It will be 65 here tomorrow and I am going to bathe horse tails. If there is any breeze at all, bathing a horse outside in the spring is risky. I say that because the ground it still cold and although the sun feels warm, it can be chilly. That being said, I'd bathe a horse in a heated barn with hot water in a second. You are SO lucky. I have a pure white horse and he is a mud ball right now!!!