Rhythm.

Monet, Morning on the Seine in the Rain, 1898

A little tap at the window, as though some missile had struck it, followed by a plentiful, falling sound, as light, though, as if a shower of sand were being sprinkled from a window overhead; then the fall spread, took on an order, a rhythm, became liquid, loud, drumming, musical, innumerable, universal. It was the rain.

Marcel Proust