The Heat Index (Feels-Like Temperature) in India Explained

The heat index, also called the feels-like or apparent temperature, is a measure that combines air temperature and humidity to show how hot the weather actually feels to the human body, and in humid parts of India it can be many degrees higher than the thermometer reading. A day with an air temperature of 35 degrees Celsius can feel far more dangerous than a dry 40 degrees if the air is heavy with moisture.

India’s heat warnings have traditionally relied on maximum temperature alone, but a growing body of evidence shows that humidity decides how well the body can cool itself. Last updated: 2 November 2026. This explainer covers what the heat index is, how it is calculated, how it differs from wet-bulb temperature and air temperature, why it matters in India’s coastal and eastern states, and how to use it to stay safe.

Key Takeaways

• The heat index combines air temperature with humidity to estimate how hot it feels, and it is higher than the air temperature when the air is humid.
• Humidity matters because sweat cools the body only when it can evaporate, and moist air slows evaporation.
• The heat index is not the same as wet-bulb temperature, which is a physical measure of the lowest temperature reachable by evaporation, nor is it the same as the air temperature shown on a thermometer.
• In dry heat, such as in Rajasthan or Delhi before the monsoon, the heat index can be close to the air temperature, while in humid coastal and eastern India it can be markedly higher.
• The IMD issues heat-index information on an experimental basis, but its official heatwave warnings still depend mainly on maximum temperature criteria.
• Heat-index values assume shade and light wind, so direct sun, hard work and poor ventilation make real conditions worse than the number suggests.

What is the heat index?

The heat index is a single number, expressed as a temperature, that estimates the apparent heat felt by a person when the effects of air temperature and relative humidity are combined. It answers a practical question: how hot does it really feel out here?

The idea rests on how the human body cools. We lose heat mainly by sweating, and sweat takes heat away as it evaporates from the skin. When the air is dry, evaporation is quick and efficient. When the air is already loaded with water vapour, evaporation slows, sweat clings to the skin, and the body struggles to shed heat. The result is that identical air temperatures can feel very different on a dry day and a humid one.

Several countries use similar indices, called by different names. In the United States it is the heat index, in Canada the humidex, and in Australia the apparent temperature. They differ in detail, but all aim to translate heat and humidity into a number that people can relate to.

How is the heat index calculated?

The heat index is calculated from the air temperature and the relative humidity using an equation fitted to a model of how the human body loses heat, and the version used by the US National Weather Service is based on regression work built on the apparent-temperature research of R. G. Steadman. Weather services also publish simple charts and online calculators so that people can read off the value.

The standard formula is a polynomial that takes temperature and humidity as inputs and returns the feels-like value, with small adjustments for very low or very high humidity. It was designed for warm conditions, generally for air temperatures from about 27 degrees Celsius upward and relative humidity above roughly 40 per cent, and it loses accuracy outside those ranges.

The index assumes a standard person of average build who is walking in the shade in light wind. It does not account for clothing, individual fitness, age or acclimatisation, and it does not include radiation from direct sun. For this reason, officials advise adding a margin of safety when working outdoors.

The table below gives approximate illustrative values for how humidity changes the feel of heat, based on the US chart. The figures are rounded and meant to show the pattern only.

How is the heat index different from air temperature and wet-bulb temperature?

The heat index differs from air temperature because it adds the effect of humidity, and it differs from wet-bulb temperature because it is a comfort and risk estimate for people, whereas wet-bulb temperature is a physical measurement of how far evaporation can cool the air. The three numbers answer different questions.

Air temperature is the reading on a standard thermometer kept in the shade. It says nothing about moisture. Wet-bulb temperature is measured by wrapping the thermometer bulb in a wet cloth and exposing it to moving air, so that evaporation cools it. It is the lowest temperature that can be reached by evaporative cooling, and it rises as the air gets both hotter and more humid.

Wet-bulb temperature has become a focus of climate research because of a physiological limit. A sustained wet-bulb temperature of around 35 degrees Celsius was long proposed as the theoretical upper limit for human survival, since above it even a healthy, resting person in shade cannot lose heat. More recent laboratory studies suggest that real people can run into trouble at lower values, especially in older people and those doing physical work. A related measure, the wet-bulb globe temperature, adds the effect of radiation from the sun and is used to set safe working hours for athletes, soldiers and labourers.

