Mother Nature abhors extremes. A very simple statement, but one that is very powerful if you want to understand what causes our weather here on Earth. When there's any imbalance in the atmosphere, Mother Nature attempts to bring the atmosphere back to an equilibrium. This is why the jet stream exists and why we have frontal systems (air masses), hurricanes and thunderstorms.
Convection is one of those tools Mother Nature uses to bring things back to an equal state. As the sun beats down on the earth (insolation), the ground temperature increases and creates an unequal situation. Initially, conduction is the primary method Mother Nature uses to overcome this imbalance. Conduction is a molecule to molecule transmission of energy. Imagine holding a metal rod. If the temperature of one end of the rod is increased (using a blow torch, for example), the temperature will increase on that end and heat will flow through the process of conduction to the other end of the rod and eventually making it too hot to hold. But, this process of conduction is very slow and not as effective to keep up with the imbalance. This is where convection kicks in and takes over.
Convection is the method of heat transfer in a fluid resulting in the transport and mixing of the properties of that fluid. Visualize a pot of boiling water. The water at the bottom of the pot is heated by conduction. It becomes less dense and rises. Cooler and denser water from the sides and the top of the pot rushes in and replaces the rising water. In time, the water becomes thoroughly mixed and equilibrium is reached. As long as heat is applied to the pot, the water continues to transfer heat by convection. The transfer of heat by convection in this case applies only to what is happening to the water in the pot. In meteorology, the term convection is normally applied to vertical transport of temperature in the atmosphere.
Convection occurs regularly in the atmosphere and is responsible for the development of atmospheric mixing which causes much of the turbulence we experience while flying. Cumuliform clouds, showers and thunderstorms occur when sufficient moisture is present and strong vertical convection occurs. Vertical transfer of heat in the atmosphere works in a similar manner. Warmer, less dense air rises and is replaced by descending cooler, denser air, which again acquires heat to repeat the process. This continual overturning of the atmosphere is one method Mother Nature uses to bring the atmosphere back to equilibrium on a more local level. On the other hand, the presence of the jet stream is the result of equalization on the larger planetary scale. A whole different topic to discuss in a later post.
Most pilots are weatherwise, but some are otherwise.™
Weather Systems Engineer
CFI & former NWS meteorologist