Cat's paws: where the wind touches still water

On a windless morning a lake reflects the sky almost evenly. Then a darker patch appears, a few metres wide, and slides across the surface faster than any current could carry it. Sailors call these cat's paws, the first marks a light gust leaves on calm water before a real breeze arrives.

The darkness is a matter of angle. A breeze of only one or two metres per second is enough to raise ripples a few millimetres high, and each tiny slope tilts its share of reflection away from the bright sky. Seen from a low angle, the roughened patch reflects more of the darker surroundings and lets more light into the water, so it reads as a shadow without anything blocking the sun.

They rarely last more than a few seconds, which makes them easy to miss and hard to photograph. A fast shutter keeps the edge of the patch crisp, the boundary where rippled water meets glass. On a screen that mix works quietly, one textured shape inside a field of smooth reflection, enough to give the eye a direction without filling the frame.