Cat's Paws on Grasmere

June 17, 2016



Photographer: John Adam
Summary Author: John Adam

When a puff of wind blows over a previously smooth surface of water, it can give rise to a patch of darkened ruffled water, caused by short-wavelength ripples (capillary waves) in its surface. These waves reduce the reflectivity of the surface (being more steeply inclined to the observer’s line of sight), so the patches usually appear darker than the surroundings, and the patches can move quite fast, driven by gusts of wind. They disappear rapidly once the wind drops because the waves they produce can no longer be sustained. These tiny waves are called Cat’s Paws, a term apparently used by sailors of old on becalmed seas. They're best seen when the observer’s line of sight is low. Sometimes these patches appear to be lighter than the surrounding water; this can occur when mountains or trees produce dark reflections in the water, and the Cat’s Paws reflect more brighter sky than the surroundings, so the distribution of background light is reversed.

The top photograph was taken above Grasmere, U.K., and shows several dark Cat’s Paws as they cross the lake. The bottom photo shows the light reversal discussed above, and well-defined capillary waves. It was taken alongside a portion the Chester Canal.

Photo Details: Top - Camera Maker: OLYMPUS IMAGING CORP.; Camera Model: SP-100EE; Focal Length: 12.7mm (35mm equivalent: 71mm); Aperture: ƒ/4.7; Exposure Time: 0.0025 s (1/400); ISO equiv: 125; Exposure: Creative Program (based towards depth of field); Software: Adobe Photoshop 7.0. Bottom - same except Focal Length: 21.2mm (35mm equivalent: 117mm), Aperture: ƒ/5.3; Exposure Time: 0.0016 s (1/640).