Do Fish Know When It Is Raining?

It is a fair question to ask on a wet afternoon, standing over a pond and watching the surface come apart into rings. Rain is the one kind of weather that arrives inside a fish's world rather than outside it, and a fish is the one animal with no obvious reason to mind.

They do notice, though. A fish reads its world through a line of pressure sensors down each flank, and heavy rain is a sustained drumming on that instrument, a sky full of tiny impacts. The water also cools at the top, softens its light, and, in a stream, begins to taste of the hillside. So rain is not invisible to them at all. It is loud, it is bright with movement, and it changes the flavour of everything.

What we cannot know is the part we actually want. Somewhere in a pond there may be a fish that recognises the pattern and thinks, in whatever way a fish thinks anything, that this again is a good hour to come up and see what the surface has knocked down. Anglers certainly believe so, and have believed it for centuries.

Which leaves the nicest possibility open. Perhaps the fish knows less about rain than we do, or perhaps it is the only one of us experiencing it from the correct side.