How Strong Is a 10, 15 or 20 m/s Wind? What Each Wind Speed Does to Umbrellas, Walking, Bikes, Cars, Trains and Typhoons, With km/h and Knot Conversions
A forecast of “maximum wind 15 m/s” does not immediately tell you how strong that wind feels. A 10 m/s wind is 36 km/h and already too strong for an umbrella. At 15 m/s you can no longer walk into the wind, at 20 m/s you cannot stay on your feet without holding on to something, and above 30 m/s trees and utility poles start to come down. This guide explains how to read the wind speed in a forecast (mean wind versus gusts, and the height it is measured at), gives a chart of what happens to people, trees, cars and buildings from 1 to 40 m/s, describes 5, 10, 15, 20 and 30 m/s in detail, and gives guidelines for umbrellas, laundry, cycling, camping, drones, work at height, trains and expressways. It also covers the force on a sign or umbrella in kgf, wind chill, how much stronger the wind is on a high apartment floor, typhoon intensity classes and hurricane categories, converting between m/s, km/h, knots and mph with mental-math shortcuts, and what to do before strong winds or a typhoon, all with concrete numbers.
The short answer: no umbrella at 10 m/s, no walking into the wind at 15 m/s, no standing at 20 m/s
Japanese forecasts give wind speed in metres per second (m/s): how many metres the air moves in one second. A 10 m/s wind moves 10 m every second, which is 10 × 3.6 = 36 km/h. Picture air moving about as fast as a bicycle ridden flat out, pressing on you without a break.
| Wind speed (m/s) | km/h | Japanese forecast term | What happens |
|---|---|---|---|
| 5 m/s | 18 km/h | (none) | Dust and loose paper are raised. Laundry dries well. Umbrellas are fine |
| 10 m/s | 36 km/h | Fairly strong wind | Hard to walk into the wind. Umbrellas cannot be used. Whole trees and power lines sway |
| 15 m/s | 54 km/h | Strong wind | Impossible to walk into the wind; some people fall. Signs and corrugated sheets start to come loose |
| 20 m/s | 72 km/h | Very strong wind | You cannot stand without holding on. Flying debris can injure. Driving at normal speed is difficult |
| 25 m/s | 90 km/h | Very strong wind | Being outdoors is extremely dangerous. Moving trucks are blown over |
| 30 m/s | 108 km/h | Violent wind | Many trees fall; some utility poles, street lights and block walls collapse |
| 40 m/s | 144 km/h | Violent wind | Some houses collapse; some steel structures are deformed |
The “what happens” column is based on the wind strength table published by the Japan Meteorological Agency (JMA). The wind speed in a forecast is a 10-minute mean, and real wind contains momentary gusts of 1.5 to 3 times that value. Even with a forecast of 10 m/s, read it as “gusts of 15 to 30 m/s are possible”.
Reading the wind in a forecast: mean wind, maximum wind and gusts
Forecasts and news reports use several different wind speeds. On the same day at the same place, the figure can differ by a factor of almost two depending on which one is quoted, so it helps to know what each term means.
| Term | Meaning | Where you see it |
|---|---|---|
| Wind speed (mean wind) | Average over the 10 minutes up to the observation time | “Wind 10 m/s” in a forecast, the Beaufort force |
| Maximum wind speed | The highest mean wind speed of the day (or period) | “Maximum wind of 25 m/s expected”, typhoon intensity |
| Gust (instantaneous wind) | Average over 3 seconds (12 readings taken every 0.25 s) | The strength of gusts |
| Maximum gust | The highest gust | “A maximum gust of 40 m/s was recorded” |
Gusts can reach 1.5 to 3 times the mean wind speed: a mean of 15 m/s can bring gusts of 22.5 to 45 m/s, and a mean of 20 m/s gusts of 30 to 60 m/s. Flying objects, people being knocked over and signs coming loose are usually caused by gusts rather than by the mean, so do not decide that it is “still fine” from the mean wind alone. Wind is concentrated and locally stronger between tall buildings, around the corners of buildings, on mountain ridges and on bridges.
