full text of the classic FAA guide
When two surfaces are heated unequally, they heat the overlying air unevenly. The warmer* air expands and becomes lighter or less dense than the cool* air. The more dense, cool air is drawn to the ground by its greater gravitational force lifting or forcing the warm air upward much as oil is forced to the top of water when the two are mixed. Figure 18 shows the convective process. The rising air spreads and cools, eventually descending to complete the convective circulation. As long as the uneven heating persists, convection maintains a continuous “convective current.”
* Frequently throughout this book, we refer to air as warm, cool, or cold. These terms refer to relative temperatures and not to any fixed temperature reference or to temperatures as they may affect our comfort. For example, compare air at −10° F to air at 0° F; relative to each other, the −10° F air is cool and the 0° F, warm. 90° F would be cool or cold relative to 100° F.
The horizontal air flow in a convective current is “wind.” Convection of both large and small scales accounts for systems ranging from hemispheric circulations down to local eddies. This horizontal flow, wind, is sometimes called “advection.” However, the term “advection” more commonly applies to the transport of atmospheric properties by the wind, i.e., warm advection; cold advection; advection of water vapor, etc.
FIGURE 18. Convective current resulting from uneven heating of air by contrasting surface temperatures. The cool, heavier air forces the warmer air aloft establishing a convective cell. Convection continues as long as the uneven heating persists.