Heat
What is apparent temperature?
Apparent temperature is what heat does to a body once humidity is taken into account, rather than what a thermometer reads in the shade. It matters because humans cool by evaporating sweat, and humid air stops that working. India has no notified threshold for it, so nothing can be over the limit — but across the stations this site reads, it is rising at more of them than the dry-bulb maximum is.
Why the thermometer is not the measurement
A thermometer measures air. A person is a wet object trying to lose heat into it.
The body's main defence against heat is evaporation. Sweat leaves the skin, takes latent heat with it, and the skin cools. How fast that happens depends on how much more water vapour the surrounding air can hold — so in dry air evaporation is efficient and in saturated air it barely happens at all.
The consequence is that two days at the same dry-bulb temperature are not the same day. A 44°C afternoon in Jodhpur at fifteen per cent humidity is survivable for a body that can drink; a 38°C afternoon in Kolkata at eighty per cent may not be, because there is nowhere for the sweat to go. Apparent temperature — sometimes published as heat index or feels-like — combines the two into one figure.
India's own national heat product agrees with this in its design. Two of its four layers add moisture rather than reporting temperature alone, which is independent evidence that dry-bulb maximum was never the right single measure.
- Dry-bulb temperature
- Air temperature as an ordinary thermometer reads it. What IMD's heatwave criteria use.
- Apparent temperature
- Temperature adjusted for humidity — closer to physiological load. No Indian threshold is set against it.
- Wet-bulb temperature
- A related and stricter measure: the lowest temperature reachable by evaporation. Above about 35°C sustained, no amount of sweating is enough.
Fourteen stations, first half of the record against the second
Counts of stations moving each way, not an average — averaging fourteen grid points does not produce a national trend.
Peak apparent temperature — stations trending up
8 of 14
ERA5, first half vs second half
— trending down
6 of 14
ERA5, first half vs second half
Peak dry-bulb maximum — stations trending up
6 of 14
ERA5, first half vs second half
— trending down
6 of 14
ERA5, first half vs second half
IMD's absolute heatwave threshold, for scale
45 °C dry-bulb
India Meteorological Department
The moisture layer in India's own heat product
Moisture and Temperature Based Heat
Bhuvan, National Remote Sensing Centre, ISRO
Notified threshold for apparent temperature
None
No Indian criterion is set against it
Read the first pair against the second. Peak apparent temperature rises at more stations than it falls at; the dry-bulb maximum splits evenly. The measure that captures what a body experiences is moving, and the measure the criteria are written on is not.
Because nothing is notified against it, apparent temperature is counted on this site as itself and never marked as a breach. That is a deliberate restriction: a reading is only shown against a limit where a limit exists, and inventing one to make a chart legible would be the same error the fire and rainfall pages refuse.
How to read a feels-like figure
It is a model of a body, and different publishers model a different body.
Which formula? Heat index, humidex, apparent temperature and wet-bulb globe temperature are four different constructions with different assumptions about shade, wind and clothing. They do not agree and should not be mixed in one series.
Shade or sun? Most feels-like figures assume shade. Direct sun adds several degrees to the load and is not in the number.
Whose body? These indices are built for a healthy adult at rest. A construction worker, an infant and someone on medication that suppresses sweating are all outside the assumption.
Modelled, always. Apparent temperature is computed from temperature and humidity, never measured directly — so it inherits the uncertainty of both, and fails the way a model fails.
What this number cannot tell you
- It cannot be compared with a dry-bulb reading. They are different quantities and only one has a legal threshold.
- It cannot be used to declare a heatwave in India. The criteria are dry-bulb, and a declaration is a defined administrative act.
- It cannot account for night-time recovery, which is a separate and arguably larger part of the danger — see warm nights.
- A reanalysis smooths local extremes hardest at exactly the tail this measure lives in, and carries no urban heat island.
Where this comes from
Every figure above is read out of one of these, at the address printed beside it.
- ERA5 reanalysis archive ECMWF, served by Open-Meteo · Temperature, humidity and apparent temperature at fourteen stations since 1991
- Heatwave / Hot Weather Outlook Bhuvan, National Remote Sensing Centre, ISRO · India's own product — two of its four layers add moisture
- Heat wave criteria India Meteorological Department · The dry-bulb thresholds a declaration is made on
Reuse freely — CC BY 4.0. The grant covers this page. Each source above keeps its own terms, which is why every one of them is named.
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