Appearance
2.4 — Storms: Thunderstorm, Tornado, Cyclone
On the night of 12 November 1970, a tropical cyclone came ashore in what was then East Pakistan. The land there is the Ganges–Brahmaputra delta: flat, barely above sea level, and densely populated. The storm pushed a wall of seawater up to about 10 metres high across it, at night, at high tide, with almost no warning reaching the villages.
Somewhere between 300,000 and 500,000 people died. It remains the deadliest tropical cyclone on record.
The relief effort by the Pakistani government was widely seen as slow and indifferent, and in the general election held a few weeks later the Awami League swept East Pakistan almost completely. The refusal to let it form a government led to a crackdown, a war, and the creation of Bangladesh. Chapter 6.26 tells that political story. It begins with a storm surge, and this page is about where such a thing comes from.
The thunderstorm: the basic unit
Everything in this chapter is a thunderstorm, or a machine built out of thunderstorms.
Three ingredients are required, always. Moisture, so there is something to condense. Instability, so that a lifted parcel keeps rising on its own (Chapter 2.3). And a trigger to give it the first push — surface heating, a front, a hill, or converging winds.
The life of an ordinary storm has three stages.
Growing. A warm moist parcel rises and reaches its condensation level. Condensation releases latent heat, which makes the parcel warmer than its surroundings, which makes it rise faster, which condenses more. This is a positive feedback and it is the whole engine. The cloud towers upward, all updraft, no rain reaching the ground yet.
Mature. The cloud hits the tropopause and spreads sideways into the flat anvil that gives a thunderstorm its shape. Water and ice have accumulated until the updraft can no longer hold them, and they fall. Falling precipitation drags air down with it, and the falling air is also cooled by evaporation of some of the rain, making it denser still. Now there is a downdraft alongside the updraft. Heaviest rain, hail, lightning, and the gust front — the cold air spreading out at the surface ahead of the storm, which is the sudden cool wind you feel a few minutes before the rain arrives.
Dying. The downdraft spreads and cuts the storm off from the warm moist air feeding it. The storm kills itself by producing rain. An ordinary single-cell storm lasts under an hour for exactly this reason.
Wind shear, and why some storms last for hours
If a storm dies by choking its own inflow, the way to make a long-lived storm is to separate the updraft from the downdraft. The thing that does that is wind shear — a change in wind speed or direction with height.
In a sheared environment the storm is tilted, so the rain falls out to one side instead of down through the updraft. The inflow stays clean. The storm can then last for hours and travel hundreds of kilometres.
With strong shear that also changes direction with height, the updraft starts to rotate. Wind changing direction with height creates horizontal spin in the air, like an invisible rolling pin lying across the landscape. When a strong updraft passes through it, the updraft tilts that rolling pin upright and stretches it — and stretching a rotating column makes it spin faster, for the same reason a spinning skater speeds up when pulling their arms in.
The result is a supercell: a single storm with one rotating updraft, typically 10 to 20 kilometres across, lasting many hours. Supercells produce almost all violent tornadoes, the largest hail, and much of the most damaging straight-line wind.
Lightning
Where the charge comes from is a question that took a long time to answer and is still not perfectly settled, but the main mechanism is well supported. Inside the storm, at temperatures around minus 10 to minus 20 °C, small ice crystals and larger soft hail pellets collide in the updraft. In those collisions charge is transferred: the lighter ice crystals tend to end up positively charged and the heavier pellets negatively charged. The updraft carries the light crystals to the top of the cloud and the heavy pellets fall toward the middle and base.
So the cloud separates into a positive top and a negative base. The negative base then repels electrons in the ground beneath, leaving the ground surface positively charged and following the storm along.
Air is an excellent insulator, so nothing happens until the voltage difference is enormous — on the order of a hundred million volts. Then a faint, invisible stepped leader works its way down from the cloud in jumps of about 50 metres, following whatever path of least resistance it finds, which is why lightning is jagged. As it nears the ground, positive streamers reach up from tall objects. When one connects, a channel is complete, and the main stroke — the return stroke — races back up that channel at about a third of the speed of light. What you see is the return stroke, and it usually flickers because several strokes follow the same channel in quick succession.
