Convection is the movement of a gas or fluid due to differences in density (gravity, natural convection) or physical force (mechanical convection). Convection can result in the transference of thermal energy, which can either be added or subtracted by the convective process.
Natural and forced convection can exist synchronously with coincidental or independent directional characteristics, velocities, and thermal authorities.
Many meteorologic and climatologic variabilities are due to convection, whether air currents or ocean currents. Relative to the variety of environmental thermographic applications microclimates are vigorously governed by convection, particularly those within the urban canyons of heat islands.
This video (8:05) explores the basics of convection in thermography.