# What kind of wave motion is described by grass moving in the wind?

You know that sort of 'rolling' illusion when wind blows across long grass, like in the "amber waves of grain" line from America the Beautiful

It's not the same motion as dropping a rock in water, which causes an up and down motion.

And if wind blows across gravel, or water, it just shears it. The water or rocks don't recover like the individual grass blades do.

It's nothing like plucking a string, but kind of like plucking a cantilever beam. Except, I'm curious about what the aggregate motion of hundreds/thousands of blades of grass is known as

• My intuition tells me that any kind of wave motion would cause such rolling effect. When wind blows periodically all grains in a local area will sway equally but periodically. Mar 2, 2017 at 23:57
• @Steeven Edited my question for clarification Mar 3, 2017 at 0:01
• Sort of answered here as longitudinal motion, but I would not agree. There is nothing like compression and rarefaction in random breezes. english.stackexchange.com/questions/372033/…. Now is your chance to create a word........
– user146020
Mar 3, 2017 at 0:41

Each blade of grass is displaced from its equilibrium position according to the local velocity of the wind. This means the waves are actually periodic variations in the windspeed. This is likely due to vortices generated as the wind passes upstream obstacles, such as hills/woodland/buildings.

Here is a more realistic rendering of vortices, albeit with a slightly different geometry (the airflow passing both above and below the obstruction). This phenomenon is known as a Kármán vortex street.

There are three basic kinds of mechanical waves: Transverse waves, longitudinal waves, and surface waves. Surface waves propagate along an interface between differing media.

One way to describe wind blowing across tall grass is as a surface wave. The grass is held in place by its roots. The tops and sides of the grass present a surface to the wind similar to water, which is held in place by gravity and surface tension. Wind blowing across the surface of water creates circular motion of water molecules that appears as surface waves. Likewise, wind blowing across grassland may create a modified circular motion of the grass stalks.

The grass rises and falls in sinusoidal waves subject to the restoring force of the grass stalks and the density of the grass.

In the case of waves through grass, most of the energy probably is carried by air, although some may be transferred from grass stalk to stalk.

Another possibility may be to describe this phenomenon as an unstable bedform. The shapes are wavelike, like ripples in sand, but grass doesn't hold its shape as long as sand because of the restoring force of the grass blade. The wave shapes are caused by variations in windspeed.

• Calling that phenomenon a wave is quite a stretch. For a wave to propagate in a medium, each part of the medium must exert a force on neighbouring parts so that a displacement of one part may induce a displacement of neighbouring parts. There is no interaction between the blades of grass of enough significance.
– user154997
Sep 9, 2017 at 2:19
• @LucJ.Bourhis : I agree with you - it's a stretch. Wind through grass looks like a wave, but it may not be a wave. English usage calls movement of grass a "wave", but that may be poetic license - misuse of a physics term. Here's a question on an English language board that seeks an appropriate word: english.stackexchange.com/questions/372033/…. This phenomenon might be described better as a series of processes, rather than as one word. Sep 10, 2017 at 7:14
• @LucJ.Bourhis : I edited the answer and added another possibility in the last paragraph. Sep 10, 2017 at 7:28