# The Heat Engine > Hurricanes, typhoons and cyclones are one kind of storm: a heat engine fuelled by warm seawater and spun by Earth's rotation. This interactive film explains how they form, why they spin, what the Saffir–Simpson categories mean, and why storm surge is often the deadliest part, with storms from Bhola and Katrina to Typhoon Gay in Chumphon. Interactive film: https://www.planetpast.com/stories/how-hurricanes-work/ Publisher: PlanetPast (https://www.planetpast.com/) · Published 2026-10-14 · Earth science, Physics ## Prologue ### The heat engine *October 2026 · Two hurricanes* In one week, two hurricanes hit North America. Both ran on the same fuel: warm seawater. On the ninth of October 2026, Hurricane Isaias came ashore near Destin, Florida, as a Category 2 storm, cutting power to more than 470,000 customers. A day later, off Mexico’s Pacific coast, Hurricane Simon reached Category 4, with winds of 240 kilometres per hour. Simon then crept slowly along the coast near Puerto Vallarta, drenching it with rain. How does a calm, warm sea turn into a machine like this? Let’s take one apart. ![Illustration: a mature hurricane with a clear eye.](https://assets.planetpast.com/a/45/45a9be485192c4a9.jpg) - **Cat 2**: Isaias at landfall near Destin, Florida, 9 Oct - **471,000+**: customers without power in Florida and Alabama - **Cat 4**: Simon, 10 Oct - **240 km/h**: maximum sustained wind (150 mph) ![Illustration: a hurricane spinning off a mountainous coast.](https://assets.planetpast.com/a/77/771e8338e39adc67.jpg) Prologue · Why the news matters #### Two hurricanes in one week On 9 October 2026 Hurricane Isaias made landfall near Destin, on Florida’s Gulf Coast, as a Category 2 hurricane with 105 mph (169 km/h) winds, after briefly reaching Category 3. More than 471,000 customers lost power in Florida and Alabama. On 10 October Hurricane Simon, off Mexico’s Pacific coast, reached Category 4 with maximum sustained winds of 150 mph (240 km/h). It weakened to Category 3 on 11 October and Category 2 on 12 October while moving slowly near Puerto Vallarta, with forecasts of extreme rain in Jalisco, Colima and Michoacán. - **Isaias, 9 Oct**: Cat 2 landfall near Destin, Florida - **Simon, 10 Oct**: Cat 4 150 mph (240 km/h) sustained This film is not a news report. It explains what is inside every storm like these, so the numbers in the next forecast make sense. The globe shows where the two storms were (markers not to scale). How to use: pick a chapter below the scene or press ← → to change step · ▶ plays the step · drag to rotate, scroll or pinch to zoom · “Play the film” runs every step full-screen. Sources: [Bernama–Xinhua: Hurricane Isaias makes landfall in Florida (10 Oct 2026)](https://bernama.com/en/world/news.php?id=2618356); [AP via WECT: Simon strengthens into a Category 4 hurricane (10 Oct 2026)](https://www.wect.com/2026/10/10/simon-strengthens-into-category-4-hurricane-it-approaches-mexicos-coast/); [AP via WECT: Hurricane Simon pounds Mexico’s Pacific coast (11 Oct 2026)](https://www.wect.com/2026/10/11/ferocious-hurricane-simon-nears-mexico-with-155-mph-winds-flooding-fears/); [CBS News: Hurricane Simon lashes Mexico’s Pacific coast (12 Oct 2026)](https://www.cbsnews.com/news/hurricane-simon-takes-aim-mexico-pacific-coast/) ### Hurricane, typhoon, cyclone *One storm, three names* The same kind of storm gets a different name in each ocean. Scientists call these storms tropical cyclones: huge, rotating systems of thunderstorms over warm tropical seas. Once the sustained wind reaches 119 kilometres per hour, about 74 miles per hour, a storm in the Atlantic or the northeast Pacific is called a hurricane. In the northwest Pacific, the same storm is a typhoon. In the Indian Ocean and the South Pacific, it is simply a tropical cyclone. Worldwide, around 86 of them a year grow at least to tropical-storm strength. ![A rotating system of clouds and thunderstorms over warm water (illustration).](https://assets.planetpast.com/a/93/93148e3a5c018169.jpg) - **119 km/h**: sustained wind for a hurricane or typhoon (74 mph) - **63 km/h**: tropical storm (39 mph) - Hurricane: North Atlantic, central and eastern North Pacific - Typhoon: northwest Pacific - Tropical cyclone: South Pacific and Indian Ocean - “Hurricane” comes from Huricán, a Taíno name; “typhoon” perhaps from Chinese or Greek - **~86**: tropical storms or stronger, per year, worldwide - **~47**: reach hurricane strength Prologue · Step 1 · Names #### One storm, three names “Tropical cyclone” is the scientific name for a rotating, organised system of clouds and thunderstorms that forms over tropical or subtropical waters. When its maximum sustained wind reaches 