here in Sonoma, we have rain. in the winter months and even into the spring ones we have wet, soggy, powder moistening rain. then, we have spring where if you are allergic to pollen like me, you can't hold a gun straight. then in the summer we have heat. that's just fine when I'm shooting, except powder stick to you if you get any on you. not bad though, i can take tons of heat but i hate the cold. maybe talk to your range owner and see if you can come over at night to shoot, or better yet see if they will hold a night shoot! it would definitely be cooler and would attract tons of people.
for summer heat i like to use paper cartridges in my rifle. seems to help with shooting because you don't seem to fumble with stuff and get sweat all over everything. just tear off the end, remove the Minnie ball, prime the pan, pour your powder down and ram the bullet home. it's real easy.