An interactive field guide
How a warm sea builds a storm
On 28 October a storm called Arnab came ashore on India's east coast. Two days earlier it had been a loose cluster of thunderstorms over the Bay of Bengal. This is how a warm ocean turns a patch of clouds into a spinning engine hundreds of kilometres wide, and what happens next.
Satellite Meteosat infrared, 27 October 2026. The brighter the cloud, the higher and colder its top.
00Arnab · 24–29 October 2026
Going by the US Navy's Joint Typhoon Warning Center, Arnab was the strongest October-to-December storm in the Bay of Bengal since Phailin in 2013. It strengthened far faster than forecast, stalled on the coast, and spread heavy rain across eastern India.
Two numbers for one storm's wind is the first clue that cyclones are easy to misread. Start at the beginning: four days earlier, in the middle of the Bay.
01Seventy-two hours
At midday UTC on 24 October, India's weather department marked a depression over the central Bay of Bengal: a patch of low pressure with winds under 50 km/h and thunderstorms starting to cluster. Most never become more than this.
By the 25th the highest, coldest cloud tops were wrapping around one point. The sea beneath was close to 30°C, warm enough to feed it.
Early on the 26th its winds passed 63 km/h and it took the next name on the list: Arnab, Bengali for ocean, put forward by Bangladesh. A storm only gets a name once it is this strong.
That day's IMD bulletin expected a very severe cyclonic storm, crossing the coast at 115–125 km/h. The dashed line on the chart is that forecast.
Overnight Arnab tightened into a small, intense ring. JTWC's estimate of its wind went from 120 km/h to 215 km/h in a day. A jump of more than 55 km/h in 24 hours is called rapid intensification, and it is the hardest thing in weather to predict.
At its peak the eye was clear and the eyewall a ring of the coldest cloud on the image. IMD put the wind at about 150 km/h, averaged over three minutes. JTWC put it at 220 km/h, averaged over one. Both are right. They measure differently.
To the east another cluster of storms was spinning, tracked by JTWC as Invest 90B. Nearby storms pull on each other. JTWC's last warning says this, with the coast, slowed Arnab and turned it.
From about 01:30 India time on the 28th, the eye crossed the coast near Narsapur, then barely moved for hours. A storm that stops keeps raining on the same towns and keeps pushing the sea inland.
Cut off from the warm sea, the storm lost its engine within hours. What was left drifted north across central India, carrying rain as far as Nepal, which stopped issuing high-altitude trekking permits for three days.
02Every storm on Earth
These are the tracks of the 4,093 storms that reached gale force from 1980 to 2025, coloured by their strongest winds.
In the Atlantic and the eastern Pacific it is a hurricane. In the western Pacific, a typhoon. Around the Indian Ocean and the South Pacific, a cyclone. The physics is the same.
North of the equator they spin anticlockwise, south of it clockwise. The spin comes from the Earth's rotation, which fades to nothing at the equator. That is why the band along it stays almost empty.
Here the sea is coloured by its normal temperature for the month and the bright line is 26.5°C. Each dot is where a storm first reached gale force in that month. Almost every dot is on the warm side of the line.
The South Atlantic is cooler, and its winds high up are strong. In 46 years only 3 storms there reached gale force.
In the Bay of Bengal storms come in two bursts, April–May and October–December. The sea is warm all summer, but the monsoon's strong winds aloft tear storms apart. In October those winds ease, and the season reopens. Arnab came ashore within a few kilometres of where Cyclone Montha did, a year earlier almost to the day.
Each region keeps its own list. For the Bay of Bengal and the Arabian Sea, 13 countries gave 13 names each in 2020, used in order and never reused. Arnab was the 27th. The next will be Murasu. Pick an ocean to see its storms.
03Born
Come down to a patch of warm sea. Every tropical cyclone starts as scattered thunderstorms like these. To grow, they need five things at once.
At least about 26.5°C, and warm through the top 50 metres, so the storm can't stir up cold water from below. Warm water evaporates fast, and the vapour is its fuel.
Hot, humid air near the sea rises through the colder air above like a bubble, building clouds 15 km tall. Dry air in the middle would choke them.
If the wind 10 km up blows much harder than the wind near the sea, it tilts the young storm and tears its top off. Forecasters call it wind shear. It kills more would-be cyclones than anything else.
With calm air above, the clouds stay stacked over one spot, and the heat they release builds up in one column instead of blowing away.
