Heat Index Calculator: Why 90F Can Feel Like 106F

Sweat only cools you when it evaporates. Humid air is already carrying water, so evaporation slows and the heat you are producing stays with you. That is the entire idea behind the number.

Two afternoons that read the same on a thermometer

Take 90F with 40% humidity, an ordinary dry afternoon in Phoenix. The heat index is 91F. Sweat evaporates freely and the air temperature is close to the truth.

Now take 90F with 70% humidity, a normal July day in Houston or Atlanta. The heat index is 106F. Nothing about the air temperature changed. What changed is that your main cooling system is running at a fraction of its capacity.

Push it a little further. At 95F with 60% humidity the heat index reaches 113F, well inside the band where heat exhaustion becomes likely during sustained activity. Five degrees separate that from the dry Phoenix afternoon on a thermometer. Twenty-two degrees separate them on the heat index.

The UK version of the same problem

British summers rarely reach those temperatures, but they are frequently humid. 30C with 70% humidity gives a heat index of about 35C. In a country where most homes have no air conditioning and the buildings are designed to hold heat in, that is the combination that makes a 30C day feel unmanageable while a dry 32C abroad does not.

How to use the heat index calculator

Two inputs: air temperature and relative humidity. Switch between Fahrenheit and Celsius as needed. The output is the apparent temperature, and most versions also show the risk band it falls into.

Finding the humidity number

Relative humidity sits on almost every weather app, usually under the hourly forecast rather than on the main screen. If yours only offers a dew point, the two relate directly: when the dew point is within a couple of degrees of the air temperature, humidity is near 100%. A dew point in the 70s Fahrenheit is what people describe as oppressive, and the dew point is arguably the better single number for judging how a day will feel.

One caution. The formula assumes shade and a light breeze. Full sun can add as much as 15F to the result, so a 105F heat index on a shaded porch is closer to 120F standing on open asphalt.

Reading the risk bands

Below 91F the National Weather Service calls it caution and leaves it there. From 91 to 103F the label is extreme caution, where cramps and heat exhaustion become possible with prolonged exposure. 103 to 124F is danger. Above 125F is extreme danger, where heat stroke is likely.

The bands assume you are doing something, not sitting still. A 100F heat index while reading in a hammock is a different proposition from the same figure while roofing. Fluid loss is the practical constraint and it climbs fast, since an hour of physical work in that range can cost well over a litre of sweat.

Common questions

Why does the calculator refuse temperatures below 80F? The standard formula was fitted to hot conditions and goes unreliable below about 80F. Under that, humidity has a small enough effect that the plain air temperature is close enough to be honest. In cold weather the adjustment runs the other way, which is what wind chill handles.

Is heat index the same as "feels like"? Not always. Weather apps often show a proprietary "feels like" figure that also folds in wind, sun and sometimes cloud cover. In hot humid conditions the two usually land close together. On a dry day in full sun they can differ by ten degrees or more, because the app is counting the sunshine and the heat index is not.

Does the heat index apply indoors? Yes, and it gets ignored there. A bedroom at 88F with 65% humidity has a heat index around 98F. Air conditioning removes moisture as well as heat, which is why a dehumidifier can make a warm room feel noticeably better without touching the thermostat.

How long is it safe to be out in a 105F heat index? There is no fixed limit, because it depends on effort, shade, how acclimatised you are and how much you drink. The rule outdoor crews work to is shorter work blocks and longer breaks as the number climbs, and treating cramp, nausea or a headache as a stop signal rather than something to push through.