To be fair, it is a mesh jacket. But that doesn't seem to help much. When it is hot (most of the time) and humid (all of the time) it is just damned brutal.
The best thing I've found for hot-weather riding is a nylon hunting vest, the kind where the whole back of the vest is used as a "game pocket". Fill a couple of big ziplock bags with ice, stuff 'em in there and snuggle 'em up right over your kidneys.