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A measure combining air temperature and relative humidity to express how hot the weather actually feels to the human body.
The Heat Index is a measure that combines air temperature with relative humidity to express the apparent temperature, or how hot the weather actually feels to a human being. It exists because the thermometer reading alone is a poor guide to heat stress: the body cools itself mainly by evaporating sweat, and when the air is already laden with moisture that evaporation slows down, so the same air temperature feels far more oppressive. The India Meteorological Department issues heat index based colour-coded warnings to help the public and administrators judge the actual risk during hot weather.
Type: IndexCombines two inputs: air temperature and relative humidity
Expresses the result as an apparent or 'feels like' temperature rather than the measured air temperature
Based on the physiological fact that high humidity reduces the rate of evaporation of sweat
The India Meteorological Department issues colour-coded heat index warnings — green, yellow, orange and red
Also known internationally as the apparent temperature or, in some usage, the humiture
Distinct from a heatwave declaration, which is based on measured temperature departures rather than on felt heat
Frequency: Heatwaves and heat stress indicators have become a recurring theme in UPSC Prelims and Mains as climate topics gain prominence
The human body maintains its core temperature largely by sweating. Sweat itself does not cool you — the cooling happens when sweat evaporates from the skin, drawing latent heat away with it. The rate of evaporation depends on how much moisture the surrounding air can still absorb. In dry air, sweat evaporates rapidly and the body sheds heat efficiently. In humid air, the surrounding atmosphere is already close to saturation, evaporation slows sharply, and the body's principal cooling mechanism begins to fail. This is why 38°C in a coastal city can be more dangerous than 44°C in a desert, and why an index that ignores humidity would badly understate the risk.
Drying clothes on a line works quickly on a dry breezy day and barely at all on a muggy one. Your skin is the clothesline, and sweat is the water that needs to evaporate.
Heat index below 35°C
Heat index of 36 to 45°C
Heat index of 46 to 55°C
Heat index above 55°C
Air temperature and relative humidity
Heat is now among the deadliest weather hazards in India, and the parts of the country most at risk are not always the hottest by thermometer. Coastal and deltaic regions with moderate temperatures but very high humidity can generate heat index values well above those of the dry interior, and a warning system based on air temperature alone would miss them entirely. The index also has direct administrative uses: it informs decisions on school closures, adjustment of outdoor working hours for construction and agricultural labour, and the readiness of hospitals for heatstroke admissions. Its main limitation is that it assumes shade and light wind, so a person working in direct sunlight faces greater stress than the index indicates.
Heat Index = apparent temperature from air temperature plus relative humidity
High humidity slows sweat evaporation, so the body cools less effectively
IMD colour codes: Green <35°C, Yellow 36-45°C, Orange 46-55°C, Red >55°C
Different from a heatwave declaration, which uses measured temperature thresholds
Assumes shade and light wind, so it understates risk in direct sunlight
Also called apparent temperature
It is a measure combining air temperature and relative humidity to express how hot the weather actually feels to the human body, also called the apparent temperature.
Green below 35°C for no action, Yellow from 36 to 45°C to stay updated, Orange from 46 to 55°C to be prepared, and Red above 55°C to take action.
The body cools by evaporating sweat. When the air already holds a lot of moisture, evaporation slows, so the body sheds heat less efficiently and the same temperature feels far hotter.
No. A heatwave is declared by IMD on the basis of measured temperature crossing defined thresholds or departing from normal, while the Heat Index describes how hot conditions feel once humidity is taken into account.