Well I'm always putting down the top and I have noticed as the air temp goes up 75-95* when I pull up to a traffic light I can feel the heat from under the car roll up over the driver door. Not a big deal but I assume it's the same for all of the cars. Just an observation.

 

I think you are right. You will especially feel the heat radiating from the undercarriage on a convertible if you have the windows down. Here in New Mexico we are currently running 95 to 110 degree temps. I barely put down the top when over 95 degrees because of the intensity of the sun. On cooler days the top goes down, but I usually have the driver and passenger window up, so I don't feel any heat eminating from the undercarraige.

Are you driving with the windows down when you feel the excess heat?

 

Have you ever driven in an Austin Healey 3000? You can cook chickens inside from the engine heat!

 

Hahaha LOL not a AH but my TR3A was a problem scraping my knuckles on the ground.