Plain-language glossary of extreme heat
87 terms that come up again and again in extreme heat, explained in a sentence or two.
Open the glossary with its filmsWhat heat does to the body
- Heatstroke
- A medical emergency. The body’s core temperature rises above about 40 °C and its cooling system fails. Without fast cooling it can kill or cause lasting organ damage. Watch the explainer (0:32)
- Heat exhaustion
- The warning stage before heatstroke: heavy sweating, weakness, dizziness and nausea. Rest, water and cooling usually bring recovery. Watch the explainer (0:33)
- Heat stress
- When the body takes in or makes more heat than it can shed, from hot air, humidity, sun and physical work. Watch the explainer (0:35)
- Acclimatisation
- The body slowly adjusting to heat over one to two weeks. New and returning workers are most at risk until it has. Watch the explainer (0:33)
- Excess deaths
- Deaths above the normal number for that place and time of year. Because death certificates rarely say “heat”, this is how the real toll of a heatwave is counted. Watch the explainer (0:34)
- Older people and heat
- With age, thirst fades and the body sweats less, so it cools more slowly and the heart works harder. Older people in a Kolkata informal settlement were 4.3 times more likely than those in a nearby village to face dangerous heat, because their homes stayed hot at night. Watch the explainer (0:42)
- Heat in pregnancy
- A pregnant body runs warm and has less room to cool. Each 1 °C rise raises the odds of a preterm birth by about 5%, and a heatwave by about 16%. Watch the explainer (0:37)
- Babies and young children in heat
- Babies and small children overheat and lose water faster than adults, and depend on someone else to notice. In 2014–2023, each infant in India faced an average of 7.7 heatwave days a year, 47% more than in the 1990s. Watch the explainer (0:37)
- Heat, kidneys and the heart
- Sweating and dehydration make the heart work harder and can damage the kidneys, especially with repeated exposure. People already living with heart, kidney or lung conditions have less room to cope. Watch the explainer (0:35)
- Sleep in the heat
- To fall asleep, the body needs to cool. On hot nights sleep comes later and ends sooner, so the next day starts already depleted. A study of 47,628 people’s sleep wristbands in 68 countries found hot nights cut sleep. Watch the explainer (0:36)
- Heat and the mind
- Heat is linked to more anxiety, distress and suicide, and to worse outcomes for people living with mental illness. Watch the explainer (0:31)
- When home is hotter than outside
- Thin metal roofs, small windows and crowded rooms can make a home hotter inside than out, worst at night. In Ahmedabad, people in informal settlements faced temperatures up to 7.6 °C higher than people in formal housing. Watch the explainer (0:38)
- Kitchen heat
- Cooking over a fire in a small, smoky kitchen during a heatwave: a daily ‘thermal labour’ that falls hardest on women in the poorest single-room homes. Watch the explainer (0:39)
- Heat illness first aid
- Heatstroke is an emergency, and cooling buys time: move the person to shade, cool them with water and fanning, call for help, and give small sips of water only if they are awake. Hot skin with confusion can mean heatstroke. Watch the explainer (0:35)
How heat is measured
- Wet-bulb temperature
- Heat and humidity combined into one number, read from a thermometer wrapped in a wet cloth. It shows whether sweat can still cool you. A wet-bulb temperature of 35 °C is often quoted as the limit of human survival; recent studies suggest the real limit is lower. Watch the explainer (0:35)
- Dry heat and humid heat
- Dry heat is high temperature with low humidity: sweat still evaporates, but the body soaks up heat fast. It peaks in South Asia’s pre-monsoon months. Humid heat is warm, wet air in which sweat can’t evaporate, so the body can’t cool itself even at lower temperatures. It dominates Southeast Asia and builds through South Asia’s monsoon, and it is rising faster than dry heat. The two need different warnings and different cooling. Watch the explainer (2:03)
- WBGT (wet-bulb globe temperature)
- The workplace measure of heat. It adds sun and wind to heat and humidity, and is used to set rest breaks and stop work. Watch the explainer (0:36)
- Heat index
- The “feels-like” temperature in the shade, combining air temperature and humidity. In the Philippines, a heat index of 42 °C or more is officially a danger level. Watch the explainer (0:41)
- Heatwave
- A spell of unusually hot weather for that place. There is no single global definition: each country sets its own thresholds. Watch the explainer (0:36)
- Hot nights
- Nights that stay too hot for the body to recover from the day. They are often more dangerous than hot days, and they are rising faster. Watch the explainer (0:35)
- Urban heat island
- Cities running several degrees hotter than the countryside around them, especially at night, because of concrete, traffic, waste heat and few trees. Watch the explainer (0:34)
- Mean radiant temperature
