I have a Saxon rain sheet – that is VERY light weight, it is also light blue which keeps it from getting hot in the sun. I find that my horse can wear this on 60+ degree days – and gallop around in turnout out without working up a sweat (she has a trace clip only)
I really like the Sunshower from Buccas, too. And I'm typically not a huge fan of Buccas. I have a mare in my care who seems to be very prone to rain induced hives. She wears her Sunshower just all summer and fall (basically from the moment she no longer needs a turnout rug to the moment she does). Very rare for her to come in sweaty, even on the hottest, most humid Virginia summer days.