Air drawn in toward the low pressure is turned by the Earth's rotation, anticlockwise in the north. Arnab formed about 15° north, far enough from the equator for that turn to take hold.
Given a disturbance to start from and a few calm days, the towers line up into spiral bands around a centre. The pressure falls, the wind rises, and the storm begins to feed itself.
Once its winds pass 63 km/h it is a named tropical storm, a cyclonic storm in India's terms. Arnab got here on 26 October.
04The engine
Cut through a mature storm and it is a machine. On the cut face, colour shows wind speed, orange fastest, and white is cloud.
Near the surface, air spirals in toward the centre, faster and faster, picking up heat and water vapour from the warm sea all the way. Each amber dot is a parcel of air.
At the eyewall the air shoots upward. As it rises it cools, and its vapour condenses into cloud and rain. Condensing gives back the heat it took to evaporate that water at the sea. This is where the storm gets its power.
Fifteen kilometres up, at about −70°C, the air spreads out in a vast sheet of cirrus. The gap between the warm sea and the cold top sets how strong the storm can get, the way the temperatures of a heat engine limit its power.
The heat released in the eyewall, and air sinking gently in the eye, make the core warmer than its surroundings. Warm air is lighter, so the pressure at the centre drops, and air rushes in harder.
Faster wind lifts more water from the sea, which releases more heat, which lowers the pressure, which speeds up the wind. While the sea stays warm and nothing disturbs it, the loop keeps going. On 26–27 October it ran away with Arnab.
05Anatomy
At the centre, air sinks, warms and dries, so the sky can clear and the wind can drop almost to nothing. The eye is usually 20 to 65 km across. People have mistaken it for the end of the storm.
Around the eye is a ring of towering cloud with the strongest winds and the heaviest rain. It slopes outward as it rises, like the seats of a stadium. When it crossed the coast, this ring is what hit Narsapur.
Spiral bands of storms wheel in toward the centre from hundreds of kilometres out. Between them the weather can ease for a while before the next one arrives.
06Build a storm
Set the ocean and the sky, and see what kind of storm they allow. Start from a real place and time, or from scratch.
This sea could fuel winds up to
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With this sky, the storm reaches
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A rule of thumb, not a forecast. The ceiling is DeMaria and Kaplan's (1994) limit from sea temperature; shear, humidity and latitude scale it down by simple rules. Preset conditions are approximate.
07Measuring a storm
Size and strength are separate things. Typhoon Tip in 1979 had gale-force winds across about 2,200 km, the largest on record. Cyclone Tracy, which flattened Darwin in 1974, had gales reaching only about 50 km from its centre. Lay them over a place you know.
Wind gusts and lulls, so agencies average it. US agencies use one minute and the five-step Saffir–Simpson scale. India's IMD uses three minutes and seven steps, from depression to super cyclone. Most others use ten. The longer the average, the lower the number for the same storm. Drag through Arnab's life to see both.
08Overnight
Arnab's leap was not unusual for a strong storm, but it was not expected. Below, three storms are lined up from the moment each became a named storm. The dashed slope is the threshold for rapid intensification, 30 knots in 24 hours.
Otis, 2023. A tropical storm expected to stay modest became a Category 5 hurricane in about a day and hit Acapulco with little warning.
Patricia, 2015. The strongest winds ever reliably measured in a tropical cyclone: about 345 km/h, off western Mexico.
Why it's hard. Track forecasts follow the big winds around a storm, which computers see well. Intensity depends on the small, fast core and on how deep the warm water goes beneath it, which they see poorly. Rapid intensification has likely become more common over the past 40 years, according to the IPCC.
09The path
A cyclone moves like a leaf in a stream, carried by the winds around it. In the tropics those winds usually run west along the edge of a great high-pressure area, then curve away from the equator around its end. That is why so many tracks hook.
Arnab was carried west along that high's southern edge, straight at Andhra Pradesh. Near the coast it slowed to a crawl. JTWC's final forecast, dashed, takes what was left north around the high's western end.
When two cyclones come within about 1,400 km of each other they start to circle a point between them. JTWC credited this pull from the system to Arnab's east, Invest 90B, for part of its slowdown and turn at the coast.
The shaded band is the uncertainty in where the centre will go: it widens with time by the agency's average track errors. It is not the size of the storm. Rain and wind reach far outside it.
India's forecasts of where a storm will be have sharpened steadily. A three-day forecast today is about as good as a two-day one was ten years ago.
