This would probably be unremarkable, except that cat’s paws are strikingly visible: you don’t just feel them; you see them. They usually take the form of a dark (occasionally lighter) patch moving across the surface of the water. This has caused some people to assign the name, cat’s paw<\/em>, to the disturbed water, itself—and indeed to fabricate explanations of it based upon the character of the disturbance. I prefer the older terminology: a cat’s paw<\/em> is a wind that gently disturbed the water’s surface, leaving a cat’s paw print<\/em>.\u00a0<\/p>\n Yet, all who discuss it agree that the disturbance appears to result from ripples—that is, capillary waves—that have been excited by the gust of air. Indeed, this seems correct. The problem I faced was: Why do the ripples make the water’s surface appear dark?<\/p>\n Waves on water fall into two overlapping groups: gravity is the restoring force (think of a swing); surface tension is the restoring force (think of a spring). When the wavelength is greater than 1.7 centimetres, gravity dominates; when it is less, surface tension dominates. Now there are a few differences in behaviour across this divide (one was discussed at hoodie ripples<\/a>), but the interesting one for cat’s paws is that while the slopes of gravity waves (\u03bb\u00a0> 1.7 cm.) never get large, the slopes of ripples (capillary waves,\u00a0\u03bb < 1.7 cm.) become quite steep.<\/p>\n To understand why the steepness of the waves makes a difference in the appearance, one can imagine a calm surface of water. The light coming from it is a mixture of light refracted from the depths and light reflected from above the surface. If the view is nearly horizontal, most of that light is reflected from a bright horizon; if the view is straight in, most of that light is refracted from dark depths.\u00a0<\/p>\n When the surface is irregular, what is seen depends upon the statistics of the surface tipping: gentle tipping, it still looks close to the brightness of a horizontal surface; steep tipping, it takes on some of the darkness of looking straight in. So, regular waves favour viewing light from the bright horizon, while ripples favour peering into stygian depths.\u00a0<\/p>\n Ok, the ripples created by the cat’s paw make the surface appear dark. Yet, another problem remains: Why does the print follow the paw (the gust)? Look at the issue this way: someone who is ski touring on virgin snow leaves a trail from the start to the finish. The cat’s paw only leaves a mark immediately below the paw—it leaves no trail. While a gravity wave can travel across oceans, a ripple (surface-tension restoration) decays rapidly. Remove the driving force of the wind gust and viscosity kills them within metres.\u00a0<\/p>\n There it is: cat’s paws are gentle gusts of wind that generate ripples on the water’s surface; the steepness of the ripples emphasizes stygian depths over horizon brilliance; the transience of ripples emphasizes tracks over trails. \u00a0<\/p>\n Dark patches move across the surface of the Lake. They are the prints from a wind called a cat’s paw. An ancient mariner’s term for a breath of air that lightly brushes the water is a cat’s paw. The name implies the gentleness of the touch; it is a zephyr, a gentle gust of wind.\u00a0 This would probably be … Continue reading ![\"\"](\"https:\/\/blog.kootenay-lake.ca\/wp-content\/uploads\/2014\/01\/catspaw131228s.jpg\")
<\/p>\n","protected":false},"excerpt":{"rendered":"