symptoms against weather, this distinction matters. Two days with identical highs can place completely different demands on your circulation, hydration and sleep. Logging the temperature alone hides that.

The physics: why humidity changes everything

Your body has four ways to lose heat: radiation, conduction, convection and evaporation. The first three all depend on the environment being cooler than you are. Once air temperature approaches skin temperature — roughly 92 to 95°F — those three routes shut down or reverse, and evaporation of sweat becomes the only mechanism left.

Evaporation depends on the vapour pressure difference between your wet skin and the surrounding air. When air is dry, that difference is large and sweat evaporates readily, carrying away about 580 calories per gram of water. When air is already close to saturated, the difference shrinks. The sweat still forms, but it drips off instead of evaporating, so you lose the fluid without getting the cooling. This is the reason a 95°F day in Phoenix and a 95°F day in Houston are not comparable experiences, and why the two cities produce very different heat-illness statistics at the same nominal temperature.

The heat index compresses this into a single "feels like" figure. It is the answer to: at what dry-ish temperature would an average person experience roughly this same physiological strain?

Where the number comes from

The heat index in general use derives from work by Robert Steadman in 1979, later fitted to a polynomial by the US National Weather Service. Steadman modelled a specific reference person and set of conditions, and the assumptions are worth knowing because they define the limits of the number:

• An adult of average build, around 5'7" and 147 lb
• Walking at about 3.1 mph
• In the shade
• Wearing long trousers and a short-sleeved shirt
• With a light breeze of roughly 5 knots
• Not acclimatised to extreme heat, and physiologically able to sweat normally

Change any of those and the real strain shifts. Direct sun can add up to 15°F to the effective load. Still air makes things worse; strong wind helps, at least until air temperature exceeds skin temperature, at which point wind starts delivering heat rather than removing it. Heavy or non-breathable clothing pushes the number up substantially, which is why occupational heat standards use different indices entirely.

The categories the National Weather Service attaches to the figure are:

• 80 to 90°F — caution: fatigue possible with prolonged exposure
• 90 to 103°F — extreme caution: heat cramps and heat exhaustion possible
• 103 to 124°F — danger: heat exhaustion likely, heat stroke possible
• 125°F and above — extreme danger: heat stroke highly likely

These describe risk for the reference person. If you take a medication that reduces sweating, have a cardiovascular condition, are over 65 or under 5, or are unacclimatised, the effective threshold moves down.

The dew point shortcut

Relative humidity is a confusing input because it depends on temperature — 70% humidity at 60°F is dry air, while 70% at 90°F is oppressive. Dew point sidesteps this by expressing absolute moisture content directly. Most meteorologists use it in preference, and it is a better thing to log:

• Below 55°F — comfortable, evaporation works well
• 55 to 60°F — noticeable but tolerable
• 60 to 65°F — becoming humid, sweat lingers
• 65 to 70°F — uncomfortable for most people
• Above 70°F — oppressive; evaporative cooling substantially impaired
• Above 75°F — genuinely dangerous when combined with high temperature

A dew point of 75°F is the same amount of water in the air regardless of whether the temperature is 80 or 100. That stability makes it far easier to compare days across a season than relative humidity is.

What this means for headache and migraine

The evidence linking heat to headache is more consistent than the evidence for many other weather variables. A frequently cited Boston study of emergency department visits found the risk of headache presentation rose roughly 7.5% for each 5°C increase in ambient temperature in the preceding 24 hours. Other work has found similar direction of effect, though effect sizes vary considerably between populations.

The mechanisms proposed are plausible and probably act together:

Dehydration. Sweat losses of one to two litres per hour are achievable in serious heat. A fluid deficit of even 1 to 2% of body mass is enough to impair cognition and is a documented headache trigger.

Vasodilation. Peripheral blood vessels dilate to shunt blood to the skin for cooling. Cerebral vessels participate in that response, and vasodilation is a recognised component of migraine pathophysiology.

Electrolyte shifts. Sweat carries sodium. Replacing heavy losses with plain water alone dilutes serum sodium, which is itself a headache mechanism at the extreme end.

Sleep disruption. Core body temperature has to fall for sleep to initiate and consolidate. Warm nights — and heat waves are increasingly defined as much by elevated overnight minima as by daytime peaks — prevent that fall. Poor sleep is one of the most consistently reported migraine triggers there is.