The numbers. Peak current around 30,000 amperes. Channel temperature about 30,000 °C, roughly five times the surface of the Sun. Thunder is that channel heating the air so fast that it expands explosively — a shock wave that decays into sound. Since light arrives essentially instantly and sound travels at about 343 metres per second, counting the seconds between flash and thunder and dividing by three gives the distance in kilometres.
Safety, stated plainly, because this is a genuine hazard in India. India records among the highest lightning death tolls in the world — commonly reported in the range of 2,000 to 3,000 deaths a year, concentrated among farm workers in the open during the pre-monsoon and monsoon months. If you can hear thunder you are within striking range. Get inside a building with wiring and plumbing, or inside a car with a metal roof — the metal shell conducts the current around the outside and the occupants are safe, which has nothing to do with the tyres. Avoid open fields, hilltops, isolated trees, water, and metal fences. A crouch is a last resort and a poor one. Wait 30 minutes after the last thunder before going back out, because the trailing edge of a storm still produces strikes.
Tornadoes
A tornado is a violently rotating column of air in contact with both the ground and the cloud base. The visible funnel is condensation caused by the pressure drop inside, plus debris — the wind itself is invisible.
Formation, in the supercell case. The rotating mesocyclone described above extends downward, tightens and reaches the surface. The details of the final step — how rotation already present near the ground is concentrated — are still an active research question, and this book will say so rather than pretending otherwise. What is clear is that the low-level rotation forms first near the ground in many cases, rather than descending ready-made from the cloud.
The Enhanced Fujita scale rates a tornado by damage, not by measured wind, because instruments are almost never in the right place. EF0 is roughly 105–137 km/h; EF5 is above 322 km/h. The strongest tornadoes can strip asphalt off roads and remove well-built houses down to the foundation slab.
Why the American Midwest gets the worst of them in the world is a matter of geography that happens to be uniquely bad. Warm humid air flows north off the Gulf of Mexico at low levels. Hot dry air flows east off the high deserts and the Rockies at mid levels, capping the moist air and letting it build up energy. Cool dry air comes down from Canada. The jet stream runs overhead providing shear. And there is no east-west mountain range to separate any of them, which is precisely what Europe and Asia do have. It is the only place on Earth where all the ingredients meet reliably, and it is why "Tornado Alley" exists.
India does get tornadoes, chiefly in West Bengal, Odisha, Bihar and Assam during the pre-monsoon season, usually embedded in the severe squall-line storms known locally as Nor'westers or Kalbaisakhi. They are less frequent and less studied than American ones but they are not rare, and the 1989 Daulatpur–Saturia tornado in Bangladesh killed around 1,300 people, the deadliest tornado ever recorded anywhere.
Tropical cyclones
The same word, different names by ocean: cyclone in the Indian Ocean and the South Pacific, hurricane in the Atlantic and northeast Pacific, typhoon in the northwest Pacific. Identical phenomenon.
Six conditions must all be met, and knowing them tells you exactly where and when cyclones can form.
- Sea surface temperature above about 26.5 °C, through at least 50 metres of depth. This is the fuel. A shallow warm layer is not enough, because the storm's own winds churn cold water up from below.
- A deep layer of moist air. Dry air entrained into the storm kills it.
- An unstable atmosphere, so convection can run.
- Low vertical wind shear. This is the opposite of the supercell requirement, and it trips people up. A tornado-producing storm needs shear to tilt and survive. A cyclone is a single vertically-stacked heat engine, and shear tears its structure apart. This is why cyclones and tornadoes want opposite environments.
- At least about 5 degrees away from the equator, because the Coriolis effect is zero at the equator and without it the inflowing air will not organise into rotation.
- A pre-existing disturbance to start from — commonly an easterly wave off Africa in the Atlantic, or a monsoon trough disturbance in the Bay of Bengal.
