How strong is a 40 m/s wind,
really?
Typhoon reports talk about "maximum gusts of 50 metres per second." It sounds fierce, but the number alone doesn't really sink in. There's a counterintuitive twist hiding in it: wind force isn't proportional to wind speed. It grows with the square of wind speed. Double the wind speed, and the force is quadrupled. That "squaring" is exactly why you should never underestimate a typhoon's wind.
It's the night a typhoon is approaching. The TV says, "By tomorrow morning, we expect maximum gusts of 50 metres per second." Outside the window, it's still just occasional strong gusts.
On the balcony sit a laundry pole, a planter, and a pair of sandals left out. "Eh, that'll probably be fine," you think — then pause. How much stronger is a 50 m/s wind than today's daytime wind, anyway?
If the daytime wind was around 10 m/s, the answer isn't "5 times." Measured by the force pushing on things, it's 25 times.
Only two things matter here
Double the wind speed, and the force becomes 2×2 = 4 times greater. Triple it, and it's 9 times. Five times the speed means 25 times the force. It intensifies far more sharply than the numbers suggest. A 40 m/s wind pushes on your whole body with a force close to an adult's body weight.
Laundry poles, planters, roof tiles — in strong wind, they all become projectiles. The impact of a flying object also grows with the square of its speed, so an object flying at over 100 km/h can punch straight through window glass.
Let's look at each of these one at a time, grounding them in familiar sensations.
What "squaring" actually means
Have you ever ridden a bicycle and noticed that doubling your speed makes the headwind resistance feel much heavier? That's the feeling of "squaring" in action.
Here's why. The faster something moves, the more air it collides with per second (that's one factor of 2), and the harder each parcel of air hits it (that's a second factor of 2). Multiply them together and you get 4 times. It's the same from the wind's point of view: a wind blowing twice as fast delivers twice the volume of air at twice the force, so the total force is 4 times greater.
Because of this property, our sense of wind strength badly undersells how the numbers actually grow. A 10 m/s wind is "hard to hold an umbrella open in." At 20 m/s, "you can't stand without holding onto something." At 30 m/s, "being outdoors is extremely dangerous." At 40 m/s, "you can't stand, and roofs can be torn off" — with every 10 m/s step, the world becomes something completely different.
"Maximum gust" is roughly 1.5 times the average
The "wind speed" in weather forecasts is the average speed over 10 minutes. But real wind isn't steady — for a few seconds at a time, it can blow far harder than the average. That's a gust, and it's typically around 1.5 times the average wind speed, and more than double in some terrain.
In other words, a forecast of "average wind speed 30 m/s" implicitly means "gusts of 45 m/s or more are possible." Converted into force, that's more than double even between the average and a momentary gust. If you think "I can still walk, so it's fine" while outdoors, the very next gust can be enough to sweep you off your feet — that's the danger of gusts.
Much of the damage from strong winds is said to come not from the wind pushing directly on things, but from flying debris striking them. The impact energy (kinetic energy) of an object when it hits something also grows with the square of its speed. An object carried by a 40 m/s wind is travelling at roughly 144 km/h — hitting a window or a person with the same force as something thrown from a car on a motorway. Clearing your balcony or garden before a typhoon isn't just about protecting your own belongings — it's about keeping your own belongings from becoming weapons.
So what should you actually do?
- Bring outdoor items inside "by the day before" the typhoon arrivesLaundry poles, planters, sandals, bike covers — anything movable. Anchor whatever you can't move. Working on the balcony after the wind has already picked up is the most dangerous time to do it.
- Once the wind picks up, stay indoors and away from windowsGusts run at least 1.5 times the average, so "it seems fine right now" can't be trusted. Close storm shutters or roller shutters — or draw the curtains if you don't have them — and stay in a room away from windows.
- Never "go and check on it"Every typhoon brings a repeat of accidents where people are hurt going to check on rice paddies, rivers, the sea, or their roof. Check conditions via TV or your phone instead, and follow your local government's evacuation guidance to decide whether you need to evacuate.
Summary
The true nature of a 40 m/s wind is a world where squaring bites twice over. ① Force grows with the square of wind speed, so it's far stronger than the raw number suggests (16 times a 10 m/s wind). ② Even flying debris becomes a weapon, its impact driven by the square of its speed. And the forecast wind speed is only the average — the right way to read it is to prepare for gusts roughly 1.5 times that, at any moment.
