Heat the body cannot sweat off
Why a humid 33 °C can be worse than a dry 42 °C, where the body's limit was measured, who crosses it first, and what the map on the first tab is telling you.
A resting adult's limit
25.75–30.55°C wet-bulb
Healthy young adults, at rest in shade, in chamber tests: the lower figure in dry air, the higher in humid air.
PSU-2022 (Vecellio et al. 2022), Table 2
Women over 65
23.33–27.08°C wet-bulb
The same limit, measured the same way. Older bodies fail two to three degrees sooner.
PSU-2024 (Leach et al. 2024), Table 4
What the textbooks said
35°C wet-bulb
Past it "dissipation of metabolic heat becomes impossible" for anyone. Real people failed well below it.
Sherwood and Huber, PNAS 2010
Where IMD's warning starts
40°C air temperature, on the plains
A maximum air temperature, with no humidity in the rule.
IMD, FAQ on Heat Wave
Children
No line
Nobody has measured a child's limit in a chamber and nobody will; the map says so rather than guess.
PSU HEAT studies, scope
Simply put
sweat only cools when it leaves
The same body, two kinds of air
- Left, dry air: sweat evaporates, and takes the heat with it
- Right, humid air: sweat stays on the skin; the heat stays in
Above about 35 °C the only way out for the body's heat is sweat, and sweat only cools you when it evaporates.
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The body makes heat all the time, even asleep, and has to get rid of it. Below about 35 °C the air takes it away by itself. Above that, the only way out is sweat. In dry air it evaporates fast. In humid air it barely evaporates at all. The number that captures both at once is the wet-bulb temperature: wrap a wet cloth around a thermometer and fan it, and it settles at a lower reading than the air, by a lot in dry air and by nothing at all in saturated air. It is the temperature a perfectly sweating body could reach.
Two afternoons, one formula
Jodhpur at 42 °C is well under the line. Howrah at 33 °C sits on top of it. The map shows how far the air is from that line, not the temperature.
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Two afternoons through the same formula the map uses (Stull 2011). Jodhpur at 42 °C and 15 % humidity has a wet-bulb of 22.0 °C; Howrah at 33 °C and 80 % sits at 30.0 °C, on top of a resting adult's limit. The map shows how far the air is from that line, for the person you choose.
Where the line was measured
on people, not from a formula
In a climate chamber
- the room, made more humid in steps
- core temperature: it holds, then rises and does not stop. That point is the limit.
- A sketch, not a measured run.
At Pennsylvania State University, volunteers sat doing light tasks while the room was made hotter or more humid in steps, until their core temperature began to rise without stopping.
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That point is the critical wet-bulb. It is lower in dry heat than in humid heat, so for each body it is a curve against the air's vapour pressure, drawn straight between the measured points; the Method tab draws the curves.
Two cautions, the authors' own
- The volunteers were healthy and acclimatised. Illness, medicines, pregnancy and dehydration lower the limit.
- The tests were at rest, in shade. Sun and work bring it down further, which is why the map has a separate working body.
Who crosses it first
the limit in humid air, lowest first
Four bodies, and one with no number
PSU-2024 (Leach et al. 2024), Table 4 and PSU-2022 (Vecellio et al. 2022), Table 2.
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The measured limits in humid air, lowest first: women over 65 27.08 °C, men over 65 29.31 °C, women 40 to 64 29.64 °C, young adults at rest 30.55 °C. Children have no bar: no chamber study has measured a child's limit, and the physiology (more surface for their weight, less sweat) says it is lower than an adult's, not higher. Garmi draws no guessed line; it says "not measured".
Working in the sun
27.8°C WBGT
A different measure, which adds the sun's load, against the US occupational limit for continuous moderate work.
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Indoors under a tin roof the air can be hotter than outside; that body is not on the map yet because it needs a model of the house.
The bodies on the map
IMD's warning, and this map
they answer different questions
IMD heat-wave warning
Looks at air temperature only
Plains: 40 °C and 4.5 °C above normal, or 45 °C outright. Hills from 30 °C, coast from 37 °C. Two stations, two days.
Catches the dry heat of May; silent on the wet heat of the monsoon coast.
Garmi
Looks at temperature and humidity together
Against a limit measured on people, per body; every 3 hours, every district, today and the next five days.
Scored against airport stations nightly; the two rules are compared side by side from March.