• Air temperature: The simple thermometer reading, with no humidity effect.
• Heat index: Temperature plus humidity, in shade, expressed as a feels-like value.
• Wet-bulb temperature: A physical limit of evaporative cooling, used in studies of survivability.
• Wet-bulb globe temperature: Wet-bulb plus radiation and wind, used for work and sport safety limits.

What are the heat index danger categories?

The heat index danger categories, as used by the US National Weather Service and widely referenced elsewhere, run from caution to extreme danger, with each band linked to a rising risk of heat cramps, heat exhaustion and heatstroke. These thresholds are guidelines and not legal limits, and local agencies may adjust them.

The thresholds were designed for conditions in temperate countries, and people in India are often acclimatised to heat. Even so, the body has biological limits, and lower-income workers who toil outdoors, such as farm labourers, construction workers and street vendors, may face dangerous conditions at values lower than the guidelines imply.

Why does the heat index matter in India?

The heat index matters in India because large parts of the country combine high temperatures with high humidity, especially along the coasts and in the east, so air temperature alone can understate the danger. Heat stress is not confined to the dry heat of the north-west.

India experiences two main kinds of hot weather. The first is dry heat, characteristic of Rajasthan, Gujarat, Madhya Pradesh, Delhi and the Vidarbha region before the monsoon, when maximum temperatures can pass 45 degrees Celsius and relative humidity is low. The second is humid heat, found in Mumbai, Chennai, Kolkata, coastal Andhra Pradesh and Odisha, parts of West Bengal and Bihar, and in the weeks before and between monsoon rains, when temperatures of 35 to 38 degrees Celsius combine with very high humidity.

In the second type, a warning based only on temperature may seem mild, yet the sweat-based cooling system of the body is overwhelmed. Heat waves in eastern India in recent years, including episodes in West Bengal, Odisha and Bihar, drew attention to the role of humidity. Nights that stay warm and muggy, giving the body no chance to recover, add to the burden, and this is a particular problem in dense cities.

Does the IMD issue heat index forecasts?

The IMD has been issuing heat-index information on an experimental basis, giving forecasters and the public a view of feels-like conditions that complements the traditional heatwave warnings. The official heatwave criteria, however, are still based mainly on maximum temperature.

IMD declares a heatwave using thresholds on maximum temperature and on how far it exceeds the normal. For plains stations, a heatwave is typically declared when the maximum reaches at least 40 degrees Celsius and is about 4.5 degrees or more above normal, with a severe heatwave when the departure is larger or the actual maximum reaches 45 degrees. Lower thresholds apply in coastal areas and hill regions. Warm-night conditions are also flagged.

The experimental heat-index product acknowledges that humidity plays a role the plain temperature criteria miss. Forecasters can use it to advise cities such as Kolkata or Mumbai, where a moderate maximum temperature can still produce dangerous feels-like conditions. Public advisories and state heat action plans continue to evolve, and users should follow the IMD website and state disaster management authorities for the latest official products and terms.

Which places in India have the highest heat index?

The places in India with the highest heat index are generally the humid lowlands where hot air meets abundant moisture, including parts of the coast, the Gangetic plain in the pre-monsoon and early monsoon weeks, and the Indus-Gangetic region when irrigation raises humidity. The highest values are not always in the places with the highest air temperature.

• Coastal cities: Mumbai, Chennai, Visakhapatnam and Kolkata often have high humidity and warm nights.
• Eastern India: Odisha, West Bengal and Bihar can see severe humid heat in April to June and again between monsoon spells.
• Gangetic plain: The combination of heat, irrigation and moisture can spike the heat index in Uttar Pradesh, Bihar and neighbouring areas.
• Dry interior: Rajasthan and Delhi have the highest air temperatures before the monsoon, but lower humidity, so the heat index is closer to the air temperature until the monsoon arrives and humidity climbs.

