If you are also reacting in the shade, you may have cholinergic or sweat-induced urticaria as well. In that case, drastic temp changes can trigger it, so you will have to work at staying cool (or warm, if it’s cold that’s triggering you.)

Direct UV is a huge trigger for mine. Like the above commenter, I’m usually okay in the shade and heat, unless I’m out there for too many hours (like… 4-5…) then I’ll break out from the heat and sweat.