It's a cold, rainy February day. A good time to blog about rain in the Middle Ages.
As any farmer will tell you (and there are fewer active farmers all the time in the US), growing food is dependent on a foot or so of topsoil and the fact that it rains. Without water falling out of the sky, agriculture is just not going to work. The vast majority of people in the Middle Ages lived on a working farm, and they knew this very well.
But how about irrigation? you say. Certainly medieval people knew about irrigation ditches, even if they didn't have the elaborate machine-driven irrigation systems used today in, for example, California's central valley. In the ancient world, the "fertile crescent" between the Tigris and Euphrates rivers, in what is now Iraq, got much of its water via irrigation ditches from the rivers. It didn't rain very much in ancient Egypt either, but the barley fields were irrigated by the Nile. But in these places, as in medieval Europe, where do you think the river water came from? That's right, rain.
Rain also provided drinking water in many places. A castle up on an inaccessible mountain, for example, would not have a decent well and thus would collect rainwater, both for drinking and cooking and for everything else water was used for.
Now in the Middle Ages, as now, too much rain could be as much of a problem as it is now. Floods in a city (and all cities were built on rivers) caused all sorts of problems. Flooded fields meant that crops couldn't be planted, or couldn't be harvested, or rotted without ripening. But overall rain was considered good.
On the other hand, it was just as uncomfortable then as it is now to be out in the rain, and they had far fewer things to keep the rain off. We reach for our umbrellas. Well, umbrellas only became available in the West in the seventeenth century. China and Japan had long had parasols (Japan's mostly made of paper, thus better to keep off the sun than the rain), and Europe probably got umbrellas from the East.
The image is a van Dyke painting of a seventeenth-century lady with an umbrella. It was a new, fashionable invention. The word umbrella comes the Latin umbra, meaning shade. The Brits often call them brolleys (derived from umbrella). Americans, for reasons obscure, may call them bumbershoots.
Medieval people also didn't have raincoats. Look at the tag in your raincoat. It is probably made at least partially from nylon or polyester or else has a plastic-based finish. Medieval people had very tightly woven boiled wool. Wrap your cloak around you and put up the hood. At least wool will continue to keep you warm even when it's wet, but it would soak through fairly soon.