During the summer, healthy horses who only go out to pasture for a few hours at a time do not usually need a turnout rug. Sick or very sensitive horses can be covered with an unlined rug. Open stable horses are normally very robust and can usually get out of the rain if it gets too much for them. Spring and autumn are a different matter. While rain alone does not affect healthy horses, the combination of rain, cold weather and wind makes rain unpleasant for the animals and ultimately dangerous to their health. In this case, a rain sheet or lightweight turnout rug is appropriate, even if the horse only spends a few hours in the meadow or paddock. The turnout rug should be lightly lined.
A lightweight horse rain rug should waterproof and breathable. If you wash the rug too often, it may no longer be waterproof. You can then treat it with a waterproofing spray. In extreme cases, you will have to replace the rug with a new one. A rug that is not waterproof will soak up rain and lie on the horse like a weight. This is not only extremely unpleasant for the animal, but will also no longer protect the horse against the elements.