July 21, 2017: HEAT DOES NOT RISE
You must have read or heard it said many times that “heat rises.” That’s a layperson’s misconception: it doesn’t.
To rise is to ascend or go up (i.e. to move in the direction opposite the pull of gravity). But heat is pure energy; hence it has no mass. Therefore, it cannot respond to gravity in any way. In vacuum, heat moves with equal ease in all directions by a process called “radiation.” (Heat also moves through a material medium by a process called conduction.)
When people say “heat rises,” what they mean to say is that heated air rises. More generally, when most fluids (liquids and gases) are heated non-uniformly, the hotter segments rise and the cooler segment falls. This happens because the hotter portion expands; the cooler portion is then relatively denser and so falls to displace the hotter portion upwards. This process is called convection and is the main engine of weather changes around us.