I'm going to disagree with most of you, and say that you can't really judge what blanket to use by temp. My mare is a tb/qh/welsh/belgian, and she pretty much has a fully developed pony winter coat (you know, the insanely thick and adorable, yet sooo annoying ones) by October. It's usually in the 60s during the day in October where I live. So she got a low trace clip so she wouldn't over heat when I rode her. Now, you can't even tell she was clipped she's so hairy. But anyway, once it gets below 40 at night, she starts getting blanketed. I show in the winter, so a huge winter coat definitely doesn't help earn points with the judges. Once its consistently below 38 or so, she gets a midweight. We've had a pretty cold winter this year- day temps between 15 and 30, night temps between 5 (windchill) and 30 depending on the day temp. So she's had her super-heavy heavy weight on all winter.
I'd bring her in at the warmest point of the day when it was about 30, and she would be obviously shivering and cold under her blanket. She's only 7, super healthy, and very fat (that's just the way she's built, and she's on almost no grain and unfortunately, since she's field kept, almost 24/7 hay) There is absolutely no reason she should be that cold, but she is. We have a 14 year old gelding who wears a midweight all winter and doesn't have a super thick coat, and the cold has never bothered him.
So my point in writing that whole novel is that blanketing really shouldn't be judged by temperature IMO. A lot of you don't even blanket your horses in the winter, and I know if I didn't blanket my mare she would pretty much freeze to death and be incredibly uncomfortable all winter. You also can't really judge when to take blankets off, because it really does just depend.
** Please don't bash me for saying that my horse needs to be blanketed in the winter. I'm not trying to be rude when I say this, but I know my horse better than you. If your horse can stand the cold comfortably, you're very lucky. You save tons of money. My horse, however, is not comfortable without being heavily blanketed, whether you choose to believe me or not.
Worth The Wait <3