# Why do flags ripple in a steady breeze? **URL:** \ **Category:** Factual Questions\ **Created:** [February 12, 2013, 5:56am UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066 "2013-02-12T05:56:16Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [February 12, 2013, 5:56am UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/1 "2013-02-12T05:56:16Z")
See subject. ---
**Author:** ![Patty\_O\_Furniture](https://avatars.discourse-cdn.com/v4/letter/p/96bed5/32.png) [@Patty\_O\_Furniture](https://boards.straightdope.com/u/Patty_O_Furniture)\ **Post date:** [February 12, 2013, 6:54am UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/2 "2013-02-12T06:54:04Z")
The weight of the flag is continuously being pulled down by gravity, while the breeze keeps it aloft. Those two forces working against each other give the fluttering or rippling effect. There may also be minor pockets of air turbulence involved. ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [February 12, 2013, 6:54am UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/3 "2013-02-12T06:54:18Z")
Gravity. ---
**Author:** ![Absolute](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@Absolute](https://boards.straightdope.com/u/Absolute)\ **Post date:** [February 12, 2013, 7:20am UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/4 "2013-02-12T07:20:57Z")
Nah, it’s not gravity. A flag held vertically in a vertical breeze would still flutter. The question is why a flag in the breeze is inherently unstable. ---
**Author:** ![Patty\_O\_Furniture](https://avatars.discourse-cdn.com/v4/letter/p/96bed5/32.png) [@Patty\_O\_Furniture](https://boards.straightdope.com/u/Patty_O_Furniture)\ **Post date:** [February 12, 2013, 7:26am UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/5 "2013-02-12T07:26:53Z")
> [@Absolute](#): > > A flag held vertically in a vertical breeze would still flutter. Not that I’m doubting you, but how do you know this? Is there a flag hanging somewhere that is subject to only a steady downward breeze (with no obstacles in the way or people walking around it)? And adding to my first post, I think the ripples in the air currents have a lot to do with it. Especially if there are any buildings or trees around, they will create swirls and little pockets of turbulence that we may not be able to perceive at ground level. ---
**Author:** ![terentii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/terentii/32/212_2.png) [@terentii](https://boards.straightdope.com/u/terentii)\ **Post date:** [February 12, 2013, 7:28am UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/6 "2013-02-12T07:28:43Z")
Bernoulli’s principle, maybe? :dubious: ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [February 12, 2013, 8:16am UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/7 "2013-02-12T08:16:52Z")
So you put a flag on a treadmill… ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 12, 2013, 8:19am UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/8 "2013-02-12T08:19:24Z")
I think a hypothetical smooth, uniform flag, subjected to a hypothetical perfectly laminar breeze would not flutter. In reality, flags aren’t perfectly smooth and uniform and the wind is turbulent - even at a small scale, and especially after being perturbed as it passes by the flagpole. Any small irregularity in the flow of air across the flag displaces the lightweight fabric and in so doing, it causes further disturbance elsewhere - amplifying the effect. ---
**Author:** ![bluezooky](https://avatars.discourse-cdn.com/v4/letter/b/e8c25b/32.png) [@bluezooky](https://boards.straightdope.com/u/bluezooky)\ **Post date:** [February 12, 2013, 11:37am UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/9 "2013-02-12T11:37:32Z")
All of the above plus “vortex shedding” ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [February 12, 2013, 12:01pm UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/10 "2013-02-12T12:01:54Z")
This is a well studied phenomenon in Fluid dynamics and there are multiple simulations available to solve this. It was first addressed in 1879. Here is an old publication discussing this : [http://www.ecs.umass.edu/mie/faculty/perot/mie821/Reviews/annurev-fluid-flapping.pdf](http://www.ecs.umass.edu/mie/faculty/perot/mie821/Reviews/annurev-fluid-flapping.pdf) If you look around on youtube you can find some simulations too. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [February 12, 2013, 1:56pm UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/11 "2013-02-12T13:56:32Z")
> [@am77494](#): > > This is a well studied phenomenon in Fluid dynamics and there are multiple simulations available to solve this. It was first addressed in 1879. Here is an old publication discussing this : [http://www.ecs.umass.edu/mie/faculty/perot/mie821/Reviews/annurev-fluid-flapping.pdf](http://www.ecs.umass.edu/mie/faculty/perot/mie821/Reviews/annurev-fluid-flapping.pdf) > > If you look around on youtube you can find some simulations too. Yeah? Well why does the US flag on the moon ripple? :dubious: Thanks. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [February 12, 2013, 4:24pm UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/12 "2013-02-12T16:24:05Z")
the flagpole creates turbulence which affects the flag in the downstream wake. ---
