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Does anyone know the word for shapes caused by wind blowing long grass, where light and dark appear to move across the field?
I'd like to say "He watched the _____ move across the field in the wind."
[Image from Chris Hawking via YouTube. Click to play the video.]
He watched the waves move across the field in the wind.
Have a look at more grass waves videos on google.
Longitudinal waves
In longitudinal waves, the oscillations are along the same direction as the direction of travel and energy transfer. Sound waves and waves in a stretched spring are longitudinal waves. P waves (relatively fast moving longitudinal seismic waves that travel through liquids and solids) are also longitudinal waves. Longitudinal waves show area of compression and rarefaction. http://www.bbc.co.uk/schools/gcsebitesize/science/aqa/waves/generalwavesrev2.shtml
The first verse of the American patriotic song "America, the Beautiful" makes reference to exactly this phenomenon in a very poetic way:
Oh beautiful for spacious skies;
For amber waves of grain;
For purple mountain majesties;
Above the fruited plain!
...
It refers to the wind blowing through wheat fields, so waves is a perfectly apt description.
Ripples, perhaps?
ripple noun [ C ]
a small wave on the surface of water:
- The stone she threw caused ripples to spread across the lake.
Cambridge Dictionary
I would say that the term doesn't just apply to water.
The ripples are referred to as metachronal rhythm. -- Wikipedia
A metachronal rhythm or metachronal wave refers to wavy movements produced by the sequential action (as opposed to synchronized) of structures such as cilia, segments of worms or legs. These movements produce the appearance of a travelling wave.
A Mexican wave is a large scale example of a metachronal wave.
The wave (known as the Mexican wave in the anglosphere outside North America) is an example of metachronal rhythm achieved in a packed stadium when successive groups of spectators briefly stand, yell, and raise their arms. Immediately upon stretching to full height, the spectator returns to the usual seated position.
The result is a wave of standing spectators that travels through the crowd, even though individual spectators never move away from their seats.
Descriptors of visual field dynamics are not too generic, and are usually related to the context.
If you want to play on the abundant or vital nature of a field of grains, gyrating is a term you might consider.
It is both a technical descriptor (the top of the grass is gyrating since it's stalk is rooted in one place at the ground), and the word alludes to human dance, which are part of mating ritual.
You may not think of it as gyrating, but if you watch tall grasses you will see that the motion is never "back and forth" — even though this may be how our mind idealizes or simplifies the memory. Look also at lemniscate curves to see what motions the grass-tops can follow.
Borrowing a term from optics, I would suggest caustic.
Definition:
a caustic or caustic network is the envelope of light rays reflected or refracted by a curved surface or object, or the projection of that envelope of rays on another surface
Source: wikipedia
Here is an image of caustics on the sea bed. I think that the patterns are similar to the lighter patches of grass asked for in the question.
