Capillary Waves

August 28, 2014

Capillary1
         Capillary2

Photographer: Dave Lynch 
Summary AuthorDave Lynch

Capillary waves are wind- and splash-generated waves on the water’s surface. They're small, less than about 3/4 in (2 cm) from crest to crest (the wavelength), and usually have wave heights of only a few mm. They frequently appear in rafts of parallel waves that die out in a second or two, to be replaced by others. Except on the calmest water, capillary waves are always present. Unlike larger waves, the ones we think of on lakes and oceans that are controlled by gravity, capillary waves are controlled by the water’s surface tension. Acting like a slightly springy coating, surface tension damps out the waves. When a bit of wind momentarily touches down on flat water, the capillary waves it makes darken the surface (cat’s paws).

The first picture shows the ocean’s surface (about 5 ft or 1.5 m across). Riding on top of the larger gravity waves is a field of capillary waves. Notice how capillary waves in each little group tend to be parallel to one another -- their wave fronts being perpendicular to the wind direction (inset at the bottom, about 4 in or 10 cm wide).

During the day, capillary waves are visible primarily because the amount of light reflected depends on its angle of incidence, and different parts of the wave are inclined by different amounts. The direction from the sky that the light originates also varies, and the sky brightness is never completely uniform. When two sets of waves that are propagating perpendicular to each other overlap, the reflection of the Sun (or Moon) can form a rectangular grid of bright spots (second photo, lower left). Most of the light from glitter on water is due to capillary waves.