How Heat Waves Work

Prologue

A toll measured after the fact

LSHTM, 23 October 2026: a study in Nature Health led by Dr Clair Barnes (Imperial College London) estimates more than 50,000 heat-related deaths in summer 2026 (June–August) across 854 urban areas of the 27 EU countries, the UK, Norway and Switzerland, home to about 40% of their population. It is the highest toll since 2000, almost double summer 2022 (about 29,000) and 2003 (about 28,000) in the same urban areas. About two-thirds is attributed to climate change. Nearly a third of the deaths fell between 17 June and 1 July.

The figures are modelled, not counted: weather data and climate models are combined with past evidence on how deaths rise with temperature. The study compares today’s climate, about 1.4 °C warmer, with one without human-caused warming. Rural areas are not covered, so news reports note that the true toll is likely higher, and the figure cannot be compared directly with Europe-wide studies such as those of 2003 (>70,000) or 2022 (61,672), which covered whole countries. Country figures reported from the study include Italy 13,303, France 7,268, Spain 6,824 and Germany 6,340 (Trending Topics).

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: LSHTM (23 Oct 2026): more than 50,000 heat-related deaths estimated in European urban areas during summer 2026; Trending Topics (23 Oct 2026): heat killed 50,000 people in Europe’s cities, two-thirds due to climate change; Democracy Now! headlines (23 Oct 2026): study, European heat waves killed 50,000 in 2026

Chapter I · What counts as hot

Hot is relative

UNDRR’s hazard profile, drawing on the WMO, describes a heatwave as a marked, unusual period of hot weather persisting for at least two to three days and nights, with a discernible impact on human activities, and says universal numeric limits are not possible because impacts vary locally.

Met Office: at least three consecutive days with maximum temperatures at or above the local threshold, which varies by county; thresholds were raised in eight counties in the 1991–2020 update. British Red Cross summary: 25 °C in places such as Scotland and Wales, 28 °C in London and parts of the East Midlands. IMD: at least 40 °C for plains (37 °C coastal, 30 °C hills) with a departure of 4.5–6.4 °C from normal (severe above 6.4 °C), or an actual maximum of 45 °C (severe 47 °C), at two stations on two consecutive days.

Try it: the curve is a schematic spread of summer afternoon temperatures; the buttons move it and its threshold to a cooler or a hotter climate.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: UNDRR / WMO hazard information profile: heatwave; Met Office (2026): UK heatwaves, how the records tell a changing story; British Red Cross: what is a heatwave? (Met Office thresholds); India Meteorological Department: heat wave criteria (morning heat bulletin, annexure)

Chapter II · The heat dome

Why high pressure means heat

The American Meteorological Society glossary describes a heat dome as an exceptionally warm air mass at middle latitudes during the warm season, under high pressure aloft that suppresses cloud and rain and can add warming as air sinks; because winds aloft are often weak, the heat can linger for days.

Dry adiabatic lapse rate: 9.8 °C per km. Air moving downward is compressed; work is done on it, so its temperature rises even though no heat is added from outside. Moist air that is condensing cools more slowly as it rises, which is why the dry rate applies to the clear, sinking air under a dome.

Try it: drop a parcel from 3 km and watch its temperature as it sinks.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: American Meteorological Society, Glossary of Meteorology: heat dome; Wikipedia: lapse rate (dry adiabatic lapse rate)

Chapter II · The heat dome

When the jet stream stalls

Met Office case study: the 2010 Russian heatwave was driven by a blocking high-pressure system that stalled the low-pressure systems that would normally break it up; the worst lasted from July into the first half of August. Moscow recorded over 14,000 deaths in July 2010, nearly 5,000 more than the previous July; pollution was about five times normal; more than 40 peat fires burned south and east of Moscow; about 9 million hectares of crops were lost in the worst drought in roughly 40 years. Highest reading in the table: Yashkul, 44.0 °C, 11 July. Nine million hectares is 90,000 km², close to Portugal’s 92,225 km² (Wikipedia). Comparing with the July before is a simple baseline: the extra deaths over a normal month, which chapter VI calls excess deaths. Peat is waterlogged soil made of old plant remains; when it dries it can smoulder for weeks.

2021: Lytton, British Columbia, set a Canadian record of 49.6 °C, far above the previous 45 °C, and was largely destroyed by a wildfire shortly afterwards (IPSL, on the World Weather Attribution study). British Columbia’s coroners’ review counted 619 heat deaths in late June and early July 2021 (CTV News).