74 mph (119 km/h), it is called a hurricane in the North Atlantic and the central and eastern North Pacific, a typhoon in the northwest Pacific, and a tropical cyclone in the South Pacific and Indian Ocean. - **Hurricane / typhoon**: ≥ 119 km/h 74 mph sustained wind - **Tropical storm**: ≥ 63 km/h 39 mph; storms are named from here - **Each year, worldwide**: about 86 tropical storms, about 47 of them reach hurricane strength long-term averages quoted in Wikipedia’s article on tropical cyclogenesis The labels on the globe mark the main basins. “Hurricane” comes from the Taíno name Huricán, through Spanish huracán; “typhoon” may come from Chinese (dà fēng, “great wind”) or from the Greek Typhon via Persian and Arabic. Sources: [NOAA Ocean Service: Hurricane, cyclone or typhoon?](https://oceanservice.noaa.gov/facts/cyclone.html); [Wikipedia: Tropical cyclogenesis](https://en.wikipedia.org/wiki/Tropical_cyclogenesis); [New World Encyclopedia: Tropical cyclone](https://www.newworldencyclopedia.org/entry/Tropical_cyclone) ## Chapter I · The ingredients ### A warm ocean *Ingredient one* The fuel tank is the top fifty metres of the tropical sea. Every tropical cyclone starts with warm water: at least 26.5 degrees Celsius, reaching down about fifty metres. Warm water evaporates quickly. The air just above it fills with water vapour, and that vapour carries hidden energy, called latent heat. Try it in the scene. Cool the sea below 26.5 degrees and the storm starves. Warm it up and the storm grows. - **≥ 26.5 °C**: sea surface temperature (80 °F) - **~50 m**: depth the warm layer must reach ![Sunlight warms the top of the tropical ocean (illustration).](https://assets.planetpast.com/a/e6/e6b5a773d18715d5.jpg) - Warm water evaporates - Vapour stores latent heat - When it condenses into rain, the heat is released Chapter I · Step 2 · Warm water #### A warm ocean NOAA’s Hurricane Research Division lists warm ocean water of at least 26.5 °C (80 °F) through a depth of roughly 50 m as the first condition for a tropical cyclone to form. Warm water evaporates readily; the vapour carries latent heat, energy that is released again when the vapour condenses into cloud and rain. - **Sea surface**: ≥ 26.5 °C 80 °F - **Warm layer**: ~50 m deep “on the order of 50 m”; the exact depth is unknown Interactive: the buttons set the sea temperature under a simplified storm. The response is schematic: a real storm also needs the other ingredients in the next step, and takes days, not seconds, to grow. Vertical scale exaggerated. Sources: [NOAA AOML Hurricane FAQ: How do tropical cyclones form?](https://www.aoml.noaa.gov/hrd/tcfaq/A15.html); [New World Encyclopedia: Tropical cyclone](https://www.newworldencyclopedia.org/entry/Tropical_cyclone) ### The right weather *Ingredients two to five* Warm water is not enough. Four more things must line up. Warm water is not enough. The air higher up, about five kilometres above the sea, must be moist, so rising clouds do not dry out. The wind must be nearly the same at all heights. Strong wind shear, more than about ten metres per second, tears a young storm apart. There must be a seed: a cluster of thunderstorms that is already turning a little. And it must be at least 500 kilometres from the equator. We will see why when we look at the spin. ![Towering clouds grow only if the air around them stays humid (illustration).](https://assets.planetpast.com/a/4b/4bcab4f1d568c1b2.jpg) - **< 10 m/s**: vertical wind shear (about 36 km/h) - Warm sea ≥ 26.5 °C, ~50 m deep - Moist air near 5 km up - Low wind shear - A seed: thunderstorms already turning - ≥ 500 km from the equator - **≥ 500 km**: from the equator Chapter I · Step 3 · The recipe #### The right weather NOAA lists the other conditions for genesis: relatively moist air near the middle of the troposphere (about 5 km up); low vertical wind shear, less than about 10 m/s (20 knots) between the surface and the upper troposphere; a pre-existing near-surface disturbance with enough spin and inflow; and a distance of at least about 500 km from the equator. Even with all of them, most disturbances never become cyclones. - Warm ocean: ≥ 26.5 °C through ~50 m. - Moist mid-level air, so clouds keep growing. - Low wind shear, so the storm’s heat stays stacked over its centre. - A seed disturbance that already turns. - ≥ ~500 km from the equator, for the Coriolis effect. On the globe, the orange tint marks tropical ocean that is typically warm enough in its warm season. It is a schematic band, not a measured temperature map. The pale strip is the equatorial zone where cyclones almost never form. Sources: [NOAA AOML Hurricane FAQ: How do tropical cyclones