Why do they say it, or why does it rise? They say it because they’re speaking carelessly. The statement is just a lot of hot air, because heat doesn’t rise. What they mean to say is that hot air rises.
Heat is one of many forms of energy; it is energy in the form of moving molecules. But it’s meaningless to say that any form of energy rises, falls or creeps along sideways. True, energy is always going places and doing things; that’s its mission. But it isn’t partial to any particular direction, except, of course, for gravitational energy, which on Earth shows a distinct preference for down.
(But that’s only because the center of Earth—lies beneath our feet, which we choose to define as “down.”)
We spend our lives engulfed in a sea of air, so when we think of something rising we mean that it’s rising through the air. Only air or other gases can rise through the air; solids and liquids can’t because they’re just too heavy, or dense. That last word, “dense,” is the key.
The density of a substance tells how heavy a given volume or bulk of it is. For example, a cubic foot of water weighs 62.4 pounds (a liter of water weighs 1 kilogram), while a cubic foot of room-temperature, sea-level air weighs about an ounce (a liter of it weighs about a gram).
In American Techspeak, we would say that the density of water is 62.4 pounds per cubic foot and the density of air is about 1 ounce per cubic foot.
Since there are about 1,000 ounces in 62.4 pounds, we could say loosely that water is 1,000 times “as heavy” (strictly speaking, “as dense”) as air. Now everyone in the world except the United States of America and three other great powers (Brunei, Myanmar and Yemen) uses the International System of Measurement (called “SI,” for S ystème I nternational in French), which is apprehensively referred to in the United States as the Metric System.
In SI units, the densities of water and air are very simple: 1 kilogram per liter for water and 1 gram per liter for air (at sea level). But that’s room-temperature air. Like most other things, air expands when it’s heated, because at higher temperatures its molecules are moving faster and require more elbow room, so they spread out, leaving more empty space between them. More empty space means that a cubic foot (or a liter) of the warmer air will weigh less. It is now less dense than it was.
But the 62.4–dollar question is, What makes that warmer, lighter air move upward through the heavier, cooler air? Well, what does “heavier” mean? It means that gravity is pulling down on the cool air more strongly than on the warm air. (There are more molecules per cubic foot or liter to pull on.)
So wherever warm and cool air find themselves next to each other, the cool air will be pulled down past the warm air. The warm air has no alternative but to get out of the way and be displaced upward.
Lo! It is risen. When one of those beautiful hot-air balloons takes off into the blue sky, people gawking upward from the ground may wonder what force is “pushing it up.” Now you know that there is no upward force. That bubble of hot air is merely being subjected to a lesser downward force, compared with the cooler surrounding air. And that has precisely, exactly, absolutely the same effect.