- Measuring height: anemometers at weather stations and AMeDAS sites are usually about 10 m above the ground. The wind right at ground level is weaker, and on a high apartment floor it is stronger
- Land and sea: under the same weather pattern, the wind is stronger over the sea and on the coast where nothing blocks it. That is why forecasts give separate values “at sea”
- Advisories and warnings: the criteria are set for each area. In many areas a strong wind advisory is issued at a mean of roughly 10 to 15 m/s and a storm warning at roughly 20 to 25 m/s
The Beaufort scale: forces 0 to 12 in m/s, km/h and knots
The Beaufort scale describes wind strength in 13 steps from 0 to 12. It began as a guide written by Admiral Beaufort of the British Royal Navy in the 19th century, based on how much sail a ship could carry, and the JMA wind force scale uses the same boundaries. “North wind, force 4” in a forecast refers to this scale.
| Force | Name | m/s | km/h | Knots | On land |
|---|---|---|---|---|---|
| 0 | Calm | 0–0.2 | 0–1 | Under 1 | Smoke rises straight up |
| 1 | Light air | 0.3–1.5 | 1–5 | 1–3 | Smoke drift shows the direction, but wind vanes do not move |
| 2 | Light breeze | 1.6–3.3 | 6–11 | 4–6 | Wind felt on the face; leaves rustle |
| 3 | Gentle breeze | 3.4–5.4 | 12–19 | 7–10 | Leaves and small twigs in constant motion; light flags extend |
| 4 | Moderate breeze | 5.5–7.9 | 20–28 | 11–16 | Raises dust and loose paper; small branches move |
| 5 | Fresh breeze | 8.0–10.7 | 29–38 | 17–21 | Small trees in leaf begin to sway; crested wavelets on ponds |
| 6 | Strong breeze | 10.8–13.8 | 39–49 | 22–27 | Large branches move; power lines whistle; umbrellas are hard to use |
| 7 | Near gale | 13.9–17.1 | 50–61 | 28–33 | Whole trees sway; walking into the wind is difficult |
| 8 | Gale | 17.2–20.7 | 62–74 | 34–40 | Twigs break off; you cannot walk into the wind |
| 9 | Strong gale | 20.8–24.4 | 75–88 | 41–47 | Slight damage to houses (roof tiles come off, for example) |
| 10 | Storm | 24.5–28.4 | 89–102 | 48–55 | Trees are uprooted; considerable damage to houses |
| 11 | Violent storm | 28.5–32.6 | 103–117 | 56–63 | Very rare; widespread damage |
| 12 | Hurricane force | 32.7 and over | 118 and over | 64 and over | (No description for land) |
The lower limit of force 8, 17.2 m/s (34 knots), is also the point at which a tropical depression is called a typhoon. The lower limit of force 12, 32.7 m/s (64 knots), is almost the same as the bottom of Category 1 on the US hurricane scale. Note that the Japanese name for force 7 translates as “strong wind”, but its range differs slightly from the forecast term “strong wind” (15 to 20 m/s) and from the strong wind advisory.
How strong are 5, 10, 15, 20, 25 and 30 m/s? A closer look
For the wind speeds people search for most often, here is how strong each one feels, what happens around you and the force on a flat 1 m² board. The force rises with the square of the wind speed, so doubling the wind speed quadruples the force (see the wind force section below).
- 1
5 m/s (18 km/h, about 10 knots, force 3)
Leaves and twigs move constantly, flags flap and dust is raised. Walking is not affected and you can use an umbrella. Laundry dries well, but light items can blow away. By the rule of thumb of about 1°C colder per 1 m/s, it feels about 5°C colder. This is the wind speed at which drone flights are usually called off.
- 2
10 m/s (36 km/h, about 19 knots, force 5)
This is where Japanese forecasts start saying “fairly strong wind”. Walking into the wind becomes hard and umbrellas cannot be used. Whole trees and power lines begin to sway, and on an expressway you feel the car being pushed sideways. The force on a 1 m² board is about 7.5 kgf, and on an open umbrella about 8 kgf. Gusts of 15 to 30 m/s may be mixed in.