form?](https://www.aoml.noaa.gov/hrd/tcfaq/A15.html); [Wikipedia: Tropical cyclogenesis](https://en.wikipedia.org/wiki/Tropical_cyclogenesis) ## Chapter II · The engine ### A chimney of rising air *How it works* Air spirals in at the sea surface, rises in the eyewall, and pours out at the top. Now the engine. We have cut the storm in half. Near the sea surface, moist air spirals inward, toward low pressure at the centre. Close to the centre it shoots upward, in a ring of towering clouds. As it rises it cools, and its vapour condenses into rain. Condensation releases the latent heat the sea gave it. That heat warms the column of air, which lowers the pressure at the surface, and pulls in even more air. At the top of the storm, the dried-out air spreads outward, in a high, thin canopy of cirrus. ![Moist air rises and builds towering clouds (illustration).](https://assets.planetpast.com/a/8b/8b416796faee0807.jpg) - 1 · Moist air spirals in over the sea - 2 · It rises in the eyewall and cools - 3 · Vapour condenses: heat released - 4 · Warm column → lower surface pressure → more inflow - **Warm core**: heat from condensation, not from clashing air masses ![At the top, air spreads out as thin cirrus, often the first sign of an approaching storm (illustration).](https://assets.planetpast.com/a/6e/6edc6de1414e66f2.jpg) Chapter II · Step 4 · The engine #### A chimney of rising air A tropical cyclone is a “warm-core” storm: its energy comes from the heat released when water vapour condenses, not from contrasts between air masses as in mid-latitude storms. Near the surface, moist air flows in toward the low pressure at the centre. Close to the centre it turns upward in the eyewall. Rising air cools, its vapour condenses, and the released latent heat warms the column. The warmer column lowers the surface pressure, which draws in more air. At the top, the air flows outward as high cirrus. - **Inflow**: near the sea surface moist air spiralling toward the centre - **Outflow**: at the top thin cirrus spiralling away The scene is a cut-away: the front half of the storm is removed. Cyan dots are inflowing moist air, orange marks where heat is released in the eyewall, pale dots the outflow. Heights are exaggerated about ten times compared with the width. Sources: [New World Encyclopedia: Tropical cyclone](https://www.newworldencyclopedia.org/entry/Tropical_cyclone); [Hurricanes: Science and Society (University of Rhode Island): Hurricane structure](https://hurricanescience.org/science/science/hurricanestructure/) ### Two hundred times our power grid *A heat engine* Almost all of a hurricane’s energy goes into heat, not wind. This makes a hurricane a heat engine. It turns heat from the ocean into motion, the way a car engine turns burning fuel into motion. NOAA’s hurricane researchers estimate that the rain of an average hurricane releases about 600 trillion watts of heat. That is about 200 times the electricity-generating capacity of the whole world, at the time of their estimate. Only about one part in 400 ends up as wind. Even that is around half the world’s generating capacity. ![Condensation inside towering clouds releases enormous heat (illustration).](https://assets.planetpast.com/a/19/192074b33d47faf8.jpg) - **6 × 10¹⁴ W**: heat released by the rain of an average hurricane - **≈ 200 ×**: world electricity-generating capacity - Heat from rain: 600 TW - Wind (kinetic energy): 1.5 TW Chapter II · Step 5 · The power #### Two hundred times our power grid NOAA’s Hurricane Research Division gives two estimates for an average hurricane. Counting the heat released by its rain (about 2.1 × 10¹⁶ cm³ a day): 5.2 × 10¹⁹ joules a day, or 6 × 10¹⁴ watts, about 200 times the world’s electrical generating capacity. Counting only the energy that keeps its winds going: 1.3 × 10¹⁷ joules a day, or 1.5 × 10¹² watts, about half of world capacity. The heat is roughly 400 times the wind energy. - **Heat released**: 6 × 10¹⁴ W ≈ 200 × world generating capacity - **Wind**: 1.5 × 10¹² W ≈ ½ × world generating capacity The comparison uses world generating capacity as given in NOAA’s FAQ, which was written some years ago; capacity has grown since, so today’s ratio would be somewhat smaller. The orders of magnitude are what matter. Sources: [NOAA AOML Hurricane FAQ: How much energy does a hurricane release?](https://www.aoml.noaa.gov/hrd/tcfaq/D7.html); [New World Encyclopedia: Tropical cyclone](https://www.newworldencyclopedia.org/entry/Tropical_cyclone) ## Chapter III · The spin ### The Coriolis effect *Why it spins* Earth’s rotation turns inflowing air into a whirl, in opposite directions north and south. Why does the storm spin? Because the Earth turns beneath it. Air