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Whenever there are high winds, such as in storms, thin metal roofs on sheds as well as concave roofs on huts are sometimes blown away.

One explanation provided to me said that the higher velocity of the air outside causes the air pressure above the roof to decrease and when it has decreased to a certain extent such that the air pressure above the roof is lesser than the air pressure beneath the roof and due to some kind of osmosis, the air particles move from the area of higher pressure (beneath the roof) to the area of low pressure. In this process, the roof is blown away.

Another explanation, specifically about the thin metal roofs, said that it was blown away due to the lift caused by the air and this is the same kind of lift you get when you blow on paper.

Both these explanations puzzle me. What really bothers me is the basis of the first one, how can an increase in velocity cause pressure to drop? I can't seem to correlate that with the Force per unit area definition of pressure.

Please, oh great physicists of the internet, help me and every other ordinary person to understand how and why roofs get blown away.

Qmechanic
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Nick
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    http://en.wikipedia.org/wiki/Bernoulli%27s_principle – akhmeteli Dec 09 '13 at 05:54
  • @akhmeteli: Oh cool, it's a relationship that includes pressure and velocity... but I see no mathematical reason why one can deduce from the incompressible flow equation that velocity is inversely proportional to pressure. – Nick Dec 09 '13 at 06:15
  • @Nick, There is a derivation on the page that akhmeteli pointed you to, just click here: http://en.wikipedia.org/wiki/Bernoulli%27s_principle#Derivations_of_Bernoulli_equation – Michiel Dec 09 '13 at 06:33
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    @Nick: why do you think it is inversely proportional? If pressure decreases when velocity increases, it does not mean they are inversely proportional. – akhmeteli Dec 09 '13 at 06:34
  • @akhmeteli: That's kind of the layman's definition of inverse proportionality. But if I am to consider that pressure decreases when velocity increases without them being inversely proportional... oh! I get it. $$a+b+c= \text{constant}$$ If any one of the parameters change, the other parameters also have to change inorder to ensure that their sum is constant. And since most of the time you can't change height and gravity, the incompressible flow equation serves as the a good relation that shows that pressure increases as velocity decreases and vice versa. Now, what about the main question? – Nick Dec 09 '13 at 06:48
  • @Nick: What about the main question? If the pressure below the roof is greater than the pressure above the roof, there is a total force directed upwards. If the roof is not attached well enough, it will be blown away. – akhmeteli Dec 09 '13 at 06:56
  • @akhmeteli: Ah yes, I get that but what about the thin roofs on sheds? How does pressure work there? – Nick Dec 09 '13 at 06:57
  • @Nick: In the same way: there is an upward force, it can tear the roof off. The only difference is that a thin roof is not as rigid as a thick roof, so the shape of the thin roof can change in the process. – akhmeteli Dec 09 '13 at 07:00
  • @akhmeteli: Oh, does that mean that the explanation above is wrong or is paper lifted in the same way? – Nick Dec 09 '13 at 07:05
  • @Nick: the paper is lifted in the same way. – akhmeteli Dec 09 '13 at 07:06
  • @akhmeteli: Meaning that air particles hit the paper and accumulate causing a high pressure area on the side you're blowing on, and pushes the paper in the direction of the low pressure area? I always thought that it was due to the reaction force from the particles deflected downwards. – Nick Dec 09 '13 at 07:15
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    @Nick: I did not say anything about "accumulation". You don't need to consider the molecular mechanism of pressure: the pressure above is lower than the pressure below - this is quite enough for the roof to go for a walk:-) As for molecular mechanism of pressure, yes, it is due to the reaction force from the particles deflected from the surface in question. – akhmeteli Dec 09 '13 at 07:29
  • The title should be "Why are wooden roofs .." since a roof made of concrete or reinforced steel is unlikely to be damaged by wind. – John Alexiou Dec 09 '13 at 18:43
  • @ja72: You are allowed to increase the wind to unnatural proportions. – Nick Dec 09 '13 at 23:06
  • @Nick then the whole house is going to fly away and not only the roof. – John Alexiou Dec 09 '13 at 23:51
  • @ja72: That is irrelevant. The wind's job was to blow the roof away. It doesn't matter what's attached to it. – Nick Dec 10 '13 at 07:46

4 Answers4

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Pressure is force per unit area, yes, but it also represents the difference in kinetic energy density across a surface - only the energy of random motion of particles, though not large-scale coherent motion like wind. Accordingly, the faster a fluid moves, the more of its kinetic energy goes into large-scale motion, and the less is left for random motion of individual particles, so all other things being equal, faster moving fluids have lower pressure.