Try it: switch between a flowing jet stream and a blocked one. The wave shape is schematic.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: Met Office: the Russian heatwave of summer 2010; IPSL: North America heatwave almost impossible without climate change, World Weather Attribution study (2021); CTV News: coroners’ review reveals true death toll of B.C.’s 2021 heat dome; Wikipedia: Portugal (area 92,225 km²)

Chapter III · Ground and city

The soil moisture feedback

Scientific American, reproducing Nature (April 2014), on a study by Diego Miralles (Ghent University) in Nature Geoscience: each of the 2003 European and 2010 Russian mega-heatwaves started over areas with low soil moisture. The ground dried further in the first heat and added more heat to the air near the surface; the heated air layer roughly doubled to over 4 km. “The heat doesn’t dissipate at night, so you start from where you left off the day before.”

Try it: switch the field between wet and dry. The arrows show where the sunshine goes: blue for evaporation, orange for heating the air. Proportions are illustrative.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: Scientific American / Nature (2014): mega-heatwaves mechanism pinpointed (Miralles et al., Nature Geoscience)

Chapter III · Ground and city

Why cities run hot

US EPA: heat islands form where structures are concentrated and greenery is limited; buildings, roads and other infrastructure absorb and re-emit the sun’s heat more than natural landscapes such as forests and water bodies. Urban areas are about 1–7 °F (0.6–3.9 °C) warmer than outlying areas by day and about 2–5 °F (1.1–2.8 °C) warmer at night.

The 2026 European estimate covers 854 urban areas only (LSHTM).

Try it: switch between day and night. The heat colours over the city and the fields are schematic.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: US EPA: learn about heat island effects; LSHTM (23 Oct 2026): more than 50,000 heat-related deaths estimated in European urban areas during summer 2026

Chapter IV · The body

How the body sheds heat

Sherwood and Huber (PNAS, 2010): a resting human body generates about 100 W of metabolic heat and keeps a core temperature near 37 °C; skin temperature is strongly regulated at 35 °C or below under normal conditions, so that heat can flow from core to skin. US National Weather Service: blood is circulated closer to the skin’s surface, and sweating does nothing to cool the body unless the water is removed by evaporation.

Try it: set the air to 25 °C or 40 °C and watch the arrows. In 40 °C air, heat flows into the skin and only evaporation carries it away. The figure is a schematic.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: Sherwood & Huber (2010), PNAS: an adaptability limit to climate change due to heat stress; US National Weather Service: heat safety (how the body cools, heat index)

Chapter IV · The body

Heat exhaustion and heatstroke

NHS 111 Wales: heat exhaustion usually does not need emergency help if the person cools down within 30 minutes; heatstroke is an emergency, and heat exhaustion can turn into it. Signs of heatstroke include still feeling unwell after 30 minutes of rest, cooling and fluids; a very high temperature; hot skin that is not sweating; fast breathing or heartbeat; confusion, poor coordination, a seizure or loss of consciousness. In those cases the service says to call the emergency number. In Thailand, the Department of Disease Control advises calling 1669 for severe cases.

This is general education, not medical advice. WHO (July 2024): heat stress worsens cardiovascular disease, diabetes, asthma and kidney disease, and in extreme heat untreated heat stress can lead to heat stroke.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: NHS 111 Wales: heat exhaustion and heatstroke; WHO (25 Jul 2024): UN Secretary-General calls for global action on extreme heat; กรมควบคุมโรค สคร.12 สงขลา (7 มี.ค. 2567): เตือนระวังฮีทสโตรก โทรสายด่วน 1669

Chapter V · Humid heat

Why humid heat is so dangerous

Sherwood and Huber (2010) estimated the survivability limit for peak wet-bulb temperature at probably close to 35 °C, “though this could be a degree or two off”, for a person out of the sun, in gale-force winds, doused with water, unclothed and not working, and said this likely overestimates what could practically be tolerated. In the data they studied, wet-bulb temperature never exceeded 31 °C. Penn State (March 2022, Journal of Applied Physiology): 24 people aged 18–34 doing light activity in an environmental chamber could no longer hold their core temperature steady at wet-bulb temperatures of 30–31 °C in warm, humid conditions and 25–28 °C in hot, dry ones; older and more vulnerable people likely have lower limits. The lower figure for dry heat is not a contradiction: to reach a wet-bulb of 25–28 °C in dry air, the air itself must be far hotter than skin, so the body gains heat directly from it. Wet-bulb is not the reading on an ordinary thermometer: by Stull’s formula, 34 °C air gives about 24 °C wet-bulb at 40% humidity and about 33 °C at 90%, and about 35 °C needs roughly 40 °C air at 70% humidity. The formula: Tw ≈ T·atan[0.151977(RH+8.313659)^½] + atan(T+RH) − atan(RH−1.676331) + 0.00391838·RH^1.5·atan(0.023101·RH) − 4.686035. The heat index, a different measure, is explained in the next step.