- The heat your body takes in from the sun and from hot surfaces around it: roads, walls and roofs. In strong sun it shapes how hot you feel more than air temperature does, which is why shade matters so much. Watch the explainer (0:29)
- Thermal comfort
- How hot or cold a person feels. It depends on air temperature, humidity, wind, sun, clothing and activity, not on air temperature alone. Watch the explainer (0:27)
- Climate attribution
- Working out how much climate change has added to an event or its harm. The 2026 Lancet Countdown attributes 71.5% of 2023’s heat deaths to climate change, and 11.3 of 13.5 heatwave days per person in 2025. Watch the explainer (0:34)
- Undercounted heat deaths
- Heat deaths that are never recorded as heat deaths, because certificates name the immediate cause, such as a heart attack. In India in 2024, 185 heatstroke deaths were confirmed against 48,385 suspected heatstroke cases, up to 28 July. Watch the explainer (0:41)
Acting before harm
- Limits to adaptation
- The point beyond which adjusting no longer prevents harm. Places used to heat cope better with days of 30–35 °C, but a study of 2.7 million firms found no adaptation to days above 40 °C. Cutting emissions keeps more places below that limit. Watch the explainer (0:30)
- Heat action plan
- A city’s or country’s plan for the heat season: who issues warnings, at what temperature, and what hospitals, employers and services do next. Watch the explainer (0:36)
- Early warning system
- A forecast that reaches people in time, in a form they understand, with clear advice on what to do. Watch the explainer (0:33)
- Anticipatory action
- Acting on a forecast, before a disaster peaks, by releasing pre-agreed money and help when a trigger is reached. Watch the explainer (0:36)
- Cooling centre
- A public place, such as a library, school or community hall, kept cool during heatwaves for people with no cool space at home. Watch the explainer (0:33)
- Chief heat officer
- A city official whose job is heat: joining up health, water, power, schools and work so that protection is planned before the hot season. Dhaka North City Corporation appointed Asia’s first in 2023. Watch the explainer (0:30)
- Heat alert levels
- Graded warnings, such as caution, danger and extreme danger, set at levels of heat that harm health locally. A good plan ties each level to named actions, so a warning switches protection on. Watch the explainer (0:29)
- Work–rest cycle
- Building shaded rest into each hour of hot, heavy work. In Singapore, employers must give at least 10 minutes’ shaded rest each hour for heavy outdoor work at a wet-bulb globe temperature of 32 °C or more. Watch the explainer (0:35)
- Ward-level heat plan
- A heat action plan that maps risk neighbourhood by neighbourhood, using felt heat, warm nights and who lives there, so help goes first where risk is highest. Thane, next to Mumbai, built its 2024 plan this way. Watch the explainer (0:30)
- Heat-resilient city
- A city that moves beyond emergency response each hot season towards lasting change in its buildings, water, transport, work, health services and cooling, with the people most exposed to heat at the centre. This is how India's Heat Resilient Cities Framework (2026) describes it.
- Heat resilience plan
- A city plan for heat that grows out of a heat action plan. It starts by mapping hotspots and who is most exposed, then sets out interventions for the next one or two years, within a longer-term perspective across city systems. Some Indian cities are setting up urban disaster management authorities to lead them.
- Core elements of a heat plan
- WHO/Europe’s checklist for a complete heat–health action plan: governance, a warning system, people at risk, communication, a resilient health system, reducing exposure, surveillance, and monitoring and learning. Watch the explainer (0:34)
- Heat and learning
- Lost schooling when heat closes schools or makes classrooms too hot to learn in. In April 2024, Bangladesh closed schools for about 33 million children, and 5,288 schools in the Philippines moved to remote learning. Watch the explainer (0:33)
- Heat check-ins
- A call or a knock for people who live alone or can’t easily leave home. In Chicago’s 1995 heatwave, living alone and not leaving home were among the strongest risks of dying. Watch the explainer (0:37)
- Heat in closed places
- Prisons, hostels, care homes and crowded shelters, where people can’t leave for shade. Tamil Nadu’s heat strategy names prisoners among the groups it is meant to reach, and proposes cool roofs and trees for its jails. Watch the explainer (0:34)
- Clinics in a heatwave
- Heatwaves bring surges of patients just as staff, power and supplies are under strain. In 2024, Indian hospitals prepared heatstroke rooms, ice baths and cooled beds before the peak. Watch the explainer (0:33)
- Warning the hospitals first
- A heat warning that reaches clinics a day or two early, so they can add staff, stock fluids and clear beds. After Ahmedabad’s plan prepared hospitals and warned the public, an evaluation estimated about 1,190 deaths a year were avoided. Watch the explainer (0:32)
