JonR, I know what you mean, man. Heat's a killer. But here's where human ingenuity kicks in. And here's where super-lightweight strategy totally fails for normal transportational cyclists. Well...

Weight and speed are two prime obsessions of racers that we don't need to care about, dude. So I flunk whichever ones I want so I can actually get the most out of my ride each time, my friend. Speed solution is obvious in the heat, just slow down; but the weight thing goes this way (for me): since I use a backpack strapped to a rear rack (call me chic), though it weighs a bit, I can store ICE COLD liquids...see! Man, one "headache cold" drink (use lotsa ice) will put fire back into your bones, like "anti-heat insulation!" Plus, wet yourself down beforehand, so you don't sweat out as much fluids...

Oh, well, a little uncontrolled rant...

NOTE: UPDATE...DUE TO THE SENSITIVITY OF SOME INDIVIDUALS TO ICE COLD DRINKS, PLEASE CONSULT A MEDICAL PROFESSIONAL BEFORE DRINKING THEM AS DESCRIBED IN THIS POST. SOME PEOPLE FACE A SERIOUS RISK FROM DOING SO, ESPECIALLY ASTHMATICS AND PEOPLE WITH ATRIAL FIBRILLATION (AF). REPEAT, THESE PEOPLE ARE SERIOUSLY AT RISK.