1. In general, mass motions within a fluid resulting in transport and mixing of the properties of that fluid. Convection, along with conduction and radiation, is a principal means of energy transfer. Distinction is made between free convection (gravitational or buoyant convection), motion caused only by density differences within the fluid; and forced convection, motion induced by mechanical forces such as deflection by a large-scale surface irregularity, turbulent flow caused by friction at the boundary of a fluid, or motion caused by any applied pressure gradient. Free and forced convection are not necessarily exclusive processes. On a windy day with overcast sky, the heat exchange between ground and air is an example of forced convection. On a sunny day with a little wind where the ground temperature rises, both kinds of convection take place. 2. (Or gravitational or buoyant convection. ) Motions that are predominantly vertical and driven by buoyancy forces arising from static instability, with locally significant deviations from hydrostatic equilibrium. Atmospheric convection is nearly always turbulent. Convection may be dry, that is, with relative humidities less than 100%, especially in the boundary layer, but is commonly moist, with visible cumuliform clouds. Most convective clouds are driven by positive buoyancy, with virtual temperature greater than the environment, but clouds with precipitation, evaporation, and/or melting can produce negatively buoyant convection. See slantwise convection. 3. As specialized in atmospheric and ocean science, a class of relatively small-scale, thermally (can be driven by salt concentration in the ocean) direct circulations that result from the action of gravity upon an unstable vertical distribution of mass. (In the case of slantwise convection, though, the motions are larger scale, and are driven by a combination of gravitational and centrifugal forces acting at an angle to the vertical. )

Almost all atmospheric and oceanic convection is fully turbulent and is generally composed of a collection of convection cells, usually having widths comparable to the depth of the convecting layer. In the atmosphere, convection is the dominant vertical transport process in convective boundary layers, which are common over tropical oceans and, during sunny days, over continents. In the ocean, convection is prominent in regions of high heat loss to the atmosphere and is the main mechanism of deep water formation. Moist convection in the atmosphere is characterized by deep, saturated updrafts and downdrafts, and unsaturated downdrafts driven largely by the evaporation and melting of precipitation. This form of convection is made visible by cumulus clouds and, in the case of precipitating convection, by cumulonimbus clouds. Moist convection and radiation are the dominant modes of vertical heat transport in the Tropics. 4. In atmospheric electricity, a process of vertical charge transfer by transport of air containing a net space charge, or by motion of other media (e.g., rain) carrying net charge. Eddy diffusion of air containing a net charge gradient may also yield a convection current.