How it intensifies. Warm sea water evaporates into the inflowing air. That air spirals in, rises, and the vapour condenses, releasing latent heat. The heating lowers the surface pressure further, which draws in more air faster, which evaporates more water. It is the same feedback as the ordinary thunderstorm, but organised around a centre and with an entire warm ocean as the reservoir. A mature cyclone releases heat energy at a rate several hundred times the world's total electricity generation.
The eye. At the centre, some of the air rising in the eyewall turns inward at the top and sinks. Sinking air warms and dries (Chapter 2.2), which clears the cloud. The eye is calm, often sunlit, sometimes with birds trapped inside it, ringed by the most violent winds in the storm. People who go outside during the eye's passage and are caught when the far eyewall arrives — with the wind now coming from the opposite direction — are a recurring cause of death.
What actually kills. Not usually the wind.
Storm surge is the biggest killer, and it caused the 1970 catastrophe. Two things raise the water. The low pressure at the centre lets the sea surface bulge up — roughly a centimetre for each millibar of pressure drop, which is under a metre. Far more importantly, the wind physically pushes water ahead of the storm, and where that water is driven into a shallowing, funnelling coastline it piles up. The Bay of Bengal is the worst-shaped coastline in the world for this: shallow, funnel-shaped, with an enormous flat low-lying delta at the closed end and tens of millions of people on it. This is why the deadliest cyclones in recorded history are overwhelmingly Bay of Bengal events.
Inland flooding is second. A slow-moving cyclone dumps extraordinary rainfall — Hurricane Harvey delivered over 1,500 mm on parts of Texas in 2017. Deaths occur hundreds of kilometres from the coast, often days later.
Wind is third, mostly through flying debris and structural failure. Tornadoes spawned in the outer rainbands are a fourth.
Cyclones in the Indian Ocean, and a genuine success story. The Bay of Bengal generates far more deadly cyclones than the Arabian Sea, because it is warmer, shallower, receives huge freshwater input which keeps a warm layer floating on top, and funnels toward a densely populated delta. The season peaks twice — before the monsoon in April–May and after it in October–November.
And the death tolls have collapsed. The 1999 Odisha super cyclone killed around 10,000 people. Cyclone Phailin in 2013 was comparable in strength and killed under 50, because roughly a million people were evacuated in advance. The change was built from unglamorous parts: satellite and radar monitoring by the India Meteorological Department, a network of concrete cyclone shelters on raised plinths, coastal warning towers and sirens, mobile alerts, pre-positioned relief, and a state disaster response force. Bangladesh made the same transition after 1970 and 1991. This is one of the clearest cases anywhere of scientific forecasting plus ordinary public administration saving lives at enormous scale, and it is worth remembering when Part 14 discusses what adaptation actually looks like.
Where this shows up in your life
Reading a forecast properly. A cyclone track forecast is drawn as a cone, and the cone is the uncertainty in the centre's position, not the extent of the damage. Effects routinely reach far outside it. Categories describe wind only and say nothing about rain or surge, which is why a weak, slow, wet storm can be far deadlier than a strong fast one.
Why coastal building rules exist. Elevation above surge level, and setback from the shoreline, are the two rules that decide whether a coastal settlement survives. India's Coastal Regulation Zone rules exist for this, and where they are ignored the surge finds out.
And the two-line summary of the whole page. A storm is a machine for moving heat from a warm surface to the upper atmosphere, using the latent heat of water vapour as the working fluid. Warmer sea, more vapour, more available energy — which is why the physics says cyclones should trend toward a higher share of intense storms and heavier rainfall in a warmer world, even if the total number does not rise. Part 14 gives that its proper treatment.
What the next page covers
There is one weather system on Earth that a billion and a half people plan their entire year around, that arrives within about a fortnight of the same date every year, and whose failure has repeatedly caused famine and changed governments. Chapter 2.5 is the Indian monsoon — what actually drives it, why the old "giant sea breeze" explanation taught in school is incomplete, how the Tibetan Plateau and the Himalaya set it up, why it advances across the country in a predictable sequence, what El Niño does to it, and how much of Indian history is written in its failures.