How do grain size and wind velocity affect the spacing of sand ripples?

In the study of sand dunes, the spacing of bed forms—including ripples—is influenced by both grain size and wind velocity, though the relationship is complex and not a simple matter of cause and effect.

Within the hierarchy of sandy features, ripples typically exhibit spacing of only a few centimetres. However, these dimensions vary based on the physical characteristics of the sediment and the flow of the wind:

• Grain Size: Larger grain sizes generally lead to wider spacing. For instance, "mega-ripples" composed of very coarse sand can be spaced almost as far apart as small dunes. Similarly, coarse sand on sand sheets can form low undulations spaced up to 500 metres apart.
• Wind Velocity: Variations in wind velocity are fundamental to the formation and spacing of these features. Regular patterns in wind flow, often organized into spiral vortices, dictate the regular repetition of dune and ripple shapes. While wind velocity is a key factor in determining the size and spacing of these features, the specific relationship is not constant across all localities or dune types.1