Heat is basically an abundance of energy. This energy causes the atoms and molecules inside your vehicle to move around at a faster pace, thus taking up more space and increasing the air's density. As we all learned from science class, denser air rises, which puts the hottest air inside any vehicle right where your head is, which is a very unpleasant circumstance!
When you get inside any hot vehicle you must resist the temptation to cool your upper body first as most people tend to do. The first thing you should do is push the hot air out, starting with the hottest air. The easiest way to do this is to set the air-conditioner on fresh-air (not recirculation), crack the windows, set the fan to high, and adjust the air to blow through the lower foot-well openings.
Pushing the air from the bottom vents helps rid the hot air more efficiently than using the upper vents. this is truly the best way to purge the stale air from the entire vehicle in the shortest amount of time. Once the majority of hot air is removed, you can close the windows and adjust the air to blow towards your upper body (if you prefer).