- 3
15 m/s (54 km/h, about 29 knots, force 7)
A “strong wind”. You can no longer walk into the wind and some people fall. Work at height becomes extremely dangerous, signs and corrugated sheets start to come loose, and some roof tiles are torn off. Storm shutters rattle. The force on a 1 m² board is about 17 kgf, about 2.3 times that at 10 m/s.
- 4
20 m/s (72 km/h, about 39 knots, force 8)
A “very strong wind”. You cannot stay on your feet without holding on to something, and flying debris can injure you. Thin tree trunks snap, signs fall or blow away, and road signs tilt. Driving at normal speed is difficult, and prefabricated sheds that are not anchored can move or topple. The force on a 1 m² board is about 30 kgf.
- 5
25 m/s (90 km/h, about 49 knots, force 10)
Being outdoors is extremely dangerous. Moving trucks can be blown over, poorly fixed metal roofing peels off and inadequately secured scaffolding can collapse. The force on a 1 m² board is about 47 kgf. If a wind this strong is forecast, prepare on the assumption that you will not go outside.
- 6
30 m/s and above (108 km/h and above, about 58 knots and above, force 11 to 12)
A “violent wind”. Many trees fall, some utility poles, street lights and block walls collapse, and cladding is blown off over wide areas. Above 40 m/s some houses collapse. The force on a 1 m² board is about 67 kgf at 30 m/s and about 120 kgf at 40 m/s, more than the weight of an adult.
Up to what wind speed can you use an umbrella, hang laundry, ride a bike, camp, fly a drone, or count on trains and expressways?
Here are reference wind speeds for everyday situations where people often hesitate. Some come from laws or regulations, some differ by operator and some are only general rules of thumb, so the right-hand column says which.
| Situation | Reference wind speed (mean) | Basis and notes |
|---|---|---|
| Using an umbrella | Under 10 m/s | By the JMA guide, umbrellas cannot be used from 10 m/s. The description of force 6 (from 10.8 m/s) also says umbrellas are hard to use. Gusts can catch an umbrella at lower speeds |
| Hanging laundry outside | Up to about 5 m/s | General guideline. Use extra pegs for light items. At 10 m/s a whole drying pole can fall |
| Cycling | Take care above 5 m/s, stop at 10 m/s | General guideline. Headwinds make pedalling heavy, and crosswinds and gusts make the bike wobble. Bikes with child seats tip over especially easily |
| Pitching a tent or tarp | Take it down at about 10 m/s | General guideline. A tarp catches the wind like a sail, so take it down first. Check the campsite's own rules too |
| Flying a drone | Under 5 m/s | The flight manual published by Japan's Ministry of Land, Infrastructure, Transport and Tourism as an example says not to fly at 5 m/s or more. The wind aloft is stronger than at ground level |
| Work 2 m or more above ground | Under 10 m/s | Under Japan's Industrial Safety and Health Regulations and the related official notice, a 10-minute mean of 10 m/s or more counts as “strong wind”, and work at height must stop |
| Trains | Varies by line and section | Many lines slow down when the anemometer (usually measuring gusts) reads around 20 m/s and suspend service at around 25 to 30 m/s |
| Expressways | Varies by road | Strong wind brings speed limits, and stronger wind closes the road. Tall vehicles and motorcycles are vulnerable to crosswinds |
| Athletics (sprints and jumps) | Tailwind up to 2.0 m/s | In the 100 m, 200 m, 100 m/110 m hurdles, long jump and triple jump, a tailwind over 2.0 m/s makes a mark ineligible for records (wind-assisted) |
- Fishing, kayaks and small boats: the wind is stronger over water than over land, and the sea starts to get choppy at about 5 m/s (white caps appear at force 3 to 4). Many small-boat users treat 5 m/s as the point to stay ashore
- Things on the balcony: flowerpots, drying poles and sandals start to move at around 10 m/s, and above 15 m/s some get blown away. Bring them indoors when strong wind is forecast
- Children and older people: the lighter the person, the more the wind pushes them around. At 15 m/s the frontal force is about 10 kgf for an adult (about 0.7 m²) and about 5 kgf for a child with roughly half the frontal area. That is one sixth of a 60 kg adult's weight but a quarter of a 20 kg child's
Wind force rises with the square of the speed: how many kg on a sign, an umbrella or a person?
