I live in the UK where the weather totally changes from one day to the next. One day it's freezing, the next it's hot. So I never know which rug to put on my horse. If I put a lightweight on and the weather turns cold he'll be cold. But if I put his mediumweight on and the sun comes out he'll be hot. Which is better?
This is why it's better not to rug at all - horses can regulate their own body temperature quite well if they're left to grow a natural coat. That said... a rugged horse can keep himself warm easier than he can keep himself cool. He can move around, he can turn his butt to the wind, he can shiver - which gets him warm without harming him. He uses more energy, true, but he IS warm. Warm enough so he won't be hurt by it, anyway. A rugged horse has no way to cool himself at all. He sweats, it makes his fur and the rug damp. This is really, really uncomfortable for him; he's hot and steamy and wet and itchy and miserable. He can't cool himself by sweating. He's also producing the ideal environment for skin conditions like rain rot, lice, and so on. Warm, dark, wet. So... letting him be a little chilly this time of year does no harm. Keeping the heavy rug on when he's over warm produces skin problems of all sorts.
I know the feeling - we are swapping and changing at the moment same as you! Personally I'd rather the horse was a bit on the warm side, at least it will help with getting the winter coat out! At the moment we are mostly going with a medium weight stable rug over night, and a no-fill outdoor one for daytime outside, on our fairly robust but elderly warmblood.
I have just heard the weather forecast and it is going to be cold for the next few days. The sun does not come out long enough to make a difference, so I think you need to be keeping him in the medium weight. I prefer horses to be a bit warm than cold, as this is more miserable and the horse will need to use so much more energy to keep himself warm - shivering etc, and he could lose weight.
Gosh, where in the UK are you that you're getting hot days?! We had a heatwave in March but since then all the north air has kept it decidedly on the cool side! (Snow due tonight!) It is better for a horse to be too cold than too hot - if he is too cold he can move about to keep himself warm, if he's got plenty to eat his gut will heat him from the inside - if he's too hot he will sweat under his rug and be even colder if the temperature drops. So long as the lightweight is windproof and waterproof he'll be fine.
It depends very much on the horse. A good doer/fatter horse such as a cob will be better off being a little cold - so long as they have the wind kept off them with a light weight then they have enough fat to keep warm and it helps to control weight as they use some energy to stay warm. A thoroughbred type/thinner horse or older one is going to be better off being in a thicker rug and possibly being a bit warm as if they get cold they will loose weight. It also depends on how you normally rug your horse. At the moment our cobs only have rain sheets on most of the time while our tb has a medium weight - thick rug on.
