Topic: [random question] what wind speed does it take to move a person? (Read 445 times)

Exactly.  I don't think the OP counted on the feet being in concrete either.  I certainly wouldn't stand out in a tornado with regular shoes...

You're much more likely to be fatally hit by flying debris before you take flight, so don't try it. 

When I was younger I used to skateboard so I was practicing my balance in trains and trams exactly like you described. Bending knees and lowering your center of balance makes you more fluid and it makes easier to use your legs and lower body as a spring while upper part remains vertical.

That won't help you though if wind is stronger then 200 kmph. It will be harder to throw you, but wind will move you anyway, if there are gushes involved especially.

I guess the frivolous answer is that it depends on the size of their feet.

More seriously, it must depend on their stance. When I was doing my Wing Chun training I used to stand in the centre of a tube train, and adopt a horse stance. I could remain upright even when the train rocked from side to side, and I didn't need to hold anything. Leaning into the wind must be another factor that could increase stability.

Thank you. Most of my time on this forum is spent arguing with xenophobes, bigots, racists, religious-nuts, anti-vaxxers, Trump-worshipers and flat-earth-trolls. It's rare and refreshing to come across a post that is based on facts and simple, evidence-based, reproducible and verifiable physics.

I would say it really depends of the type of the wind.

We have a hurricane wind in Croatia each year between October and April with winds that blow over 220+ km/h. It is called Bora (Bura). It blows with varying intensity during those months, sometimes weak low intensity like 80 km/h, but there are periods when it is almost unbearable.

The highest speed measured 220 km/h, then Windmasters start giving errors So gusts are the ones that are scary. You get Bora blowing around 200 km/h and then gusts go way over 220 km/h.

Worse area is near Velebit mountain and channels under it.I have a house on an island there so I witnessed it many times

Japanese are studying it for more then 10 years now as this is a place on earth where they basically have lab conditions to study high speed winds. You get it each year on schedule and with guaranteed intensity. Best Prosciutto comes from this areas 

I'll throw in a few videos so you can get an idea.

https://www.youtube.com/watch?v=eKyQ9JwBp-Y

https://www.youtube.com/watch?v=BnGMCGIHVFU

https://www.youtube.com/watch?v=lksKSo5Qg4A

Knocking somebody over is all about balance, so there really is no good answer to that question. However, if you want to know the wind speed necessary to lift a person, then here you go:

If you are not afraid of math, this will tell you: Wikipedia article on drag

If you don't want to deal with math, then the answer is 200 km/h. That's the terminal velocity of a skydiver.

As for severity, both tornadoes and hurricanes are common in certain areas of the U.S and cause major damage.

There are an average of about 10 major tornadoes (F4 and F5, with wind speeds exceeding 350 km/h) each year. (https://www.ustornadoes.com/2012/04/10/violent-f4ef-4-and-f5ef-5-tornadoes-in-the-united-states-since-1950/)

Likewise, an average of about 6 major hurricanes (catagories 3, 4, and 5, with wind speeds exceeding 178 km/h) hit the U.S. each decade, or roughly one every other year.  (https://www.nhc.noaa.gov/pastdec.shtml)

It depends on the surface area of the person orthogonal to the wind. I can give you an idea without getting into gross math, but I'll explain the assumptions made and where gross math would change things and why. If you have a background in vector calculus, we can run this again in a more realistic scenario.

The average adult male has a surface area of around 2m^2. We'll say that you're only facing the wind with either your front or back, and ignore the surface area from the top of your head/sides/etc. 50Kg would be a very thin "average man" but lets roll with it. Wind speed is our unknown. One assumption that we'll make is that the wind is completely orthogonal to you, so there is no vertical component. The person is standing straight up and the wind is coming and hitting them head on at a 90 degree angle like in a wind tunnel. If that isn't the case, we just need to find the horizontal and vertical components of the force of the wind separately with some easy trig functions, but we'll quickly end up in more complicated territory if we follow that.

How much it would take to push you backwards is based on a lot of things, center of gravity, what you're standing on, what type of shoes you're wearing, but we can make an approximation again to avoid gross math. If you have a mass of 50Kg, then your weight is your mass times the acceleration due to gravity, 50Kg * 9.80m/s^2. We'll round that to 500N. For the wind to push you, it must overcome the static friction of your feet on the ground and your weight. Google says the coefficient of friction of pavement is about 0.7, and we are going to once against ignore the interface between the pavement and whatever type of shoes you're wearing. The force required to move you would be f = (coefficient of friction)*(Normal Force) where normal force is your weight. so 0.7 * 500N = 350N of force is the threshold for moving you if you stood still.

To get the force of the wind, you need the density of the medium (air) , the surface area its acting on (the person), and velocity of the wind. The density of air is 1.23kg/m^3 and we estimated the front surface area of a person to be about 1m^2. With that we can solve for the velocity squared. (sanity check, Force (Newtons) has units Kgm/s^2. If we multiply (kg/m^3) * (m^2) * ( m/s)^2 we get kgm/s^2)

So to push you back, we needed 350N. F = 1.23kg/m^3 * 1m^2 * (X)^2. X ends up being ~16.9m/s or about 37.8 miles per hour.

To actually lift that person up, you'd need the force of the wind to be greater than their weight of 500N. As mentioned about horizontal/vertical components of the force, A wind coming straight at you will never launch you up and into the air no matter how strong it is. In reality wind is a vector field and its not going to hit you with uniform velocity or direction. The best approximation for understanding the concept would be again the wind tunnel scenario above, but this time its more like one of those indoor diving simulators where you get a suit and the wind blowing up from the ground keeps you in the air. That would occur when the force of the wind is >500N or at  20.2 m/s or 45 miles per hour.

That isn't to say that a 45 mile per hour wind is going to pick you up if you are walking down the street, but its the minimum possible wind speed to lift a 50kg average man. If you turn to your side against the wind, it'll take significantly faster wind speeds to push or lift you. If you are walking forward against the wind and not just standing still, it'll impede your walking but not push you back. If you have rubber soles on your shoes, that'll change the coefficient of friction. Lots of other considerations, but I'll hold off on doing those unless you specifically want to know.


Theres a zone known as tornado alley in the southern United States where they're a pretty common occurrence. Elsewhere they're relatively rare. I don't know the exact amount of damage and fatalities tornadoes cause across the states, but they aren't a usual weather pattern. I'm reading over this now as I post it https://www.iii.org/fact-statistic/facts-statistics-tornadoes-and-thunderstorms

Apparently there was a tornado in the US that reported 140 mile an hour winds and I'm now curious how much force it'll take to 1 knock someone over and 2 move the off the ground in wind speed...

The two should be taken similarly...
Whenever I look this up I get vastly different figures as this stuff is done in a lab (I guess) for example I've seen a 50 kilo person should get knocked down at 40mph but there are enough 50kg people here that go out in 100mph storms (UK).

Also do hurricanes and tornados cause a huge problem in the US? I don't think they're more than a minor inconvenience here and doing little more than knocking trees over...