rushing toward low pressure is nudged sideways by Earth’s rotation: to the right in the Northern Hemisphere, to the left in the Southern. This is the Coriolis effect. So northern storms turn counterclockwise, and southern storms clockwise. Use the buttons to switch hemispheres. At the equator, the sideways push drops to zero. Air flows straight in, and no spin can build. That is why storms almost never form there. ![Spiral cloud bands wind in toward the centre (illustration).](https://assets.planetpast.com/a/93/93148e3a5c018169.jpg) $$f = 2\,\Omega\,\sin\varphi$$ - $f$: Coriolis parameter: strength of the sideways push - $\Omega$: Earth’s rotation rate (one turn a day) - $\varphi$: latitude sin 0° = 0, so f vanishes at the equator. It is positive in the north and negative in the south. - **↺**: Northern Hemisphere: counterclockwise - **↻**: Southern Hemisphere: clockwise - No spin can build at the equator - Myth: sinks do not drain the other way “because of Coriolis” - In a sink, the force is far too weak; the basin’s shape decides Chapter III · Step 6 · The Coriolis effect #### Why storms spin On a rotating Earth, moving air is deflected: to the right in the Northern Hemisphere and to the left in the Southern. Air converging on a low-pressure centre is therefore turned into a counterclockwise whirl in the north and a clockwise one in the south. The strength of the effect is set by the Coriolis parameter f = 2Ω sin φ, which is zero at the equator. That is why cyclones normally form at least about 500 km (around 4.5–5° of latitude) from it; a few rare exceptions exist. - **North**: ↺ counterclockwise - **South**: ↻ clockwise Interactive: the scene is a simplified flat patch of ocean with a low in the middle (schematic, not to scale). Each dot is a parcel of air pulled toward the low and pushed sideways by the Coriolis effect, with friction. Switch between north, equator and south. NOAA notes that the effect is far too weak to decide which way a sink drains. Sources: [NOAA AOML Hurricane FAQ: Why do tropical cyclones rotate?](https://www.aoml.noaa.gov/hrd/tcfaq/D3.html); [University at Albany ATM 317: Apparent forces and the Coriolis force](https://www.atmos.albany.edu/facstaff/kristen/ATM317/Reference_Material_files/ApparentForces_Coriolis.pdf); [Wikipedia: Tropical cyclogenesis](https://en.wikipedia.org/wiki/Tropical_cyclogenesis) ### The eye and the wall *Anatomy of a hurricane* The calmest place in the storm sits right next to the most violent. Look down on a mature hurricane. At its centre is the eye, usually 20 to 50 kilometres across, with light winds and often a clear sky. Around it stands the eyewall, a ring of towering clouds with the strongest winds and the heaviest rain. Spiral rainbands curl in toward it, bringing bursts of rain and gusts, with calmer gaps between them. The whole storm is usually a few hundred kilometres across. The largest on record, Typhoon Tip in 1979, measured 2,220 kilometres. - **20–50 km**: typical width of the eye ![From inside the eye, the eyewall rises like a stadium (illustration).](https://assets.planetpast.com/a/cf/cfe8234e3221f1ed.jpg) ![Spiral bands of heavy rain with drier gaps (illustration).](https://assets.planetpast.com/a/3e/3ec406d9b600a213.jpg) - **2,220 km**: Typhoon Tip, 1979: largest tropical cyclone on record - **~100 km**: Cyclone Tracy’s whole wind field at landfall Chapter III · Step 7 · Structure #### The eye and the wall The eye of a hurricane is generally 20–50 km wide, nearly free of cloud, with light winds. The eyewall around it holds the strongest winds and heaviest rain; it forms where warm, inflowing air turns upward. Outside it, spiral rainbands bring heavy rain, wind and sometimes tornadoes, with gaps of little rain between them. Hurricane-force winds typically span about 160 km; tropical-storm winds can reach almost 500 km from the eye of a large hurricane. - **Eye**: 20–50 km light winds, often clear - **Strongest winds**: 10–50 km from the centre, in the eyewall - **Largest on record**: Typhoon Tip, October 1979: 2,220 km across nearly double the previous record The coloured streaks are wind tracers: they move fastest (red) in the eyewall and slow down outward. Their speeds are scaled down for viewing. On the side to the right of the storm’s motion, the forward speed adds to the wind, which is why that quadrant is usually the most destructive. Sources: [Hurricanes: Science and Society (University of Rhode Island): Hurricane structure](https://hurricanescience.org/science/science/hurricanestructure/); [Wikipedia: Typhoon Tip](https://en.wikipedia.org/wiki/Typhoon_Tip) ## Chapter IV · How strong ### Category 1 to 5 *Measuring a storm* The Saffir–Simpson