Convection heat transfer occurs through the movement of fluids like air, because warm fluids are lighter (less dense) than cold fluids, and float!
It is because hot air climbs over cold air, that a hot air balloon lifts into the sky over the cold air below it.This thermography image shows how hot air climbs through the house (called the stack effect) to the highest point before conducting through the roof and walls to the outside, and radiating its heat away (which is why the camera can detect it).