- Cool classrooms
- A hot classroom is hard to learn in, and when it becomes unsafe, schools close. In 2024, heatwaves disrupted the schooling of 171 million students; South Asia was the most affected region. Watch the explainer (0:38)
Money and protection
- Parametric insurance
- Insurance that pays out automatically when a measured trigger, such as a temperature threshold, is crossed. There is no claim form and no damage assessment. Watch the explainer (0:36)
- Basis risk
- When the trigger and the real loss don’t match: people suffer from the heat, but the threshold isn’t crossed and no payout comes. Watch the explainer (0:36)
- Risk pool
- A group of countries, cities or insurers that share the cost of disasters, so no single member carries a catastrophe alone. Watch the explainer (0:35)
- Public support such as pensions, cash transfers and unemployment benefits that keeps households out of poverty when income stops. Watch the explainer (0:35)
- Lost work hours
- Work that heat makes unsafe or impossible. The 2026 Lancet Countdown estimates 573 billion potential work hours were lost to heat in 2025, with farm workers losing twice as many hours as any other sector. Watch the explainer (0:41)
- Informal work
- Work without a formal contract, sick pay or social insurance, often outdoors: street vending, construction, farm and home-based work. An OECD report estimates heat costs informal workers about 2.4% of working hours, against 1.4% for formal workers. Watch the explainer (0:32)
- Adaptation finance gap
- The difference between what countries say they need to adapt to climate change and the money that arrives. Forty-eight countries put their health adaptation needs at about US$3.3 billion a year; international funding committed in 2024 covered about a third. Watch the explainer (0:30)
- Heat and credit
- How heat travels from wages to lenders: lost working hours mean missed repayments, and lenders face a cash crunch. About 60% of India’s ₹3.81 trillion microfinance portfolio is in farming and allied work. Watch the explainer (0:32)
Cooling, land and systems
- Passive cooling
- Cooling a building without using power: shade, airflow, insulation, orientation and reflective surfaces. Watch the explainer (0:33)
- Cool roof
- A roof painted or built to reflect sunlight, so less heat reaches the room below. Watch the explainer (0:33)
- Refrigerant
- The fluid inside air conditioners and fridges that carries heat away. Many common ones are powerful greenhouse gases if they leak. Watch the explainer (0:35)
- Black carbon
- Soot from diesel, wood, coal and brick kilns. It warms the air, and when it settles on snow and ice it darkens them so they melt faster. Watch the explainer (0:36)
- Vicious cycle
- A harm that feeds itself: each turn of the loop makes the next one worse. This map shows where those loops could be interrupted. Watch the explainer (0:35)
- Maladaptation
- A response to heat that makes things worse elsewhere or later, such as air conditioners that cool a room but heat the street. Watch the explainer (0:30)
- Albedo
- How much sunlight a surface reflects. A white roof has a high albedo and stays cooler; dark asphalt has a low albedo and stores heat. Watch the explainer (0:28)
- Aquifer
- Underground layers of rock or sand that hold groundwater, the water that wells and pumps draw from. Watch the explainer (0:33)
- The cooling gap
- The difference between people who can stay cool and those who can’t. Air conditioning may have prevented about 360,000 deaths a year in 2022 and 2023, but only about 3% of households in the poorest countries have it. Watch the explainer (0:34)
- Cooling demand
- How much cooling people need, and the energy it takes. Southeast Asia’s air conditioners could rise from about 40 million to 300 million by 2040, half of them in Indonesia. Watch the explainer (0:33)
- Peak power demand
- The highest electricity load of the day or year, which in hot countries comes with heat and air conditioning. In May and June 2023, heat drove demand up in northern Viet Nam as hydropower reservoirs ran low; blackouts cost about US$1.4 billion. Watch the explainer (0:31)
- Glacier loss
- Mountain ice shrinking faster than snow can replace it. In the Hindu Kush Himalaya, glaciers lost ice 65% faster in the 2010s than in the decade before, and could lose 30–50% of their volume by 2100. Watch the explainer (0:33)
- Peat fires
- Fires that smoulder in drained peatland, the thick, wet soil of tropical swamps. Once dry, peat burns underground for weeks and fills the air with haze. Indonesia’s 2015 forest and peat fires cost an estimated US$16.1 billion. Watch the explainer (0:32)
- Cold chains in the heat
- Every fridge and cool box from the factory to the patient or the plate. Without reliable power, fridges for food, medicines and vaccines fail; 526 million tonnes of food are lost each year for lack of refrigeration. Watch the explainer (0:34)
- When phones go quiet
- Phone masts and networks need power and cooling too. In Bangladesh in 2026, mobile signals weakened as blackouts spread; warnings, mobile payments and calls for help all ride on them. Watch the explainer (0:34)
