Answer:
While conduction can be a painfully slow method of energy transfer, convection is more like a speedy locomotive. Convection is the transfer of heat energy via the vertical movement of the air. Remember those very thin layers of air in contact with paved surfaces on hot summer days? They can approach 140 degrees Fahrenheit thanks to conduction, but convection limits the thickening of those blazing hot layers of air. Just like a hot-air balloon lifting off the ground, blobs or "parcels" of hot air rise from the ground, carrying hot air skyward. This transfer of heat energy away from the ground by the vertical movement of air is called "free convection" or "natural convection." To understand the nuts and bolts of free convection, let's with the concept of buoyancy. Suppose that, while taking a swim, you submerge your favorite beach ball and then let it go. In a heartbeat, the beach ball will bob to the surface of the water. In scientific terms, the beach ball is positively buoyant. Now submerge a rock and then release it. It falls to the bottom of the pool because the rock lacks sufficient positive buoyancy to keep it afloat. Formally, we say that the rock has negative buoyancy.What makes the difference in the buoyancy between a rock and a beach ball? The answer is density. Formally, the density of an object is its mass (akin to weight) divided by its volume. The beach ball has a relatively large volume and small mass, making its density rather small and far less than the density of water. A rock, on the other hand, has a greater density than water, so it sinks. We can generalize these observations in this way: an object immersed in a fluid (water, air, etc.) is positively buoyant if the density of the object is less than the density of the fluid. Moreover, the magnitude of the buoyancy force depends on the difference in densities between the submersed object and the fluid - the greater the difference, the greater the buoyancy force.Okay, let's take our discussion out of the water and into the air. For the time being, let's start with a "parcel" of air ("parcel" is just a fancy name for a generic blob of air that we assume does not interact with surrounding air). Several factors can cause the density of the air to change, but we're going to focus on the effects of temperature on air density.
