Believe it or not, the idea behind wool jackets in summertime was, ... the jacket would fill up with sweat and when a breeze went across it, it would have a cooling effect. I reenacted back in the day, and this theory really works. But it's hard to get used to, and at first doesn't make sense. The temps drop at night usually during camp and the wool jackets are good for that too. Guess it just serves double duty, with the emphasis being on night time temps.
Les