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Cats Paws
11/07/2002
Listen in RealAudio 
In the early dawn, a sailboat anchored in the still lagoon rocks slightly, its image mirrored
on the calm water. Above, a gossamer fog hangs like a veil around the deck and mast.
A puff of wind crosses the water to disturb the calm, and it raises nearly imperceptible waves
-- called capillary waves -- on the lagoon's surface. Unlike waves which form under stronger,
persisting winds, capillary waves arise from light winds pushing smooth undulations in the
springy water surface.
Hi, I'm Bryan Yeaton for The Weather Notebook.
In his poem "Fog," Carl Sandburg wrote: "The fog comes on little cat feet." If the fog veil
that hangs over the lagoon is a descendent of Sandburg's fog, then its little cat feet mark
the lagoon surface with cat's paws, the popular name given to those capillary waves.
Cat's paws leave their mark as small dark-water patches that litter the aquatic surface,
coming and going at the whim of the capricious breeze's touch. They darken the water surface
because sun- or moonlight reflected from portions of the cat's paws wave is directed away from
our eyes. Meanwhile, light reflecting toward us brightens the water outlining the cat's
paw.
The winds that form these waves are also known as cat's paws. Over land, cat's paws winds
often reveal their presence as they surf the land by leaving their distinctive touch upon the
tall prairie grass and wheatfields, causing -- yes -- amber waves of grain.
On the lagoon, the cat's paws presence is fleeting and quite random. With each calming of the
gentle wind, the cat's paws waves disappear like the Cheshire cat, as the water's surface
tension almost instantly pulls the water surface taut and flat again -- a near-perfect
mirror.
Thanks again to Subaru of America, and The National Science Foundation for support.
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