On ripple formation (winds)

Question 1

What do ripples on sand originate from?

Answer 1

They do not result from some predetermined pattern in the wind that is somehow impressed on the surface, but rather from the dynamics of individual grains in motion across the surface.

Question 2

When do ripples arise?

Answer 2

Whenever wind blows strongly enough over a sand surface to entrain grains into the wind.
Rising relatively steeply off the bed, their path is then stretched downwind as they are accelerated by drag forces.
They impact the sand typically at a low angle.
It is this beam of wind-accelerated grains impacting the sand surface at a low angle that is responsible for ripples.

Question 3

What is saltation?

Answer 3

The subsequent hopping and leaping of these grains.

Question 4

In order to march downwind what must happen?

Answer 4

Deposition must be occurring on the downwind faces of the ripples, and erosion must be occurring on the upwind faces

Question 5

When does erosion and deposition occur?

Answer 5

Erosion occurs where there are more grains leaving the surface than are arriving, and deposition occurs where more grains are arriving than are leaving

Question 6

What controls the wavelength?

Answer 6

The wind speed, in the following way. The wind speed determines the impact angle of the longer trajectories: the greater the wind speed, the more the saltation trajectories are stretched out, and the lower the resulting impact angles.