10Landfall
Wind does the visible damage, but most people who die in cyclones drown, in the storm surge or in the floods that follow. This is the real sea floor off Kakinada, where Arnab's surge was forecast, from a little inland to 120 km out to sea.
Air pressure presses on the sea. Under a cyclone's low pressure the sea bulges up, about a centimetre for every hectopascal of drop. Under a strong storm that is half a metre, over the open ocean.
Wind blowing toward the shore drags the surface water with it. In deep water the push spreads down and fades. Over a shallow sea floor it has nowhere to go but up. Off Kakinada the sea is 50 m deep just 8 km out, so the pile stays modest.
Storm waves break on that raised water, and a high tide can add more. IMD forecast 1.5–2.5 m above the tide over Kakinada and Yanam, reaching up to half a kilometre inland at Yanam.
Off the Meghna estuary in Bangladesh, the sea stays shallower than 50 m for more than 100 km, and the land behind is barely above sea level. Here the same wind raises the water several times higher and pushes it kilometres inland. The 1970 Bhola cyclone's surge was reported at up to 10 m. It killed somewhere between 300,000 and 500,000 people.
Off Muscat in Oman the sea floor falls away fast. The wind has little shallow water to push, so the surge stays small. The shape of the sea floor, more than the storm, decides how high the water comes.
Change the wind and the coast.
A textbook estimate: the pressure bulge plus steady wind setup over this sea floor. It leaves out waves, tides, the bend of the coast and friction, so real surges differ.
A stalled storm rains on the same ground for hours. In the day Arnab came ashore, these IMD stations measured:
11The end
Over land there is no warm water to evaporate, and the rough ground slows the wind. The engine loses its fuel. Within hours the eye fills in and the eyewall falls apart.
Cooler water does the same, including the cold water a slow storm churns up from beneath itself. So do strong winds aloft and dry air. Most storms die at sea, never touching land.
Even dead, a storm carries an enormous amount of water. What was left of Arnab drifted north across India toward the Himalaya, bringing the risk of rain and snow to mountains more than 1,000 km from where it came ashore.
In September 2021 Cyclone Gulab crossed India from the Bay of Bengal. Its remains reached the Arabian Sea, found warm water again, and rebuilt themselves into Cyclone Shaheen, which hit Oman a week after Gulab's landfall.
12Rule-breakers
Everything so far is the usual story. A few storms broke it, and each one shows what the rules depend on.
It formed 1.4° north of the equator, off Singapore, where the spin from the Earth's rotation is almost nothing. A long, steady surge of north-easterly wind wrapping around a low near Borneo supplied the spin instead.
The first hurricane ever recorded in the South Atlantic. It hit southern Brazil, where forecasters had never had to warn of one.
Freddy formed off north-west Australia, crossed the entire southern Indian Ocean, hit Madagascar and then Mozambique, wandered back out to sea, and hit Mozambique again. It lasted 36 days, the longest of any tropical cyclone on record, and killed more than 1,400 people, most in Malawi.
John travelled about 13,000 km across the Pacific over 31 days, crossing the international date line and then crossing back.
Over unusually warm water off Mexico, Patricia's winds reached about 345 km/h, the highest reliably measured in any tropical cyclone. It weakened sharply before landfall, and far fewer people died than feared.
The Arabian Sea is smaller and usually drier than the Bay of Bengal, so strong storms there are rare. Gonu became a super cyclone and struck Oman and Iran. Since 2001 the Arabian Sea has been producing more storms.
13The toll
The Bay of Bengal holds a small share of the world's cyclones, but most of the deadliest. Its storms have not got weaker. What changed is warning and shelter. Each dot is one storm on this coast, on a scale where every step is ten times the last.
1999, Odisha. A super cyclone came ashore with about two days' warning. Nearly 10,000 people died. The state then built more than 800 cyclone shelters and sirens in over 1,200 coastal villages.
2013, Phailin. A storm of similar strength hit the same coast with about four days' warning. Around a million people were moved. 45 died.
Bangladesh. After 1970 and 1991, volunteers trained to carry warnings village to village with megaphones and flags, and thousands of raised shelters were built. Deaths from comparable storms have fallen by orders of magnitude.
14A warmer sea
The IPCC's assessment of the evidence, in its own words of confidence:
No study has yet asked how much climate change shaped Arnab itself, so this guide doesn't say.
15Your coast
Search for a place to see every storm that came within 200 km of it since 1980.