How Strong Can a Typhoon Get?
― The Limit Is Set by Sea Temperature and the Cold of the Upper Air
A typhoon cannot grow stronger without end. It is an "engine" that runs on heat taken from warm seas. There is a limited amount of fuel, and a brake that bites harder the faster the wind blows. The balance between these two puts a ceiling on wind speed.
In an autumn news report you hear the words "violent typhoon." The central pressure has dropped below 900 hectopascals, and the maximum wind speed is close to 60 meters per second. On the screen you see a neat spiral with a clear hole of an eye.
A thought comes to mind. Over an even hotter sea, could a typhoon with winds of 100 or even 200 meters per second form?
In fact, it cannot. Even the strongest typhoons and hurricanes on record top out at about the same wind speed. There is a clear reason for this.
There are just two reasons for the limit
A typhoon's power comes from the heat released when water vapor evaporated from the sea turns into cloud. The sea temperature fixes how much water vapor the air can pick up.
If the wind doubles, the heat gained only roughly doubles. But the loss to friction with the sea surface grows about eightfold. Sooner or later the loss catches up.
In other words, a typhoon is like a car with the accelerator and the brake pressed at the same time. The sea temperature sets how hard the accelerator pushes, and the brake bites harder the faster you go. Let's take them one at a time.
A typhoon is an "engine" driven by the heat of the sea
Around a typhoon's center, winds skim just above the sea surface and spiral in toward the middle. On the way, the wind picks up plenty of heat and water vapor from the warm sea.
Air that reaches the center shoots up the wall of cloud around the eye. As it rises it cools, and the water vapor turns into cloud droplets. The heat taken up during evaporation is then released into the air, pushing it still higher.
The air reaches a height of about 15 kilometers, where it flows outward. It is a very cold place, below −70℃. There the air dumps its heat into space and sinks quietly far away. See Figure 1.
Take in heat where it is warm, and throw it away where it is cold. This is the principle of a "heat engine," the same as in a car engine or a thermal power plant. The bigger the temperature gap between the hot side and the cold side, the larger the share of heat a heat engine can turn into motion.
For a typhoon, the hot side is the sea surface at around 27℃. The cold side is the upper air at about −73℃. This gap of 100℃ gives a typhoon a large efficiency. But the fuel is limited. The sea temperature sets an upper limit on how much water vapor the air can hold.
As the wind strengthens, the brake suddenly starts to bite
So, as long as there is fuel, can the wind grow without limit? Here the second reason comes in.
The faster the wind blows over the sea, the more heat it takes up, because fast wind swaps the air above the surface for fresh air again and again. But the gain grows only in proportion to wind speed.
The energy lost to friction with the sea surface, on the other hand, grows with the cube of the wind speed. Double the wind and the loss is 8 times; triple it and the loss is 27 times. While a typhoon is weak, the heat gained wins out and the wind keeps strengthening. But at some wind speed, the loss catches up with the gain. That point is the limit over that sea.
Figure 2 shows the link between sea temperature and this limit, calculated with the simplest form of the formula. Over a 27℃ sea, the limit is about 74 meters per second. Over a sea warmer than 30℃, it passes 80 meters per second. However favorable the conditions, a typhoon far beyond 100 meters per second is thought unlikely to form over Earth's oceans today.
Real records agree well with this estimate. Typhoon Tip in 1979 is said to have recorded a central pressure of 870 hectopascals. Hurricane Patricia in 2015 is estimated to have had maximum winds of about 95 meters per second. Both were among the strongest storms ever observed, and both formed over warm seas of around 30℃.
Strong winds stir the sea and pull cold water from the depths up to the surface. The slower a typhoon moves, the more it chills the sea under its own feet, and the fuel dwindles. A band of cooled water is known to remain in the sea after a typhoon passes. This is one reason few typhoons ever reach the limit.
The seas near Japan are often colder than the tropics, so the fuel runs low. On top of that, strong westerly winds aloft tilt the typhoon's column of cloud. But in years when the sea south of Japan is warmer than usual, typhoons are said to approach more easily while keeping their strength.
A forecast of a "violent typhoon" means the storm has grown close to its limit. And a limit is no reason to feel safe. Even at 54 meters per second, the force of the wind on an object is about 7 times that of a 20-meter-per-second wind. When you hear these strength terms in a forecast, take them as a cue to prepare early.
