It really does depend on the horse BUT! As a rule my trainer refuses to stand in the indoor and teach at temps below 20F so I figure turnout at temps below 15F is not necessary. I realize this is just for my own comfort but at that temp I’ve noticed my horses stand still, turn their back to the wind, and that’s pretty much it. I don’t feel like they are moving around enough to keep warm so in the stall they go.
Now, I have to admit, if it’s 15F during the day I will put the horses out. I just figure in they day the sun is out creating warmth (even though it’s not since the temps are 15F WITH the sun but it makes me feel better).
My horse and one other horse on the farm wear a blanket. There is another horse who wears one sometimes and a couple that wear a sheet if they are needed clean for some reason. Other than that they are nekkid regardless of temps. Our horses are older (mostly in their 20’s) and are TB’s, QH’s, and Walkers.
I will agree some horses can’t handle the temps but mosty it’s the wind that eliminates their ability to keep warm. Horses handle the cold very well so long as they have shelter from the wind and aren’t wet. It’s such a testament to horses when I get to the farm and find inches of snow accumulating on their backs - the hair traps the heat in so the snow doesn’t melt
If the snow is melting then the horse isn’t insulating and therefore is probably cold. If it’s not windy and not precipitaing they are generally fine. Those in a cozy blankie are fine no matter what the temp/precip (usually).