scale ranks hurricanes by wind alone. Forecasters rank hurricanes on the Saffir–Simpson scale, from Category 1 to 5, using the highest wind averaged over one minute. Category 1 starts at 119 kilometres per hour. From Category 3, at 178, a storm counts as a major hurricane. Category 5 begins at 252. Press the categories, and watch the storm tighten. But the scale measures only wind. It says nothing about rain or storm surge, which often do the most harm. ![Reconnaissance aircraft fly into storms to measure wind and pressure (illustration).](https://assets.planetpast.com/a/0c/0cfdf06e9a30f5d8.jpg) - Cat 1: 119–153 - Cat 2: 154–177 - Cat 3: 178–208 - Cat 4: 209–251 - Cat 5: ≥ 252 - 1971: Herbert Saffir, an engineer, devises the scale - 1973: Robert Simpson’s version is made public - 2010: Pressure and surge removed: a pure wind scale - Not in the scale: storm surge - Not in the scale: rainfall and floods - Not in the scale: size of the storm Chapter IV · Step 8 · The Saffir–Simpson scale #### Category 1 to 5 The Saffir–Simpson Hurricane Wind Scale rates a hurricane by its highest one-minute average wind at 10 m above the surface. Civil engineer Herbert Saffir devised it in 1971; Robert Simpson, then director of the US National Hurricane Center, added storm surge and flooding, and it went public in 1973. Since 2010 it has been a pure wind scale. Category 3 and above are “major hurricanes”. - **Cat 1**: 119–153 km/h 74–95 mph - **Cat 2**: 154–177 km/h 96–110 mph - **Cat 3**: 178–208 km/h 111–129 mph · major - **Cat 4**: 209–251 km/h 130–156 mph · major - **Cat 5**: ≥ 252 km/h ≥ 157 mph · major Interactive: the category buttons change the simulated storm’s maximum wind. The scale does not account for storm surge, rainfall or the size of the storm; forecasters warn about those separately. Other agencies, such as the Japan Meteorological Agency, use 10-minute average winds and their own categories. Sources: [Wikipedia: Saffir–Simpson scale](https://en.wikipedia.org/wiki/Saffir%E2%80%93Simpson_scale) ### Rapid intensification *Explosive growth* Over very warm water, a storm can gain strength frighteningly fast. Sometimes a storm strengthens explosively. When its wind jumps by at least 30 knots, about 55 kilometres per hour, within 24 hours, forecasters call it rapid intensification. The record belongs to Hurricane Patricia, off the same Mexican coast as Simon. In October 2015, its winds rose from 137 to 330 kilometres per hour in just 24 hours. At its peak, Patricia reached 345 kilometres per hour, the strongest sustained wind ever reliably measured. The lowest pressure on record, 870 hectopascals, belongs to Typhoon Tip. - **≥ 30 kt**: wind increase within 24 h (≈ 55 km/h) - 22 Oct 2015, 06 UTC: 137 km/h - 23 Oct 2015, 06 UTC: 330 km/h - **345 km/h**: Patricia, 23 Oct 2015: highest sustained wind reliably measured - **870 hPa**: Typhoon Tip, 12 Oct 1979: lowest pressure on record Chapter IV · Step 9 · Rapid intensification #### Explosive growth The US National Hurricane Center calls an increase of at least 30 knots in maximum one-minute sustained wind within 24 hours “rapid intensification”. It is typically associated with warm sea surface temperatures and high ocean heat content. Hurricane Patricia went from 85 to 205 mph (about 137 to 330 km/h) between 06 UTC on 22 October and 06 UTC on 23 October 2015, while its central pressure fell 95 hPa: the fastest such increase the NHC has observed. It peaked at 215 mph (345 km/h), the highest one-minute sustained wind reliably observed or estimated anywhere, and made landfall in Jalisco, Mexico, as a Category 4 storm. - **Patricia, 2015**: 345 km/h 872 hPa, the Western Hemisphere’s most intense - **Tip, 1979**: 870 hPa world’s lowest recorded pressure The scene replays the storm’s 24 hours of growth in a few seconds (schematic). Researchers have questioned whether a few later typhoons were even more intense than Tip, but conclusive measurements are lacking, so Tip’s record still stands. Sources: [Wikipedia: Rapid intensification](https://en.wikipedia.org/wiki/Rapid_intensification); [Wikipedia: Hurricane Patricia](https://en.wikipedia.org/wiki/Hurricane_Patricia); [Wikipedia: Typhoon Tip](https://en.wikipedia.org/wiki/Typhoon_Tip) ## Chapter V · The sea rises ### Storm surge *The water* In many of history’s worst storms, the killer was the sea itself. In many of history’s worst storms, the killer was not the wind, but the sea. Storm surge is the rise in water that a storm causes, above the normal tide. Wind drives water toward the shore. And low air pressure lets the sea bulge upward: about one centimetre for every hectopascal the pressure drops. A wide, shallow sea floor