Summary
A typhoon is a heat engine that takes in heat over a warm sea and throws it away in the cold upper air. The sea temperature sets the amount of fuel, and the stronger the wind, the faster the loss to sea-surface friction grows. The wind-speed ceiling lies where these two balance. Over today's oceans, that ceiling is about 75 to 85 meters per second.
A typhoon's ceiling is set by the sea's temperature and the cold of the sky.
So if the sea gets warmer, the ceiling rises a little too.
If you want to learn more about how typhoons work, try also Why Is the Eye of a Typhoon Clear?, Why a Typhoon Makes the Sea Surface Rise and How Strong Is a 40-Meter-per-Second Wind?.
- While a typhoon is active, check its central pressure, maximum wind speed and intensity category on the Japan Meteorological Agency's typhoon information page.
- Open a sea-surface temperature map for the same day (such as the JMA's "Health Check Chart of the Ocean") and see whether the sea ahead of the typhoon is warmer or colder than 26℃.
- Record this each day and check whether the typhoon strengthens over warm water and weakens once it enters colder water.
On a sea-temperature map made after a typhoon has passed, you may find a band of cooled water along its track.
For those who want to know more ― terms, formulas and links to textbooksWe state clearly what level each item belongs to, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school physics or earth science
- High school+Advanced high school material, or textbook sidebar content
- UniversityNot taught in high school; university courses (atmospheric dynamics, thermodynamics)
- ResearchNot yet taught even at university as settled fact; questions researchers are investigating now
Junior highTerms: this phenomenon has names
- Latent heat: The heat taken up when water evaporates and released when water vapor turns back into water. This is a typhoon's fuel.
- Heat engine: A mechanism that turns part of the heat into motion as it moves from a high-temperature place to a low-temperature place.
- Maximum potential intensity: The greatest strength a typhoon could theoretically reach under given sea and atmosphere conditions. Also called potential intensity.
Junior highHigh schoolChecking with a formula: what is the wind limit over a 27℃ sea?
We find the wind speed at which the force generated by heat from the sea balances the force lost to sea-surface friction. Wind speed is in meters per second, and the heat difference is in joules per kilogram of air.
| In symbols | V² ＝ （Ck ÷ Cd）×（Ts − To）÷ To × Δk |
| In words | Square of the wind-speed limit ＝ ratio of how easily heat is taken up to how strongly friction acts × engine efficiency × heat difference between sea surface and air |
| Where it comes from | A balance between the amount of heat taken from the sea surface (proportional to wind speed) converted to wind power at the engine's efficiency, and the amount lost to sea-surface friction (proportional to the cube of wind speed). Divide both sides by the wind speed and the square of the wind speed is left |
| Symbol | Meaning and unit |
| V | Maximum wind speed near the sea surface (m/s) |
| Ck, Cd | A coefficient for how efficiently heat moves from the sea surface, and a coefficient for friction (no units) |
| Ts | Sea-surface temperature (K, kelvin; the ℃ value plus 273) |
| To | Temperature at the height where the air flows out (K) |
| Δk | Difference in heat content between the moist air just above the sea and the surrounding air (J/kg) |
| Sea-surface temperature | 27℃ (about 300 K) |
| Temperature at the outflow height | −73℃ (about 200 K), taken as given |
| Ratio of Ck to Cd | about 1, taken as given |
| Water vapor the air can hold at 27℃ | about 21.9 g per kg |
| Humidity of the surrounding air | about 80% (the shortfall is 20%) |
| Heat to evaporate 1 g of water | about 2500 J |
| Temperature difference, sea surface to upper air | 300 − 200 ＝ 100 |
| Engine efficiency | 100 ÷ 200 ＝ 0.5 |
| Extra water vapor the air can take up at the sea surface (g) | 21.9 × 0.2 ≒ 4.4 |
| Heat difference Δk (J/kg) | 4.4 × 2500 ＝ 11000 |
| Square of the wind speed | 0.5 × 11000 ＝ 5500 |
| Wind-speed limit (m/s) | The square root of 5500 is about 74 |
| Converted to km per hour (km/h) | 74 × 3.6 ≒ 266 |
Over a 27℃ sea, the wind speed a typhoon could reach is about 74 meters per second, close to 270 kilometers per hour. The slider in Figure 2 repeats this calculation for each sea temperature.
| Ratio to the violent-typhoon threshold | 74 ÷ 54 ≒ 1.37 |
| Ratio of wind force on an object (square of wind speed) | 1.37 × 1.37 ≒ 1.88 |
| Ratio of the violent-typhoon threshold to a 20 m/s wind | 54 ÷ 20 ＝ 2.7 |
| Ratio of the pushing force in that case | 2.7 × 2.7 ≒ 7.3 |
A typhoon at its limit pushes on objects about 1.9 times harder than the wind at the threshold of a "violent typhoon."