The force with which wind pushes on an object is proportional to the square of the wind speed. First the dynamic pressure is found from the air density (about 1.225 kg/m³) and the wind speed, and then it is multiplied by the area of the object and a drag coefficient that depends on its shape.
| Wind speed | Dynamic pressure | Sign 1 m² (Cd 1.2) | Open umbrella 0.9 m² (1.4) | Standing person 0.7 m² (1.0) |
|---|---|---|---|---|
| 5 m/s | 15 Pa | About 1.9 kgf | About 2 kgf | About 1.1 kgf |
| 10 m/s | 61 Pa | About 7.5 kgf | About 7.9 kgf | About 4.4 kgf |
| 15 m/s | 138 Pa | About 17 kgf | About 18 kgf | About 9.8 kgf |
| 20 m/s | 245 Pa | About 30 kgf | About 32 kgf | About 18 kgf |
| 25 m/s | 383 Pa | About 47 kgf | About 49 kgf | About 27 kgf |
| 30 m/s | 551 Pa | About 67 kgf | About 71 kgf | About 39 kgf |
| 40 m/s | 980 Pa | About 120 kgf | About 126 kgf | About 70 kgf |
At 10 m/s an open umbrella takes about 8 kgf, which is hard to hold with one hand for long, and a gust can break the ribs or pull you along with it. A 1 m² sign takes about 30 kgf at 20 m/s and about 67 kgf at 30 m/s. Anything that catches the wind with a flat surface, such as banners, bicycle covers or balcony screens, becomes more dangerous the larger its area.
The force on a standing adult seen from the front (about 0.7 m²) is about 10 kgf at 15 m/s and about 18 kgf at 20 m/s. On paper that sounds manageable, but the wind keeps rising and falling and gusts are several times stronger. Being thrown off balance by sudden changes in strength is the main reason people fall in strong wind.
How much colder does each 1 m/s of wind make it feel?
Wind strips away the layer of warm air around the body and carries heat away, so it feels colder than the actual temperature. In hiking, the rule of thumb is that each 1 m/s of wind makes it feel about 1°C colder, but the result varies considerably with the formula used.
| Temperature and wind | 1°C per 1 m/s rule | Wind chill (North America and UK) | Steadman formula (60% humidity) | Linke formula |
|---|---|---|---|---|
| 10°C, 5 m/s | 5°C | About 7.6°C | About 4.9°C | About 1.1°C |
| 10°C, 10 m/s | 0°C | About 6.2°C | About 1.4°C | About −2.6°C |
| 5°C, 10 m/s | −5°C | About −0.4°C | About −4.3°C | About −7.6°C |
| 0°C, 5 m/s | −5°C | About −4.9°C | About −6.3°C | About −8.9°C |
| 0°C, 10 m/s | −10°C | About −7.1°C | About −9.8°C | About −12.6°C |
| −5°C, 10 m/s | −15°C | About −13.7°C | About −15.2°C | About −17.6°C |
The wind chill index is the formula used by the North American and UK weather services and is defined for temperatures of 10°C or below and winds of 4.8 km/h (about 1.3 m/s) or more. The Steadman formula, used by the Australian Bureau of Meteorology, also takes humidity into account, and the Linke formula (temperature − 4√wind speed) is often quoted in Japanese explanations. Whichever formula you use, a 10 m/s wind makes it feel roughly 5 to 13°C colder, so a windproof layer is essential for winter fishing, hiking or cycling to work. How to estimate the temperature and feels-like temperature on a summit is explained in the summit temperature article.
How much stronger is the wind on a high apartment floor or at drone altitude?