- Power grids in a heatwave
- Demand peaks as everyone switches on cooling, just as hot lines and transformers struggle. An early World Bank estimate put the cost of northern Viet Nam’s power cuts in May and June 2023 at US$1.4 billion. Watch the explainer (0:41)
- Water for power, power for water
- Many power plants need water to cool, and water pumps need power to run, so in a heatwave each strains the other. In 2016, India lost 14 TWh of electricity because thermal power plants ran short of water. Watch the explainer (0:29)
- The diesel fallback
- When the grid fails, homes, shops and hospitals that can afford it switch to diesel generators, which keep the lights on and add to the heat and the smoke. There are 20 to 30 million back-up generator sites in low- and middle-income countries. Watch the explainer (0:32)
- Roads and rails in the heat
- Steel rails expand and can buckle, so trains slow; roads soften; hot, thin air means planes need longer runways. Journeys, shifts and supplies, including medicines and food, are delayed. Watch the explainer (0:31)
Nature in the city
- Evapotranspiration
- Water evaporating from soil and leaves. It draws heat out of the air, so well-watered plants cool their surroundings. Plants short of water stop cooling. Watch the explainer (0:32)
- Nature-based solutions
- Using living systems such as trees, wetlands, soils and water to tackle problems like heat and flooding while also benefiting people and nature. A park only works as one if it is planned for those benefits and kept alive. Watch the explainer (0:27)
- Ecosystem services
- The benefits people get from nature: shade and cooling, cleaner air and water, rain soaking into the ground, food and places to rest. The same service is worth most to those with the fewest alternatives, such as cooling for people without air conditioning. Watch the explainer (0:29)
- Blue-green infrastructure
- A network of water (blue) and plants and soil (green), such as rivers, ponds, wetlands, parks and planted drains, designed to manage rain and heat together. Watch the explainer (0:27)
- Grey infrastructure
- Engineered, mostly concrete systems such as pipes, drains, embankments and pumps. Many good solutions mix grey and green. Watch the explainer (0:27)
- Sponge city
- A city designed to soak up, store, clean and slowly release rainwater through parks, wetlands, permeable ground and planted drains, not only pipes. China piloted it in 30 cities from 2014 to 2019; Chennai and Bangkok are building sponge parks. Watch the explainer (0:28)
- Co-benefits
- The extra gains a measure brings beyond its main aim, such as a park built to hold floodwater that also cools the street and gives children somewhere to play. Watch the explainer (0:29)
- Ecosystem-based adaptation
- Using nature to help people adapt to climate change, such as mangroves against storm surges or trees for shade. A kind of nature-based solution. Watch the explainer (0:28)
- Green gentrification
- When a new park or greening raises rents and land prices so much that the poorer residents it was meant to help are pushed out. Watch the explainer (0:29)
- Environmental justice
- A fair share of environmental benefits and harms, and a fair say in decisions about them. Poorer neighbourhoods are often hotter and less green. Watch the explainer (0:30)
- Agroforestry
- Growing trees alongside crops or livestock. The trees shade the soil, help it hold moisture and give farmers another source of income. Watch the explainer (0:24)
- Hot nights and crops
- Crops need cool nights too. At the International Rice Research Institute in the Philippines, rice yields fell about 10% for each 1 °C rise in growing-season night temperature. Watch the explainer (0:33)
- City trees under heat
- Heat, drought and missing upkeep can kill street trees just when their shade is needed. Across 164 cities, 56% of urban tree and shrub species already face temperatures beyond their natural range, and 76% could by 2050. Watch the explainer (0:40)
- Shade follows wealth
- Trees and parks cluster in better-off neighbourhoods, which can pay for them and their upkeep, so poorer districts stay hotter. In Jakarta, green space is concentrated in wealthier areas while dense low-income neighbourhoods act as heat islands. Watch the explainer (0:36)
- When dry soil heats the air
- Moist soil cools the air as its water evaporates. When soil dries, that cooling stops, so the air heats faster and dries the soil further: a loop that water kept in the ground can break. Watch the explainer (0:28)
- Asia’s last tropical ice
- On Puncak Jaya in Papua, Indonesia, 99% of the glacier area has gone since 1850. About 0.17 km² was left in 2024, and it will very likely disappear around 2030. Watch the explainer (0:31)
- Heat, drought and salt water
- Heat and drought lower rivers and canals, and the sea pushes inland. In Viet Nam’s Mekong Delta in February 2024, farmers who pumped salty canal water onto their rice lost up to half their crop. Watch the explainer (0:31)