Try it: change the air temperature and humidity. The surface plots wet-bulb temperature from Stull’s 2011 formula, an approximation at standard sea-level pressure; the heat index in the readout uses the US National Weather Service equation. The red sheet marks 35 °C, the orange one 31 °C.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: Sherwood & Huber (2010), PNAS: an adaptability limit to climate change due to heat stress; Penn State (2022): humans can’t endure temperatures and humidities as high as previously thought (Vecellio et al., J. Appl. Physiol.); Stull (2011), J. Appl. Meteor. Climatol.: wet-bulb temperature from relative humidity and air temperature (calculator and validity range); NOAA Weather Prediction Center: the heat index equation; US National Weather Service: heat safety (how the body cools, heat index)

Chapter V · Humid heat

Thailand’s hottest month on record

Tak recorded 45.4 °C on 15 April 2023, the highest temperature ever measured in Thailand (Wikipedia; Yale Environment 360). On 22 April 2023 the Thai Meteorological Department reported 42 °C in Bang Na district, Bangkok, and a record heat index of 54 °C; at least 28 provinces were forecast above 40 °C (AFP via The Jakarta Post). The Thai Meteorological Department grades the heat index in four levels: watch 27–32 °C, caution 32–41 °C, danger 41–54 °C, extreme danger above 54 °C (Manager Online, April 2024). Bang Na’s 54 °C therefore sits at the top of the danger band. Three different numbers: air temperature is what a thermometer in the shade reads; the heat index is how hot it feels once humidity is counted; the wet-bulb is what a wet thermometer reads.

World Weather Attribution (May 2023, reported by Yale Environment 360): the April 2023 humid heat in Laos and Thailand would have been virtually impossible without climate change; at 2 °C of global warming such heat would become about ten times more likely again.

The scene starts at 40 °C and 45% humidity as an example, not a measurement. The readout shows the wet-bulb temperature, the heat index and the Thai level for whatever you set.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: Wikipedia: Tak, Thailand (national record 45.4 °C, 15 April 2023); The Jakarta Post / AFP (22 Apr 2023): Thai authorities issue extreme heat warnings for dozens of provinces; ผู้จัดการออนไลน์ (13 เม.ย. 2567): ระดับดัชนีความร้อนของกรมอุตุนิยมวิทยา; Yale Environment 360 (17 May 2023): April heat wave in South Asia made 30 times more likely by climate change; NOAA Weather Prediction Center: the heat index equation

Chapter VI · Counting the toll

Why heat deaths are estimated, not counted

AP (August 2023): coroners may not link contributing conditions to heat, and confirming heat’s role can take weeks or months. The excess-deaths method compares deaths in a period with the number expected; epidemiologist Kristie Ebi says it captures “people who would not have suffered a fatal heart attack or renal failure without the heat”.

Robine et al. (2008): daily regional death counts in 16 European countries for summer 2003 were compared with the same days in the reference period 1998–2002, giving more than 70,000 additional deaths; the August crisis ran from 3 to 16 August, with 12 August the deadliest day. The 2026 study, which covers only 854 urban areas and uses a modelled temperature–mortality method, puts 2003 at about 28,000 deaths in those cities (LSHTM): different coverage and method, not a contradiction.

Try it: show the expected line, then the observed one; the gap between them is the excess. The curves are schematic, not the 2003 data.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: AP via PBS NewsHour (13 Aug 2023): inconsistent methods for counting U.S. heat deaths; Robine et al. (2008), Comptes Rendus Biologies: death toll exceeded 70,000 in Europe during the summer of 2003; LSHTM (23 Oct 2026): more than 50,000 heat-related deaths estimated in European urban areas during summer 2026

Chapter VI · Counting the toll

Who heat kills

ISGlobal (July 2023), on Ballester et al. in Nature Medicine: 61,672 heat-attributable deaths in 35 European countries between 30 May and 4 September 2022, from epidemiological models fitted to temperature and mortality data for 823 regions. Estimated deaths: 4,822 under 65, 9,226 aged 65–79 and 36,848 over 79; 35,406 women and 21,667 men.