Near the ground, wind is slowed by friction with buildings and trees. That effect fades with height, so the wind gets stronger the higher you go. Building design describes this increase with a power law.
| Surroundings | 20 m | 30 m (about the 10th floor) | 50 m | 100 m |
|---|---|---|---|---|
| Open sea or coast (I) | About 10.7 m/s | About 11.2 m/s | About 11.8 m/s | About 12.6 m/s |
| Farmland, scattered houses (II) | About 11.1 m/s | About 11.8 m/s | About 12.7 m/s | About 14.1 m/s |
| Residential, suburbs (III) | About 11.5 m/s | About 12.5 m/s | About 13.8 m/s | About 15.8 m/s |
| Dense city centre (IV) | About 12.1 m/s | About 13.5 m/s | About 15.4 m/s | About 18.6 m/s |
The table assumes a 10 m/s wind at 10 m above the ground. In a residential area it is about 12.5 m/s at 30 m (around the 10th floor), and because force goes with the square of the speed, the force is about 1.55 times as large. This is why things blow off the balconies of tall apartment blocks so easily: a “fairly strong wind” in the forecast can be close to a “strong wind” on a high floor. If you fly a drone up to 100 m, expect a 5 m/s wind at ground level to be about 6 to 9 m/s up there. Real wind can be locally much stronger still around the corners of and gaps between buildings.
What wind speeds make a typhoon “strong”, “very strong” or “violent”? Comparing with hurricanes
A tropical cyclone becomes a typhoon when its maximum wind speed (10-minute mean) reaches 17.2 m/s (34 knots). The JMA describes a typhoon's intensity by its maximum wind speed and its size by the radius of the area with winds of 15 m/s or more.
| Class | Maximum wind (m/s) | Knots | km/h |
|---|---|---|---|
| Typhoon (no intensity label) | 17.2–32.9 | 34–63 | About 62–118 km/h |
| Strong typhoon | 33–43.9 | 64–84 | About 119–158 km/h |
| Very strong typhoon | 44–53.9 | 85–104 | About 158–194 km/h |
| Violent typhoon | 54 and over | 105 and over | About 194 km/h and over |
- Size: a strong-wind radius of 500 km to under 800 km is “large”, and 800 km or more is “very large”
- Storm area: the area with mean winds of 25 m/s or more. Check it together with the forecast circle to see whether your area will be inside it
- A typhoon's maximum wind speed is the value near the centre. Even with the same “very strong typhoon”, the wind far from the centre may be much weaker, or it may be strengthened by terrain
US hurricanes are classified on the Saffir-Simpson scale (Categories 1 to 5), which uses the 1-minute mean wind. A 1-minute mean comes out higher than a 10-minute mean, so to compare, multiply the 10-minute mean by about 1.07 (1 ÷ 0.93). On that basis, the lower limit of a “strong typhoon” (33 m/s) is Category 1, that of a “very strong typhoon” (44 m/s) is Category 2, and that of a “violent typhoon” (54 m/s) falls around the boundary between Categories 3 and 4. Category 5 (a 1-minute mean of 137 knots or more) corresponds to roughly 65.5 m/s or more as a 10-minute mean.
Converting between m/s, km/h, knots and mph, with mental-math shortcuts
Japanese forecasts use m/s, marine and aviation weather uses knots (kt), and forecasts abroad use km/h or mph. One knot is one nautical mile (1,852 m) per hour, and 1 mph is one mile (1,609.344 m) per hour.
| Conversion | Multiply by | Shortcut | Example |
|---|---|---|---|
| m/s → km/h | × 3.6 | × 4, then take off 10% | 10 m/s → 36 km/h |
| km/h → m/s | ÷ 3.6 | ÷ 4, then add 10% | 60 km/h → about 16.7 m/s |
| m/s → knots | × 1.944 | Roughly double | 10 m/s → about 19.4 knots |
| Knots → m/s | × 0.514 | Roughly half | 20 knots → about 10.3 m/s |
| m/s → mph | × 2.237 | Double, then add a bit over 10% | 10 m/s → about 22.4 mph |
| mph → m/s | × 0.447 | A little under half | 40 mph → about 17.9 m/s |
| Knots → km/h | × 1.852 | Double, then take off 10% | 30 knots → about 55.6 km/h |
It is handy to remember a few common pairs: 5 m/s ≈ 18 km/h ≈ 10 knots, 10 m/s = 36 km/h ≈ 19.4 knots ≈ 22 mph, 15 m/s = 54 km/h ≈ 29 knots, 20 m/s = 72 km/h ≈ 39 knots, and 30 m/s = 108 km/h ≈ 58 knots ≈ 67 mph. A forecast of 50 km/h abroad is about 13.9 m/s, towards the top of what Japanese forecasts call a “fairly strong wind”.
