I don't know what kind of climate we're talking about, but here in the northwest I've never had a problem with melting rosin, short of leaving it in a hot car or my pocket...

I've used most types of rosin and not had problems with melting, and now I use Carlson most of the year, and Nymans when it's really hot or for solo playing because it's a little smoother. The Carlson has a nice bite for orchestra playing. I imagine the Pop's rosin would melt in relatively hot weather though, but I would never use that junk. IMO, it just ruins your bow hair and makes a mess of your bass.