You might then wonder why all other things should be equal; in particular, why would air on both sides of a roof have roughly the same overall kinetic energy density? That's because air tends to come to equilibrium over long time scales: the pressure and temperature inside and outside the house, and thus the kinetic energy density, will tend to be similar. When a strong wind kicks up, the pressure outside drops, but there isn't time for air to flow in and out of the house to cause the corresponding drop in the interior pressure. That's why you get a pressure gradient across the roof.

David Z
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  • A Wonderful answer but I don't understand how pressure can represent the difference in KE across a surface. On dimensionally analysing pressure and energy, I see that pressure has the same physical dimensions as Energy per unit Volume. Although, I can see the relation, I can't intuitively grasp it. – Nick Dec 09 '13 at 07:35
  • I'm sure there are derivations of that out on the internet - or if you can't find one, it would make for a good separate question. – David Z Dec 09 '13 at 08:59
  • There's something wrong with this explanation. If the strong wind blows all day, allowing enough time for the "air to flow in and out of the house to cause the corresponding drop in interior pressure", Bernoulli's principle still holds. – Peter Shor Dec 09 '13 at 12:14
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    Well, but when a strong wind actually blows for a whole day, it's usually repeated gusts on top of a weaker wind. That still allows for a pressure differential. In theory, if there were constant velocity airflow (and somehow no turbulence) outside a house for an extended period, wouldn't the pressure inside drop to match the pressure outside over time? Or is it something else that equilibriates? – David Z Dec 09 '13 at 16:39
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Bernoulli to the rescue!

enter image description here

Does this answer the question?

Keep in mind, hurricane speeds are often twice small aircraft stall speeds, and typical aircraft wing loading is in the range of $50 kg/m^2$, so a roof could see 4 times that. Roofing material would have to be really heavy not to be lifted by that.

Mike Dunlavey
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Refer Bernoulli's Theorem. Watch this video for demonstration http://dornsife.usc.edu/labs/lecture-support-lab/wind-storm/.

Brief explanation: When the velocity of the wind is great enough, the air pressure above the surface is lower compared to that underneath. This cause the roof to blow off. The aeroplane work in the same principle (lower pressure on top surface).

Brandon Enright
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albedo
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    True for roof; false for airplanes. This is a common misunderstanding. Aircraft get lift by directing flow downwards, not from a (basically nonexistent) pressure differential between upper and lower wing surfaces. Google around; you'll find some interesting articles. – Carl Witthoft Dec 09 '13 at 12:37
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    @CarlWitthoft Your assertion that the pressure difference is basically nonexistent is false. The common misunderstanding that you refer to is the equal times fallacy. The misunderstanding is not that the static pressure on the upper surface is lower because the air is moving faster, the misunderstanding is why that air is moving faster. – OSE Dec 09 '13 at 13:45
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    Airfoils deflecting the flow downwards is one way to interpret lift, but (as I discuss here) there are other, equivalent, ways to look at lift as well...including a pressure differential between the upper and lower surfaces. – OSE Dec 09 '13 at 13:46
  • @OSE but that pressure differential is a function of attack angle, not airfoil shape. Airplanes fly upside down, etc. – Carl Witthoft Dec 09 '13 at 13:54
  • @CarlWitthoft It is a function of angle-of-attack, but it is also a function of airfoil shape. Cambered airfoils provide greater lift at zero angle-of-attack solely due to the change in airfoil shape, leading to a different boundary-layer edge velocity distribution and hence a different pressure differential. – OSE Dec 09 '13 at 14:01
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    @CarlWitthoft: You're right about angle of attack, but not about no-pressure-difference. OSE is also right. If you're a wing (symmetrical for argument's sake) and you're generating lift, yes you are deflecting air downward due to angle of attack, and your reward for doing this is to see a pressure difference. There's no other way for you to feel force. – Mike Dunlavey Dec 09 '13 at 14:04
  • +1 on the assumption you meant to say "pressure above the surface is lower", as your video illustrates. – Mike Dunlavey Dec 09 '13 at 18:22
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High speed winds are accompanied by reduced air pressure So high pressure from inside the house pushes roof to low pressure and gets blown away.

Pratham
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