Like Poseidon said, negative temps and worse windchills. Yesterday the windchills were in the -30's. Ours are out 24/7 with just trees for shelter. They have a heated water trough and free choice hay. No blankets. When it snows, the snow forms a natural "blanket".
If your horse is wet and shivering, you could put a cooler or blanket on until they warm up and dry off. I wouldn't leave it on too long though.
Even with no blankets and no shelter, I haven't seen any of our horses shiver this year. Not even yesterday with how cold it was. The only time I did see it was as the temps fell from 60's one week to 30's the next. There was also rain involved.
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