leaves the water nowhere to go, so the surge piles higher. Add a high tide, and you get the storm tide. Use the buttons to change the tide and the shape of the sea floor. ![Seawater pushed over a seawall into a coastal street (illustration).](https://assets.planetpast.com/a/25/25fa76c232c99e57.jpg) $$\Delta h \approx 1\ \text{cm} \times \Delta p\,[\text{hPa}]$$ - $\Delta h$: rise of the sea from low pressure - $\Delta p$: drop in air pressure A 100 hPa drop raises the sea about 1 m. Wind pushing water ashore usually adds more. - Wide, shallow shelf → higher surge, smaller waves - Narrow shelf, deep water → lower surge, bigger waves - Storm tide = storm surge + the normal tide - **surge**: height above the predicted tide - **storm tide**: total water level: surge + tide Chapter V · Step 10 · Storm surge #### When the sea comes ashore Storm surge is the rise of water generated by a storm, above the predicted astronomical tide; storm tide is the total water level, surge plus tide. Wind pushes water toward the shore, and the main meteorological factor is strong wind over a long fetch. Low pressure also lets the sea rise, by about 10 mm for every hectopascal of pressure drop. A wide, shallow continental shelf produces a higher surge with smaller waves; a narrow shelf with deep water close to shore produces a lower surge but bigger waves. - **Pressure effect**: ~1 cm / hPa ~1 m for a 100 hPa drop - **Storm tide**: surge + tide what actually floods the coast Interactive: a schematic cross-section of a coast (heights in metres, distances not to scale). The buttons switch between low and high tide and between a wide shallow shelf and a steep one. The surge heights in the readout are illustrative values chosen to show the effect, not a forecast for any real place. Sources: [Wikipedia: Storm surge](https://en.wikipedia.org/wiki/Storm_surge); [NOAA National Weather Service: Defining storm surge, storm tide and inundation](https://ocean.weather.gov/defining_storm_surge.pdf) ### The deadliest storms *When the sea comes in* The worst tropical cyclone disasters were floods from the sea. In November 1970, the Bhola cyclone drove a surge about ten metres high into the low delta of what is now Bangladesh. Between 300,000 and 500,000 people died. It is the deadliest tropical cyclone on record. In 2005, Hurricane Katrina’s surge reached about eight and a half metres on the Mississippi coast. Katrina killed 1,392 people. Low, flat coastlines facing wide, shallow seas are where a surge reaches farthest inland. ![Low river deltas like this face the Bay of Bengal (illustration).](https://assets.planetpast.com/a/53/5357bf96a6ffafe1.jpg) - **300,000–500,000**: estimated deaths, Bhola cyclone, 12 Nov 1970 - **~10 m**: storm surge - **~8.5 m**: Katrina’s surge at Bay St. Louis and Pass Christian, Mississippi - **1,392**: deaths ![Where land is flat and the sea shallow, water travels far inland (illustration).](https://assets.planetpast.com/a/89/89f2022507ef90ce.jpg) Chapter V · Step 11 · The deadliest storms #### When the sea comes in The Bhola cyclone struck East Pakistan (now Bangladesh) and West Bengal on 12 November 1970 with a storm surge of about 10 m. An estimated 300,000 to 500,000 people died, making it the deadliest tropical cyclone on record. The slow official relief response fuelled anger in East Pakistan, part of the chain of events that led to the 1971 Bangladesh Liberation War. Hurricane Katrina came ashore on 29 August 2005 as a Category 3 storm; its surge was 25–28 ft (7.6–8.5 m) along the Mississippi coast and 10–20 ft in southeast Louisiana. It killed 1,392 people. - **Bhola, 1970**: ~10 m surge 300,000–500,000 deaths (estimate) - **Katrina, 2005**: ~8.5 m surge 1,392 deaths Katrina was “only” Category 3 at landfall, a reminder that the wind category does not measure surge. Markers on the globe show where these storms struck (not to scale). Sources: [Wikipedia: 1970 Bhola cyclone](https://en.wikipedia.org/wiki/1970_Bhola_cyclone); [NOAA National Weather Service New Orleans: Hurricane Katrina](https://www.weather.gov/lix/katrina_anniversary); [Wikipedia: Storm surge](https://en.wikipedia.org/wiki/Storm_surge) ## Chapter VI · Closer to home ### Storms in the Gulf of Thailand *Closer to home* Over land a storm loses its fuel. The coast takes the hardest blow. Over land, a storm loses its fuel, warm seawater, and weakens fast. That is why coasts take the hardest blow. In 1962, Tropical Storm Harriet struck Laem Talumphuk, in Nakhon Si Thammarat. Its surge and winds killed more than 900 people. In November 1989, Typhoon Gay hit Chumphon: the first typhoon to make landfall in Thailand since 1891. More than 500 people died. A storm also