Compounding air quality. Heat and stagnant air favour ozone formation, and heat waves often coincide with elevated pollutant levels. Some of what gets attributed to temperature may be air quality riding alongside it.

Note that most of these operate through the humidity-adjusted load rather than the raw temperature. That is a direct argument for tracking heat index or dew point rather than the daily high.

Where the heat index stops being useful

It is a model, and models have edges.

It assumes shade. Full sun with a high solar angle can add 10 to 15°F. If you work or exercise outdoors, the published heat index systematically understates your exposure. The wet bulb globe temperature, which incorporates radiant heat and wind directly, is the standard used in sports and military settings for this reason.

It assumes a functioning sweat response. Anticholinergic medications, some antihistamines, certain blood pressure drugs, and conditions affecting autonomic function all reduce sweating capacity. For someone in that group the index understates strain considerably.

It handles very high dry heat poorly. At extreme temperatures with very low humidity the Steadman equations were extended by extrapolation, and researchers have argued for years that the values become unreliable. Revised formulations published in recent years give substantially different — generally higher — figures in that regime.

It says nothing about acclimatisation. Ten to fourteen days of consistent heat exposure produces real physiological adaptation: earlier sweat onset, higher sweat rate, lower sodium concentration in sweat, expanded plasma volume. The same heat index in June and September represents quite different subjective loads. This is why early-season heat waves cause disproportionate harm.

Practical use if you track symptoms

Log dew point or heat index, not the high temperature. If your tracker only offers temperature, note the humidity separately.

Pay attention to the overnight minimum. A day peaking at 95°F that falls to 65°F overnight gives your body a recovery window. The same peak with a 78°F minimum does not, and multi-day heat waves accumulate strain precisely through that mechanism.

Watch for the first hot spell of the season specifically. If your log shows symptoms clustering in early-season heat and then easing despite continued high temperatures, that is acclimatisation, and it tells you the useful intervention is gradual exposure rather than avoidance.

Separate heat from pressure. Summer heat waves usually sit under stagnant high pressure with a very flat barometer, which makes them a clean test case: if your symptoms spike during a heat wave, pressure is not the explanation. Pressure Pal logs both alongside each other, so you can check whether a given bad day was a heat day, a pressure day, or both. If you are in a city where the two rarely coincide — see the Phoenix forecast page for an example of a place with intense heat and very little barometric movement — the separation is unusually clear in the record.

Hydrate to a plan rather than to thirst. Thirst lags fluid deficit, particularly in older adults. In heavy sweating conditions, include sodium, not just water.

FAQ

Is heat index the same as "feels like" temperature?

Usually, in summer. Most weather apps display heat index as "feels like" when it is warm and wind chill when it is cold. Some services use a proprietary blend that also accounts for sun and wind, so two apps can show different "feels like" values for the same conditions. The underlying air temperature will match; the adjustment will not.

Why does my app show a heat index lower than the temperature?

When relative humidity is low, the heat index can fall below air temperature — dry air lets sweat evaporate more efficiently than the reference conditions assume. Many services simply suppress the display below a threshold, so you may just see the raw temperature.

Does high humidity alone cause headaches, or does it need heat?

Both patterns appear in symptom logs. Humidity without much heat is associated more with sinus congestion and fatigue than with acute headache. The strongest heat-headache associations in the literature involve temperature and humidity together, which is exactly what the heat index captures.

What heat index should I stop exercising at?

There is no single number, because it depends on intensity, acclimatisation, clothing and individual physiology. As general guidance, most sports medicine bodies advise modifying activity above a heat index of about 90°F and moving indoors above roughly 103°F. If you have a condition affecting thermoregulation, ask your clinician for a personal threshold rather than using a public one.

Is the heat index getting less accurate as the climate warms?

The concern raised by researchers is that the original equations were extrapolated beyond their tested range, and conditions that used to be rare are now occurring often enough to matter. Revised formulations exist and generally produce higher values under extreme conditions. For everyday use in the 85 to 105°F band the standard index is still reasonable.

The short version

Air temperature tells you about the air. Heat index tells you about the demand placed on your body, and on humid days those two numbers diverge dramatically. If you are trying to work out what triggers your headaches, the second one is the more informative variable — and dew point, which is simpler and more stable, is an even better thing to write down.