Wind strength doesn't add up.
Double the wind speed number, and the danger is four times greater.
The same "squared force" physics reaches its extreme in tornadoes. For how a broad, gentle swirl gets squeezed into a violently narrow, ferocious wind, see this article. And for why the "eye" at a typhoon's centre stays clear, see this article.
- Hold a flat board or a hand fan up to an electric fan set to "low," facing it directly, and note how hard it pushes back
- Switch to "high," and compare the push at the same distance and angle
If the wind speed has roughly doubled, the pushing force should be four times greater. See for yourself that the "difference in force" feels much bigger than the "difference in speed." You can feel the same squared effect by comparing headwinds while walking, running, or cycling at different speeds.
For those who want to know more ― terms, formulas, and how this connects to textbooksWe label which level each part belongs to, from middle-school science to university-level specialised courses
- MSCovered in middle-school science
- HSCovered in high-school "Physics Basics" or "Physics"
- HS+Advanced high-school "Physics" content, or textbook sidebar material
- UnivContent not taught in high school — university-level specialised courses (fluid dynamics, meteorology)
- ResearchNot yet taught as settled fact even at university — an active research question
MSTerms: getting the news vocabulary right
- Wind speed: In Japanese weather forecasts, this refers to the average wind speed over 10 minutes [m/s].
- Maximum instantaneous wind speed (gust): The peak wind speed measured as a 3-second average. It's said to run roughly 1.5 times the average wind speed, and more than double in some terrain.
- Wind pressure (dynamic pressure): The force per unit area with which wind pushes on a surface. This is the basis of the "wind force" discussed in the main text.
- Airborne debris: Objects carried by the wind. Considered one of the main causes of damage in strong winds.
MSHSChecking with formulas: the force a 40 m/s wind exerts on a person
As a rough guide, the pressure a wind exerts on a surface can be estimated as Pressure [N/m²] ≈ 0.6 × (wind speed)² (this approximation bundles together the air density and a shape coefficient — its origin is explained below).
| In symbols | F = ½ × ρ × v² × C × A |
| In words | Force from wind = ½ × air density × (wind speed)² × a shape-dependent coefficient × the area facing the wind |
| Where this comes from | Moving air carries kinetic energy of "½ × density × (speed)²" per cubic metre. When that air is brought to a stop in front of a body, it becomes pressure (dynamic pressure) pushing on it. This is the drag equation from fluid dynamics. In the calculation below, we take the coefficient C to be roughly 1, and bundle ½ × 1.2 = 0.6 into a single figure. |
| Wind speed squared | 40 × 40 = 1600 |
| Estimated pressure (per 1 m²) | 0.6 × 1600 = 960 (N/m²) |
| Area a person presents from the front (estimate) | About 0.5 m² |
| Total force on the whole body | 960 × 0.5 = 480 (N) |
| Converted to a felt weight | 480 ÷ 9.8 ≈ 49 (kgf) |
The calculation works out to being pushed sideways by a force close to a single adult's body weight, continuously. It's easy to see, from the numbers alone, why it's impossible to stand and why people get swept off their feet.
| Ratio of wind speeds | 50 ÷ 25 = 2 (times) |
| Ratio of forces (squared) | 2 × 2 = 4 (times) |
| 50 m/s converted to km/h | 50 × 3.6 = 180 (km/h) |
50 m/s is 180 km/h — close to the wind you'd feel sticking your head out of a bullet-train window. The difference between "25 and 50" doesn't hint at how different those two worlds really are; squaring is what creates that gap.
HSWhere the approximation "0.6 × (wind speed)²" comes from
The dynamic-pressure formula taught in physics is Dynamic pressure = ½ × air density × (wind speed)². Air density is about 1.2 kg/m³, so ½ × 1.2 = 0.6 — that's the source of this approximation. The actual force on an object is this dynamic pressure multiplied by the surface area and a shape-dependent coefficient (roughly 1 to 1.3 for a flat plate, smaller for a streamlined shape). The calculation in the main text approximates this coefficient as about 1 and omits it.