High schoolHigh school+A typhoon as a heat engine
High schoolIn high school thermodynamics you learn that the upper limit of a heat engine's efficiency is set by the absolute temperatures of the hot and cold sides. For a typhoon, the hot side is the sea surface and the cold side is the tropopause (the top of the troposphere). The saturation water-vapor content rises by about 6 to 7% for each 1℃ rise in temperature, so the warmer the sea, the more latent-heat fuel there is.
High school+The reason the formula above uses (Ts − To) ÷ To for the efficiency, rather than (Ts − To) ÷ Ts, is said to be that the heat produced by friction is used again as fuel. Friction heat warms the air near the sea surface, and that air goes back into the engine.
UniversityThe theory of maximum potential intensity
The formula in this article is the simplest form of the maximum potential intensity theory that Emanuel presented in 1986. It treats a typhoon as a Carnot cycle that takes in heat isothermally at the sea surface and gives it up in the upper air. The ratio of the enthalpy exchange coefficient to the drag coefficient at the sea surface, the temperatures of the sea surface and the outflow layer, and the moist enthalpy disequilibrium at the surface together determine the maximum wind speed that can be reached. How saturation water-vapor content changes with temperature is described by the Clausius–Clapeyron equation. In real analyses, the outflow temperature is found from the vertical profile of air temperature and humidity, and the result is drawn as a map for each sea region.
📖 Derivations and further reading: Carnot cycle (Wikipedia) ／ Clausius–Clapeyron equation (Wikipedia)
ResearchWhat is still not clear
- Why do some typhoons exceed the limit? Observations sometimes find typhoons slightly above the calculated limit. Explanations have been proposed, such as warm air in the eye making up for some of the heat, and research continues.
- Predicting rapidly intensifying typhoons. "Rapid intensification," in which the central pressure falls by dozens of hectopascals in a day, is said to be harder to get right than forecasting the track.
- How many more strong typhoons will warming bring? The limit is thought to rise as the sea warms. But how the number of typhoons, and the share that actually grow close to the limit, will change is still debated.
In other words, this article is also an "explanation within what is known today." The limit formula tells us the "ceiling" of strength, but it does not tell us how far any individual typhoon will grow.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Junior high | Grade 8 science, "Weather and Its Changes" (water vapor, how clouds form) | Water vapor releasing heat as it turns into cloud |
| High school | Physics "Thermodynamics" and Earth Science "Motion of the Atmosphere" | Heat-engine efficiency and temperature difference |
| High school+ | Advanced physics (form of the efficiency formula) | Friction heat returning as fuel |
| University | Atmospheric dynamics and tropical meteorology | The theory of maximum potential intensity |
| Research | Typhoon intensity forecasting and climate change | Typhoons above the limit, rapid intensification, effects of warming |
| ― | Links to daily life | What a "violent typhoon" forecast means, and preparing early |
- Emanuel, K. A. (1986) An Air-Sea Interaction Theory for Tropical Cyclones. Part I: Steady-State Maintenance. Journal of the Atmospheric Sciences
- Japan Meteorological Agency (気象庁), "Types and Expressions of Typhoon Information" (intensity categories)
- Emanuel, K. (2005) Divine Wind: The History and Science of Hurricanes. Oxford University Press
- Kimberlain, T. B., Blake, E. S., Cangialosi, J. P. (2016) Hurricane Patricia. National Hurricane Center Tropical Cyclone Report
Note: This article is a general-audience science explainer. The figures given are rough estimates meant to help you understand how things work. The wind-speed limit is an estimate from a simple formula; for the actual strength of a typhoon, please check the Japan Meteorological Agency's announcements. When a typhoon approaches, follow the information and instructions of the Japan Meteorological Agency and your local authorities.