IMD's criteria in full
From its own FAQ: on the plains a station maximum of 40 °C that is 4.5 to 6.4 °C above normal, or 45 °C outright; "severe" from 6.4 °C above normal or 47 °C; hills from 30 °C, coast from 37 °C with a 4.5 °C departure; two stations in a subdivision for two days. It also names "hot and humid weather" when a station is 3 °C above normal with above-normal humidity, but that is not a heat wave. The official warning is a temperature rule for dry heat; Garmi is a humidity-aware rule for the body.
Sun on skin: the UV index
heat and ultraviolet do not rise together
Five bands
Low (up to 2), moderate (3 to 5), high (6 to 7), very high (8 to 10), extreme (11 and above). The advice changes at 3 and again at 8.
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A cool, clear day in Ladakh can carry more ultraviolet than a sweltering one on the coast. The map's second reading, Sun on skin, shows the UV index: in the World Health Organization's words, “a measure of the intensity of UV radiation on the Earth's surface that is relevant to effects on the human skin”. It is reported as the day's highest value, a whole number, in five bands. The guide names the colours by print reference; the screen colours here and on the map are our match to them.
One district's day
Chamarajanagar, Karnataka, 25 October 2026, as forecast: with the day's cloud the index peaks at 1 at 11:30; with the cloud taken away, 11.
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The map shows the solid line's peak and gives the other beside it. That day cloud took at least one point off the peak in 185 of 783 districts.
No “minutes to burn”
The guide is blunt: “Reporting burn times is not recommended.” Its reason: “people tend to interpret burn times to mean that there is a safe level of unprotected sun exposure.” So the map gives the index and the hours it stays high, and no countdown for any skin.
Darker skin
The basic messages were written for “fair-skinned people who tend to burn”. The same guide: “Even though the incidence of skin cancer is lower in dark-skinned people, they are nevertheless susceptible to the damaging effects of UV radiation, especially to the effects on the eye and immune system.” At 8 and above the advice is for everyone.
What this reading cannot see
A forecast from the Copernicus atmosphere service, on cells about 45 km across, for today and the next four days; heat reaches a day further. It cannot see one cloud, a valley or a snowfield, and snow and height both raise the real figure, so the high Himalaya is likely to read low. It is not medical advice.
Comfortable days
how much of the year is easy to live in, and how that has moved
A year, counted in days
- comfortable day
- too cold
- too hot
Chennai, on the average of 2016 to 2025: a row a month, a dot a day. The map's third reading, Comfortable days, draws this for every district and marks where the count stood in the 1990s.
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Each day since 1990 is read hour by hour from the weather record and put in one of three boxes: comfortable, too cold or too hot. The two drawings count the same days by two different rules, which is why they differ.
Two rules, your choice
Indian comfort moves with what people in a place are used to: the warmer the last month, the warmer the day they still call comfortable. International is one fixed band for the whole world. The switch is beside the reading's name.
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The Indian rule comes from surveys of people in offices in five Indian cities. Because it knows nothing of humidity, a day also has to stay at least 3 °C below the body's limit, the line the Heat reading draws, to count.
India since 1990
The average district had 185 comfortable days a year in the 1990s and has 162 now by the Indian rule; 167 and 147 by the international one.
What this reading cannot see
The record's cells are about 25 km across, so a hill town reads like the slopes around it and a beach like the sea beside it. It counts the district's whole area, not its towns, and it does not look at rain. It says how days compare, not how any one day felt to you.
What to do
IMD's and the National Disaster Management Authority's advice
Drink before you are thirsty
Water, ORS, lassi, lemon water, buttermilk
all daysweat is the only way out, and it needs water
Out of the sun, noon to 3 pm
any day the map shows a district within a degree of the linethe sun adds its own load on top of the air's
Light, loose cotton
And cover the head
outdoorslets sweat evaporate, keeps the sun off
No alcohol, tea, coffee, fizzy drinks
hot daysthey dehydrate
Heavy work in the cooler hours
And more breaks
the working body on the mapwork raises the heat the body must shed
Curtains by day, windows at night
Windows open at night
at homekeeps the indoor air below the outdoor peak
Never a child or an animal in a parked vehicle
evera closed car runs far past any line on this map
Call 108
For a body temperature of 40 °C with confusion or collapse
at onceheat stroke is an emergency
The advice is in their words; the "when" line in bold is ours.
Papers
Every figure above was read from the source named beside it. The limits and the wet-bulb values are computed from the model's own file and formula at build, so this page cannot drift from the map.