stirs up cold water from below, leaving a cooler trail in the sea. A storm that stalls over its own cold trail tends to weaken. ![Fishing boats pulled ashore as a storm approaches (illustration).](https://assets.planetpast.com/a/4a/4a65db5824e60312.jpg) ![A low sandy cape in Pak Phanang, Nakhon Si Thammarat. Harriet’s surge killed more than 900 people (illustration).](https://assets.planetpast.com/a/da/da0bec5fef7c1ce0.jpg) - 1891: last typhoon landfall in Thailand before Gay - 1962: Harriet, Laem Talumphuk: more than 900 deaths - 4 Nov 1989: Typhoon Gay, Chumphon: more than 500 deaths - Over land: no warm water → the engine stops - Cold wake: churned-up deep water cools the surface - Cooler than ~26.5 °C → the storm loses its tropical character Chapter VI · Step 12 · Closer to home #### Storms in the Gulf of Thailand Over land a tropical cyclone loses its supply of heat and moisture and weakens quickly. It can also weaken over the sea: its winds churn up colder water from below, and a storm that lingers can cool the surface too much to sustain itself. In 1962 Tropical Storm Harriet made landfall at Laem Talumphuk in Pak Phanang, Nakhon Si Thammarat; its surge killed more than 900 people and left over 10,000 homeless. On 4 November 1989 Typhoon Gay struck Chumphon, the first typhoon to make landfall in Thailand since 1891. The day before, it capsized the drilling ship Seacrest in the Gulf; only 6 of its 97 crew survived. - **Harriet, 1962**: 900+ deaths Laem Talumphuk, Nakhon Si Thammarat - **Gay, 1989**: 500+ deaths Chumphon; first typhoon landfall since 1891 Death tolls for Gay differ: Wikipedia’s article on the storm gives 558 deaths on land and 44 offshore in Thailand; its list of Thai disasters gives 833. Both are well above 500. Markers on the globe are not to scale. Sources: [Wikipedia: Typhoon Gay (1989)](https://en.wikipedia.org/wiki/Typhoon_Gay); [Wikipedia: List of disasters in Thailand](https://en.wikipedia.org/wiki/List_of_disasters_in_Thailand); [Khaosod English: South warned of first tropical storm since 1962 disaster (2019)](https://www.khaosodenglish.com/news/2019/01/01/south-warned-of-first-tropical-storm-since-1962-disaster/amp/); [New World Encyclopedia: Tropical cyclone](https://www.newworldencyclopedia.org/entry/Tropical_cyclone) ## Chapter VII · A warmer ocean ### What climate change does to storms *A warmer ocean* Not necessarily more storms, but stronger and wetter ones. What happens as the oceans warm? NOAA’s Geophysical Fluid Dynamics Laboratory, which reviews the research, summarises projections for two degrees of global warming. The strongest winds of tropical cyclones are projected to rise by about 1 to 10 percent, and the rain near a storm by about 14 percent. A larger share of storms is projected to reach Category 4 or 5, while the total number of storms may stay the same, or fall. And because sea level is rising, the same storm surge will reach farther inland. ![Each surge starts from a higher sea level (illustration).](https://assets.planetpast.com/a/7f/7f6876d9e6dccd00.jpg) - **+1–10 %**: maximum wind speed - **≈ +14 %**: rain rate within ~100 km of the centre - More of the storms reach Category 4–5 - Total number: same or fewer (most studies) - Higher sea level → more coastal flooding from the same storm Chapter VII · Step 13 · A warmer ocean #### What climate change does to storms NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL) summarises the research for about 2 °C of global warming: tropical cyclone wind speeds rise globally by about 1–10%, and rain rates within about 100 km of the storm by about 14% (range +6 to +22%), both with medium to high confidence. The proportion of storms reaching Category 4–5 is projected to increase, while most studies project the total number of tropical cyclones to fall or change little. Rising sea level, which the IPCC says has very likely been driven mainly by human activity since 1971, makes coastal flooding from a given storm worse. - **Maximum wind**: +1–10 % for ~2 °C of warming - **Rain near the storm**: ≈ +14 % range +6 to +22 % - **Category 4–5 share**: projected to rise total number: same or fewer in most studies Interactive: “Warmer ocean” raises the simulated storm’s wind by about 5% and its rain, the middle of the projected ranges. It is a schematic comparison, not a forecast for any storm. Whether one particular storm such as Simon was made stronger by warming is a separate question that needs a dedicated attribution study. Sources: [NOAA GFDL: Global warming and hurricanes](https://www.gfdl.noaa.gov/global-warming-and-hurricanes/) ## Epilogue ### Reading the forecast *Epilogue* Now you know what the numbers in the news