Studies of India’s changing climate suggest that humid heat stress has been increasing, particularly in eastern and coastal areas, as warming raises both temperature and atmospheric moisture.

How is climate change affecting the heat index in India?

Climate change is raising the heat index in India because a warmer atmosphere carries more water vapour and because average and extreme temperatures are climbing, so feels-like temperatures are rising faster than the thermometer alone. Projections point to more days of dangerous humid heat later this century if warming continues.

Two effects combine. Higher temperatures push up the air temperature part of the formula, and a warmer atmosphere can hold roughly 7 per cent more moisture for every degree Celsius of warming, which raises humidity in regions with a moisture source. Irrigation and the expansion of cities add local effects. Urban areas trap heat, and their concrete and traffic keep nights warm.

The result is a growing risk to labour productivity, health and energy demand. India has a large outdoor workforce, and humid heat reduces how much safe work can be done in a day. Researchers who study the limits of survivability warn that, under high-emission scenarios, parts of the Indo-Gangetic plain could eventually approach dangerous wet-bulb levels, though this depends on emissions choices and on adaptation.

How can people use the heat index to stay safe?

People can use the heat index to stay safe by checking the feels-like value rather than only the temperature, timing outdoor work for cooler hours, drinking water often, and treating any rising reading as a reason to rest and cool down. Because the number is calculated for shade, it is wise to add a margin in sunshine.

• Check feels-like values: Look at the heat index or humidity in the forecast, not only the maximum temperature.
• Shift work hours: Avoid strenuous outdoor work between late morning and late afternoon on hot, humid days.
• Hydrate: Drink water before you feel thirsty, and use oral rehydration solution if sweating heavily.
• Cool the body: Wear light cotton clothes, use wet cloths, fans and shade, and take breaks in cool places.
• Watch vulnerable people: Children, older people, pregnant women, outdoor workers and people with heart or kidney conditions need extra care.
• Know the signs: Dizziness, headache, nausea, confusion and a very high body temperature need urgent medical help.

City and state heat action plans, starting with the model developed in Ahmedabad, combine early warning, public cooling spaces and health-system preparedness. Following local advisories is the best way to apply these ideas.

Bottom Line

The heat index tells you what the thermometer does not, which is how heat and humidity together affect your body. With much of India both hot and humid, and with warming raising both factors, feels-like temperature is becoming a vital companion to the maximum temperature in understanding, forecasting and surviving heat.

Frequently Asked Questions

What is the heat index in simple words?

The heat index is the temperature that the weather feels like once humidity is taken into account. It is higher than the actual air temperature when the air is moist, because sweat evaporates more slowly. It is also called the feels-like or apparent temperature.

Why does humidity make it feel hotter?

Humidity makes it feel hotter because the body cools mainly by sweating, and sweat evaporates poorly in moist air. When sweat cannot evaporate, it cannot carry heat away from the skin. The body then retains more heat, and the same air temperature feels hotter and more dangerous.

What is a dangerous heat index?

A heat index of about 39 degrees Celsius or higher is generally considered dangerous, and values above roughly 51 degrees are classed as extreme danger. At these levels, heat exhaustion and heatstroke become likely with prolonged exposure. Workers and vulnerable people can be at risk at lower values.

Is the heat index the same as wet-bulb temperature?

The heat index is not the same as wet-bulb temperature. The heat index estimates how hot the weather feels to people, while the wet-bulb temperature is a physical measure of the cooling that evaporation can achieve. Wet-bulb readings are used to assess the limits of human survival in extreme heat.

Does the IMD give a heat index for India?

The IMD provides heat-index information on an experimental basis, alongside its standard heatwave warnings that rely mainly on maximum temperature. Users should check the IMD website and state disaster management advisories for the latest products. Official heatwave declarations still follow the department’s temperature criteria.

Which Indian cities feel the most humid heat?

Coastal and eastern cities such as Mumbai, Chennai, Kolkata, Visakhapatnam and Bhubaneswar feel the most humid heat, especially in the weeks before the monsoon. The air there is both warm and moist, so feels-like temperatures run well above thermometer readings. Dry cities such as Jaipur or Delhi feel more extreme during dry spells before the rains.

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