What to do before strong winds or a typhoon, by wind speed
- 1
When a mean of 10 m/s (fairly strong wind) is forecast
Take rainwear instead of an umbrella and reconsider cycling. Bring light items, flowerpots and drying poles in from the balcony, or tie them down.
- 2
When a mean of 15 m/s (strong wind) is forecast
Avoid going out unless necessary. Close storm shutters and keep things away from windows. Lay down and secure outdoor signs, banners and bicycles, or move them indoors.
- 3
A mean of 20 m/s or more (very strong wind) or an approaching typhoon
Prepare on the assumption that you will not go outside. Fully charge your phone and power banks in case of a power cut, and check torches, water and food. Wait until the wind has died down before going out to inspect the roof or antenna.
- 4
Right after the wind eases
The wind can return from the opposite direction, so stay alert for several hours after the centre of a typhoon has passed. Keep away from fallen power lines and tilted signs, and report them to the local authority or power company.
How many days a portable power station can keep a fridge and lights running during a long power cut is calculated in the portable power station article. When to take shelter in stormy weather with thunder is covered in the lightning distance article. In the Wind Speed Converter you can enter the forecast wind speed and check what it does, the range of gusts and the wind at the height of your home, so use it to decide how far to prepare.
FAQ
- How strong is a 10 m/s wind?
- It is 36 km/h or about 19 knots, force 5 on the Beaufort scale (fresh breeze), and the point where Japanese forecasts start to say “fairly strong wind”. Walking into it is hard and umbrellas cannot be used. Whole trees and power lines start to sway, and an open umbrella takes about 8 kgf. Gusts of 15 to 30 m/s may be mixed in.
- Can you walk outside in a 15 m/s wind?
- It is strong enough that you cannot walk into the wind and some people fall (the forecast term “strong wind”, Beaufort force 7). Signs and corrugated sheets start to come loose and some roof tiles are torn off, so avoid going out unless necessary.
- What happens at 20 m/s?
- You cannot stand without holding on to something, and flying debris can injure you. Thin tree trunks snap, signs fall or blow away, and driving at normal speed becomes difficult. A 1 m² sign takes about 30 kgf.
- What is the difference between mean wind speed and gusts?
- The wind speed in a forecast is a 10-minute mean, while a gust (instantaneous wind speed) is a 3-second mean. Gusts can reach 1.5 to 3 times the mean, so even with a mean of 15 m/s there may be momentary winds of 22.5 to 45 m/s.
- How do I convert m/s to km/h?
- Multiply by 3.6 (an hour has 3,600 seconds and a kilometre has 1,000 m). 10 m/s is 36 km/h and 20 m/s is 72 km/h. To go from km/h to m/s, divide by 3.6. To get knots, multiply m/s by about 1.944.
- Up to what wind speed can I use an umbrella?
- By the JMA guide, umbrellas cannot be used from 10 m/s (fairly strong wind). The description of Beaufort force 6 (from 10.8 m/s) also says umbrellas are hard to use. Even below that, a gust can catch the umbrella. If the forecast mean is over 10 m/s, switch to rainwear.
- What wind speed is a “violent” typhoon?
- A maximum wind speed (10-minute mean) of 54 m/s (105 knots) or more. 44 m/s or more is “very strong”, 33 m/s or more is “strong”, and 17.2 m/s or more makes it a typhoon. On the US hurricane scale, the lower limit of a violent typhoon falls around the boundary between Categories 3 and 4.
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