HS+The impact of flying debris: kinetic energy also squares
The source of a flying object's impact is its kinetic energy, ½ × mass × (speed)². Squaring shows up here too. A 1 kg object carried by a 40 m/s wind (about 144 km/h) has an energy of ½ × 1 × 1600 = 800 J. That's the same energy as dropping a 1 kg object from a height of about 80 metres — far beyond what an ordinary window pane can withstand. Much of the window damage during typhoons is said to come from flying debris rather than wind pressure itself, which is why storm shutters and roller shutters are recommended.
UnivWhy gusts happen: turbulence and the gust factor
Wind near the ground is disturbed by friction with the terrain and buildings, and blows while carrying eddies (turbulence) of various sizes. Gusts far exceeding the average occur because wind speed spikes as one of these eddies passes through. In meteorology, the ratio of maximum gust to average wind speed is called the gust factor, said to run roughly 1.2 to 1.5 over the sea, and sometimes exceeding 2 over uneven terrain or in built-up urban areas. Wind-resistant building design incorporates the statistical properties of this turbulence (turbulence intensity, the distribution of eddy sizes) to set a design wind speed for each structure.
📖 For the derivation of the formulas and further reading: Drag (Japanese Wikipedia) ／ Turbulence (Japanese Wikipedia)
ResearchWhat's still not fully understood
The physics of wind force is well established, but unresolved questions remain in predicting when, where, and how strong the wind will actually blow.
- Building-scale gust prediction still isn't possible. Forecasting a typhoon's overall track has advanced enormously, but predicting exactly which spot in a city will see a gust of a given speed is said to be beyond even current computing power, because of the complexity of turbulence generated by buildings. Detailed wind-flow simulation of urban areas remains an active research field.
- How climate change will alter typhoon intensity is still debated quantitatively. Several studies point to a rising share of intense typhoons, but estimates of how frequency and regional patterns will change vary between studies, and monitoring and reanalysis continue.
- Modelling the conditions under which debris starts flying is also a work in progress. At what wind speed a given object starts to move, and how far it travels, depends heavily on its shape, how it's placed, and the surrounding buildings. Research is working backward from damage surveys of tornadoes and typhoons, with reflecting this in disaster-prevention standards remaining a challenge.
In other words, even this article is "an explanation based on what's currently known." Precisely because prediction isn't perfect, the surest defence remains simple: finish your preparations before the wind picks up.
Connections to the curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science: Force and pressure / weather | The force of wind pushing on objects, reading forecast wind speeds |
| HS | Physics Basics: force balance; Physics: kinetic energy | The dynamic-pressure formula, the origin of 0.6×(wind speed)², calculations ① and ② |
| HS+ | Physics: advanced energy topics | Estimating the kinetic energy of flying debris |
| Univ | Fluid dynamics, meteorology, architectural engineering | Turbulence and the gust factor, principles of wind-resistant design |
| Research | Meteorology, wind engineering (unresolved) | Urban gust prediction, climate change and typhoons, modelling flying debris |
| ― | Disaster prevention / safety education | Preparing by the day before, reading gust speeds, never "going to check" |
- Japan Meteorological Agency, "Wind Strength and Behaviour" (森林総合研究所) — wait, correcting: Japan Meteorological Agency (気象庁), "Wind Strength and Behaviour" (guide to perceived effects and damage by wind speed, and the relationship between average and gust wind speed).
- Japan Meteorological Agency (気象庁), "About Typhoons," and Cabinet Office (内閣府) disaster-preparedness materials (preparation before a typhoon's approach; staying indoors and not going to check on conditions).
- Standard physics textbook explanations of dynamic pressure, drag, and kinetic energy.
- Architectural Institute of Japan (日本建築学会), commentary on the "Building Load Guidelines" (design wind speed, gust factor, and treatment of turbulence).
- Knutson, T. et al., Tropical Cyclones and Climate Change Assessment, Bulletin of the American Meteorological Society 101(3), 2020 (assessment of climate change and typhoon intensity).
※This article is a general-audience science explainer. For actual disaster preparedness, please follow warnings and advisories from the Japan Meteorological Agency and evacuation information and instructions from your local government and fire department. The figures given here are approximations intended to help explain the underlying mechanism.