British Columbia coroners’ review of the 2021 heat dome (CTV News): 619 deaths; 67% aged 70 or older; 56% lived alone; 98% died indoors; most lacked adequate cooling. CDC lists older adults, infants and young children, people with chronic conditions, people without air conditioning, athletes, outdoor workers and pregnant people as at increased risk. Different studies use different areas and methods, so their totals are not directly comparable: the 2026 study counted about 29,000 deaths in 2022 for its 854 urban areas alone.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: ISGlobal (10 Jul 2023): record-breaking heat in the summer of 2022 caused more than 61,000 deaths in Europe (Ballester et al., Nature Medicine); CTV News: coroners’ review reveals true death toll of B.C.’s 2021 heat dome; US CDC: people at increased risk for heat-related illness; LSHTM (23 Oct 2026): more than 50,000 heat-related deaths estimated in European urban areas during summer 2026

Chapter VII · A warming world

What the IPCC found

IPCC AR6 Working Group I (2021), as reported by the WMO Bulletin and Carbon Brief: hot extremes, including heatwaves, have become more frequent and more intense across most land regions since the 1950s; it is virtually certain that human-induced greenhouse gas forcing is the main driver of observed changes in hot and cold extremes globally; some recent hot events would have been extremely unlikely without human influence. Hottest days in some mid-latitude and semi-arid regions are projected to warm at about 1.5 to 2 times the rate of global warming.

Reuters (9 August 2021), on the IPCC report: heatwaves that used to occur once every 50 years now happen about once a decade. IPCC AR6 Figure SPM.6, as reported by Actuaries Digital: a hot extreme that came once in 50 years in 1850–1900 becomes 8.6 times as frequent at 1.5 °C of warming, 13.9 times at 2 °C and 39.2 times at 4 °C. The “~5” for today is about once a decade instead of once in 50 years. Three warming figures appear in this film because they measure different periods: about 1.1 °C for 2011–2020 against 1850–1900 (IPCC, 2021); about 1.3 °C for the present climate used by World Weather Attribution; about 1.4 °C in the 2026 European study. How greenhouse gases warm the planet is explained in PlanetPast’s film on the greenhouse effect.

Try it: switch between the past and today. The bell curve is a schematic of daily summer temperatures; the share of days past the line is worked out from it and is not an IPCC figure.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: WMO Bulletin 71(1), 2022: regional trends in extreme events in the IPCC 2021 report; Carbon Brief (2021): what the new IPCC report says about extreme weather and climate change; Reuters via TimesLIVE (9 Aug 2021): once-in-50-year heatwaves now happening every decade, UN climate report; IPCC AR6 WGI, Figure SPM.6: projected changes in hot temperature extremes (baseline 1850–1900); Actuaries Digital (Aug 2021): IPCC AR6, hotter, wetter, faster, stronger (50-year hot extremes at 1.5, 2 and 4 °C)

Chapter VII · A warming world

Blaming the weather on the climate, carefully

Stott, Stone and Allen (Nature, 2004): the summer of 2003 was probably the hottest in Europe since at latest AD 1500, and it is very likely (confidence level >90%) that human influence has at least doubled the risk of a heatwave exceeding that threshold magnitude. World Weather Attribution methods: select and define the event; use observations to estimate how often it occurs today and how that has changed; use many climate models to simulate today’s world (about 1.3 °C of warming) and a cooler world without it; combine the results with 95% confidence intervals. WWA says its methods are peer reviewed and have been applied in more than 100 rapid studies, 26 of which were also published in peer-reviewed journals.

Pacific Northwest 2021 (IPSL, on the WWA study): the heat would have been virtually impossible without human influence and was made at least 150 times more likely; even at 2 °C of warming, such a heatwave would occur about once every 5 to 10 years. The 2026 European estimate applies the same two-worlds idea to deaths: it compares modelled deaths in today’s climate with those in a climate without human-caused warming (LSHTM).

Try it: switch between a world without warming and today’s world. The curves are schematic.

The 3D scenes are teaching models, not measurements: shapes, colours and curves are schematic unless a number is shown with a source.

