Wind, rain, sun, light snow - mines out.
Unless its hailing, blowing the roofs off or deep snow then mine goes out.
If it snowed bad he'd still get a couple of hours leg stretch. Luckily another livery has his 3 in the next field all year round so mines never totally alone if his field companion is brought in
Horses don't melt in the rain
So long as they are rugged or fed sufficiently I don't see a prob turning out.