A "feels-like" temperature that combines air temperature and humidity. At 35°C with 70% humidity, the heat index reaches 46°C — dangerous.
measurementWhat is Heat Index?
The heat index (HI) — also called “feels-like temperature” or apparent temperature — is a measure of how hot the air actually feels when combined effects of temperature and humidity are accounted for. It was developed by Robert Steadman in 1979 and adopted by the US National Weather Service for public heat warnings; it is now used globally.
The principle is simple: humans cool themselves primarily by sweating, which depends on evaporation of sweat into the surrounding air. When humidity is high, evaporation slows down, sweat doesn’t cool you efficiently, and your body temperature climbs even though the air thermometer reads the same number.
A dry 40°C day feels uncomfortable but survivable. A humid 35°C day with 70% humidity has a heat index of 46°C — and is far more dangerous for prolonged outdoor activity.
How heat index is calculated
The standard formula combines air temperature (T, in °F) and relative humidity (RH, in %):
HI = -42.379 + 2.04901523*T + 10.14333127*RH
- 0.22475541*T*RH - 0.00683783*T² - 0.05481717*RH²
+ 0.00122874*T²*RH + 0.00085282*T*RH² - 0.00000199*T²*RH²This Steadman regression is calibrated to a person walking outdoors in shade with a slight breeze. Actual perceived heat in full sun can be 8–10°C higher than the heat-index value.
The formula is valid only when air temperature ≥ 27°C and RH ≥ 40%. Below those thresholds, dry air does not cause significant heat stress.
Heat index danger categories
The US NWS classifies heat index values into four risk levels:
| Heat Index | Risk | Effect on healthy adults |
|---|---|---|
| 27–32°C | Caution | Fatigue possible with prolonged exertion |
| 32–41°C | Extreme caution | Heat cramps and exhaustion possible |
| 41–54°C | Danger | Heat exhaustion likely; heat stroke possible |
| Above 54°C | Extreme danger | Heat stroke imminent |
In coastal South Asia (Mumbai, Karachi, Chennai, Dhaka, Chittagong, Kolkata), heat-index values of 45–55°C are routine during summer and the early monsoon — placing hundreds of millions of people in the “Danger” zone every year.
Heat index in South Asia
South Asia experiences some of the world’s most extreme heat-index conditions, especially in two scenarios:
Dry inland heat (Rajasthan, Sindh, Balochistan, central Maharashtra):
- Air temperature can reach 48–52°C, but humidity is often only 10–20%.
- Heat index is similar to or only slightly above air temperature.
- Sweating remains effective; risk is mostly dehydration.
Humid coastal and monsoon heat (Mumbai, Karachi, Kolkata, Chennai, Dhaka):
- Air temperature 33–37°C is paired with humidity 70–85%.
- Heat index can reach 45–55°C.
- Sweating fails; heat-stroke risk is very high, especially during exertion.
Pre-monsoon coastal heat (April–May) in Chennai, Vizag, Karachi, Dhaka regularly produces heat indices that meet or exceed the US NWS “Extreme Danger” threshold.
Pakistan’s heat hotspot Jacobabad has recorded wet-bulb 35°C (corresponding to heat index above 65°C) — one of the few places on Earth to have crossed what scientists call the “survivability limit” for humans.
Heat index vs wet-bulb vs feels-like
Three closely related “felt heat” concepts often get confused:
- Heat index — a US NWS engineering formula combining temperature and RH. Used for public warnings.
- Wet-bulb temperature — a physical measurement: the lowest temperature you can reach by evaporative cooling. Used by climate scientists; 35°C wet-bulb is considered human survivability limit.
- Feels-like temperature — a general consumer term used by weather apps. Most apps use heat index in summer and wind chill in winter. Mausam Online’s “feels-like” is the same concept.
For practical use during a heatwave, heat index is the right metric to check. For deep climate-risk analysis, scientists use wet-bulb.
Mausam Online and heat warnings
Mausam Online displays both air temperature and feels-like (heat index) for every South Asian city. When the values diverge by 5°C or more, humidity is the cause — a signal that outdoor exertion is more dangerous than the thermometer alone suggests.
During IMD heatwave alerts (orange / red codes), check feels-like before going outside. If feels-like exceeds 42°C, treat outdoor work as a serious risk.
Frequently asked questions
Why is humid heat more dangerous than dry heat? Sweating cools you only when sweat evaporates. High humidity slows or stops evaporation. Your sweat stays on your skin instead of carrying heat away. Your body temperature climbs even though the air temperature hasn’t changed.
Is heat index calculated in the shade or sun? The US NWS heat index assumes shade with light wind. Full sun adds 8–10°C to the perceived heat. A 40°C heat-index value in sun feels like 48–50°C.
Does heat index apply to indoor environments? Yes — the same physiology applies. Indoor heat index above 35°C without active cooling causes heat exhaustion and stroke risk, especially in poorly ventilated rooms in coastal South Asian cities during monsoon nights.
What is a “safe” heat index for outdoor work? Below 32°C, healthy adults can work outdoors with normal hydration. Between 32°C and 41°C, take regular shade and water breaks. Above 41°C, limit exposure to short periods. Above 54°C, avoid all but emergency outdoor work.
Where can I see live heat-index data for my city? Mausam Online shows feels-like temperature alongside air temperature on every city page. See Mumbai, Karachi, Chennai, Dhaka, Kolkata, Delhi.