Sources: Stott, Stone & Allen (2004), Nature: human contribution to the European heatwave of 2003; World Weather Attribution: methods; IPSL: North America heatwave almost impossible without climate change, World Weather Attribution study (2021); LSHTM (23 Oct 2026): more than 50,000 heat-related deaths estimated in European urban areas during summer 2026

Epilogue · Sources

Sources

Every fact in this film comes from the sources below. The chapter stills and pictures are AI-generated illustrations. The globe uses NASA Blue Marble imagery. The 3D scenes, bell curves and mortality curves are schematic; wet-bulb temperatures use Stull’s 2011 approximation and heat index values the US National Weather Service equation. Health content is general education, not medical advice: if someone may have heatstroke, call the emergency number (1669 in Thailand, per the Department of Disease Control). Much of heat mortality is preventable: the Global Heat Health Information Network summarises WHO’s call for heat-health action plans, early warning systems, urban planning and access to cooling.

Sources: LSHTM (23 Oct 2026): more than 50,000 heat-related deaths estimated in European urban areas during summer 2026; Trending Topics (23 Oct 2026): heat killed 50,000 people in Europe’s cities, two-thirds due to climate change; Democracy Now! headlines (23 Oct 2026): study, European heat waves killed 50,000 in 2026; UNDRR / WMO hazard information profile: heatwave; Met Office (2026): UK heatwaves, how the records tell a changing story; British Red Cross: what is a heatwave? (Met Office thresholds); India Meteorological Department: heat wave criteria (morning heat bulletin, annexure); American Meteorological Society, Glossary of Meteorology: heat dome; Wikipedia: lapse rate (dry adiabatic lapse rate); Met Office: the Russian heatwave of summer 2010; IPSL: North America heatwave almost impossible without climate change, World Weather Attribution study (2021); CTV News: coroners’ review reveals true death toll of B.C.’s 2021 heat dome; Scientific American / Nature (2014): mega-heatwaves mechanism pinpointed (Miralles et al., Nature Geoscience); US EPA: learn about heat island effects; Sherwood & Huber (2010), PNAS: an adaptability limit to climate change due to heat stress; US National Weather Service: heat safety (how the body cools, heat index); NHS 111 Wales: heat exhaustion and heatstroke; WHO (25 Jul 2024): UN Secretary-General calls for global action on extreme heat; Global Heat Health Information Network: WHO heat and health; Penn State (2022): humans can’t endure temperatures and humidities as high as previously thought (Vecellio et al., J. Appl. Physiol.); Stull (2011), J. Appl. Meteor. Climatol.: wet-bulb temperature from relative humidity and air temperature (calculator and validity range); NOAA Weather Prediction Center: the heat index equation; Wikipedia: Tak, Thailand (national record 45.4 °C, 15 April 2023); The Jakarta Post / AFP (22 Apr 2023): Thai authorities issue extreme heat warnings for dozens of provinces; ผู้จัดการออนไลน์ (13 เม.ย. 2567): ระดับดัชนีความร้อนของกรมอุตุนิยมวิทยา; Yale Environment 360 (17 May 2023): April heat wave in South Asia made 30 times more likely by climate change; AP via PBS NewsHour (13 Aug 2023): inconsistent methods for counting U.S. heat deaths; Robine et al. (2008), Comptes Rendus Biologies: death toll exceeded 70,000 in Europe during the summer of 2003; ISGlobal (10 Jul 2023): record-breaking heat in the summer of 2022 caused more than 61,000 deaths in Europe (Ballester et al., Nature Medicine); US CDC: people at increased risk for heat-related illness; WMO Bulletin 71(1), 2022: regional trends in extreme events in the IPCC 2021 report; Carbon Brief (2021): what the new IPCC report says about extreme weather and climate change; Reuters via TimesLIVE (9 Aug 2021): once-in-50-year heatwaves now happening every decade, UN climate report; IPCC AR6 WGI, Figure SPM.6: projected changes in hot temperature extremes (baseline 1850–1900); Actuaries Digital (Aug 2021): IPCC AR6, hotter, wetter, faster, stronger (50-year hot extremes at 1.5, 2 and 4 °C); Stott, Stone & Allen (2004), Nature: human contribution to the European heatwave of 2003; World Weather Attribution: methods; กรมควบคุมโรค สคร.12 สงขลา (7 มี.ค. 2567): เตือนระวังฮีทสโตรก โทรสายด่วน 1669; Wikipedia: Portugal (area 92,225 km²)