> hot air rises, so pick a campsite at higher elevation so that it will be warmer at night."
This is not entirely silly. In our Alpine region the valleys are often grass-lands, while the hills are tree-covered. That is because the cold air sinks into the valleys, creating what we call 'frost-hollows'. They are so cold that trees can't grow there.
Camping at the bottom of one of these valleys can be a chilly experience - I know! In fact, I have some photos of an open frost-covered valley with the frost cutting out just a little up the surrounding hillsides.