Tagging along w/ the other replies & preivious experience, I've noticed that heat buildup also seems to be related to the size of the vehicle/glass area ratio; i.e., the smaller the vehicle, the larger-by-default glass area = quicker heat buildup...
For an eye opening (leg warming?) experience, try driving an MGB (if you can find one) for a while...you will appreciate why top-down mode is the preferred method of travel...my $.02...