**Author:** ![RickG](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rickg/32/5174_2.png) [@RickG](https://boards.straightdope.com/u/RickG)\ **Post date:** [February 12, 2013, 4:34pm UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/13 "2013-02-12T16:34:47Z")
> [@Leo\_Bloom](#): > > Yeah? Well why does the US flag on the moon ripple? It doesn’t anymore. It did initially because the astronauts were moving the pole/frame about and that made the material of the flag flap around. After a bit, internal friction would have damped the motion, and, in the absence of further disturbance by time-varying external forces (like wind) it should remain quite still. I think _Mythbusters_ put a flag in a big vacuum chamber to test this out a while back. -Rick ---
**Author:** ![ski](https://avatars.discourse-cdn.com/v4/letter/s/bbe5ce/32.png) [@ski](https://boards.straightdope.com/u/ski)\ **Post date:** [February 12, 2013, 8:18pm UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/14 "2013-02-12T20:18:53Z")
Well, first I’d say there’s no such thing as a “steady breeze”, the wind always has small but continuous changes in velocity. But that said, two reasons - boundary layer and the flagpole. 1. The aerodynamic effect called the “boundary layer.” > **[Boundary layer](https://en.wikipedia.org/wiki/Boundary_layer)** > > In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces a no-slip boundary condition (zero velocity at the wall). The flow velocity then monotonically increases above the surface until it returns to the bulk flow velocity. The thin layer consisting of fluid whose velocity has not yet returned to the bulk flow velocity is called the vel... which causes Boundary Layer Separation > **[Flow separation](https://en.wikipedia.org/wiki/Boundary_layer_separation)** > > In fluid dynamics, flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake. > A boundary layer exists whenever there is relative movement between a fluid and a solid surface with viscous forces present in the layer of fluid close to the surface. The flow can be externally, around a body, or internally, in an enclosed passage. Boundary layers can be either laminar or turbulent. A reasonable assessment of whether the boundary layer will be lam... The wiki articles are pretty mathy, but basically, airflow along a flat surface (like a wall, or a perfectly stiff flag) will eventually get turbulent - no longer smooth airflow. The flag has this happening on both sides, but since it would almost never happen exactly identically, the flag ripples in the breeze. 1. The flagpole is an obstruction to the flow of the air (wind). The air spills around the flagpole and creates small-scale turbulent whirlpools and eddys of air. Thus the flag ripples. Note that 1) would happen even if you didn’t have a flagpole, so the flag would still ripple. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [February 12, 2013, 9:38pm UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/15 "2013-02-12T21:38:04Z")
> [@RickG](#): > > It doesn’t anymore. It did initially because the astronauts were moving the pole/frame about and that made the material of the flag flap around. After a bit, internal friction would have damped the motion, and, in the absence of further disturbance by time-varying external forces (like wind) it should remain quite still. I think _Mythbusters_ put a flag in a big vacuum chamber to test this out a while back. > > -Rick That’s what they want you to believe. ---
**Author:** ![Manlob](https://avatars.discourse-cdn.com/v4/letter/m/96bed5/32.png) [@Manlob](https://boards.straightdope.com/u/Manlob)\ **Post date:** [February 13, 2013, 4:01am UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/16 "2013-02-13T04:01:38Z")
A flag extended by a breeze is a thin flexible membrane under tension and will therefore act to transmit waves, i.e. ripple, especially if not loaded in a perfectly symmetrical way. There really isn’t a reasonable way for it to be loaded symmetrically by the wind. A non rippling flag extended by a laminar flow of wind would probably not have enough viscous forces to maintain an extended configuration and it will flop down, but it will wrinkle asymmetrically as it drops causing asymmetrical forces and rippling. Of course a waving flag will interact with the flow and may create vortices, but I wouldn’t say that turbulence and vortices cause the waving. Imagine hold a towel at 2 corners and letting it hang under its own weight in a vacuum. If you lightly shake that towel it will ripple much like a flag waving in the wind, because you would have the thin flexible membrane under tension. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [February 13, 2013, 4:52am UTC](https://boards.straightdope.com/t/why-do-flags-ripple-in-a-steady-breeze/650066/17 "2013-02-13T04:52:25Z")
Isn’t that called a transmission wave? When I snap the bullwhip and the force travels over less and less mass–propelling it as the the force barely attenuates on the thinning whip–till past sound speed. What is the initial “flip” energy? How can that be maintained over a flag?–eg, when you whip up high a sheet to cover your bed, the flip is always difficult because you have to figure in the air that the flip moves, and how it affects the travel of that sheep (the course of its transmission wave–right?).