High twist wool, unlined, adds basically zero heat (“summer weight wool” is a term that may include all these qualities). Doesn’t even do much to block wind (bu

High twist wool, unlined, adds basically zero heat (“summer weight wool” is a term that may include all these qualities). Doesn’t even do much to block wind (but does block the sun, a bit!). Totally fine in that range. That’s before resorting to something like an unstructured linen jacket (ever seen Lawrence of Arabia? Those guys are wearing plenty of clothing in the heat, a linen jacket is nothing) or warm-weather cottons like a seersucker (I’ve not bothered with any of that for jackets, myself, as light wool does fine for me, though I have several other pieces in linen). Hell I own sweaters that are totally comfortable up to about that point, and higher if there’s a breeze. It’s all about construction and fabric.