mean. So, when the news says a Category 4 hurricane is coming, you know what it means: winds of more than 209 kilometres per hour, wrapped around a calm eye. Behind it is a heat engine, fuelled by warm seawater, spun by the turning Earth, and most dangerous where it pushes the sea onto low coasts. Hurricane, typhoon or cyclone, it is the same machine. When the warnings come, follow the advice of local authorities. - 1970: Bhola cyclone: deadliest on record - 1979: Typhoon Tip: 870 hPa, 2,220 km - 1989: Typhoon Gay hits Chumphon - 2005: Hurricane Katrina - 2015: Hurricane Patricia: 345 km/h - 2026: Isaias and Simon - Category = 1-minute sustained wind only - Surge and rain are warned about separately - The right-front side of a storm is usually the worst - Follow local official warnings ![All stills in this film are AI-generated illustrations, not photographs.](https://assets.planetpast.com/a/00/0060d7923f1143ea.jpg) Epilogue · Reading the forecast #### One machine, many names - 1891 Last typhoon landfall in Thailand before 1989 - 1962 Tropical Storm Harriet, Laem Talumphuk: more than 900 deaths - 1970 Bhola cyclone: ~10 m surge, 300,000–500,000 deaths - 1971–73 Saffir–Simpson scale devised and made public - 1979 Typhoon Tip: 870 hPa, 2,220 km across - 1989 Typhoon Gay hits Chumphon - 2005 Hurricane Katrina: ~8.5 m surge, 1,392 deaths - 2015 Hurricane Patricia: 345 km/h, fastest intensification observed - 2026 Hurricanes Isaias (Florida) and Simon (Mexico) Simplified models: the 3D storm, the Coriolis patch and the coastal cross-section are schematic, with exaggerated heights and slowed winds. The globe uses NASA Blue Marble imagery; the warm-water band is schematic. All full-screen stills and media pictures are AI-generated illustrations, not photographs. This film is education, not a forecast: in a real storm, follow your local meteorological service and civil-defence authorities. #### Sources [Bernama–Xinhua: Hurricane Isaias makes landfall in Florida (10 Oct 2026)](https://bernama.com/en/world/news.php?id=2618356) [AP via WECT: Simon strengthens into a Category 4 hurricane (10 Oct 2026)](https://www.wect.com/2026/10/10/simon-strengthens-into-category-4-hurricane-it-approaches-mexicos-coast/) [AP via WECT: Hurricane Simon pounds Mexico’s Pacific coast (11 Oct 2026)](https://www.wect.com/2026/10/11/ferocious-hurricane-simon-nears-mexico-with-155-mph-winds-flooding-fears/) [CBS News: Hurricane Simon lashes Mexico’s Pacific coast (12 Oct 2026)](https://www.cbsnews.com/news/hurricane-simon-takes-aim-mexico-pacific-coast/) [NOAA Ocean Service: Hurricane, cyclone or typhoon?](https://oceanservice.noaa.gov/facts/cyclone.html) [NOAA AOML Hurricane FAQ: How do tropical cyclones form?](https://www.aoml.noaa.gov/hrd/tcfaq/A15.html) [NOAA AOML Hurricane FAQ: How much energy does a hurricane release?](https://www.aoml.noaa.gov/hrd/tcfaq/D7.html) [NOAA AOML Hurricane FAQ: Why do tropical cyclones rotate?](https://www.aoml.noaa.gov/hrd/tcfaq/D3.html) [University at Albany ATM 317: Apparent forces and the Coriolis force](https://www.atmos.albany.edu/facstaff/kristen/ATM317/Reference_Material_files/ApparentForces_Coriolis.pdf) [Wikipedia: Tropical cyclogenesis](https://en.wikipedia.org/wiki/Tropical_cyclogenesis) [New World Encyclopedia: Tropical cyclone](https://www.newworldencyclopedia.org/entry/Tropical_cyclone) [Hurricanes: Science and Society (University of Rhode Island): Hurricane structure](https://hurricanescience.org/science/science/hurricanestructure/) [Wikipedia: Saffir–Simpson scale](https://en.wikipedia.org/wiki/Saffir%E2%80%93Simpson_scale) [Wikipedia: Rapid intensification](https://en.wikipedia.org/wiki/Rapid_intensification) [Wikipedia: Hurricane Patricia](https://en.wikipedia.org/wiki/Hurricane_Patricia) [Wikipedia: Typhoon Tip](https://en.wikipedia.org/wiki/Typhoon_Tip) [Wikipedia: Storm surge](https://en.wikipedia.org/wiki/Storm_surge) [NOAA National Weather Service: Defining storm surge, storm tide and inundation](https://ocean.weather.gov/defining_storm_surge.pdf) [Wikipedia: 1970 Bhola cyclone](https://en.wikipedia.org/wiki/1970_Bhola_cyclone) [NOAA National Weather Service New Orleans: Hurricane Katrina](https://www.weather.gov/lix/katrina_anniversary) [Wikipedia: Typhoon Gay (1989)](https://en.wikipedia.org/wiki/Typhoon_Gay) [Wikipedia: List of disasters in Thailand](https://en.wikipedia.org/wiki/List_of_disasters_in_Thailand) [Khaosod English: South warned of first tropical storm since 1962 disaster (2019)](https://www.khaosodenglish.com/news/2019/01/01/south-warned-of-first-tropical-storm-since-1962-disaster/amp/) [NOAA GFDL: Global warming and hurricanes](https://www.gfdl.noaa.gov/global-warming-and-hurricanes/)