Convection

As heat transfer: energy transfer associated with motion of mass of fluid
"Turbulent transfer" if motion is small-scale in the boundary layer.
"Advection" if just part of circulation is considered.
As a form of motion (broader sense)
In a field of gravity, inhomogeneity of density is somehow made
(Note: "density" is potential one including pressure effect),
lighter parts rise and heavier parts sink.
The motion acts to reduce the inhomogeneity of density.
As a form of motion: Benard convection
Fluid is kept at high temperature at bottom and low temperature at top;
Cell-type or roll-type circulation is generated.
(Horizontal and vertical scales of the circulation are similar.)
Cumulus convection
Difference from Benard convection come from phase change of water.
Rain and snow drop => asymmetry between up and down
=> stronger updraft, broader downwelling area
Part of rain evaporates
=> upward motion does not perpetuate but appear elsewhere