absolute vorticity The vorticity of a fluid particle defined with respect to an absolute reference frame. The vertical component (perpendicular to the horizontal plane) of the absolute vorticity, the one to which reference is normally made, is defined as the sum of the vertical component of the vorticity of the fluid with respect to Earth's surface (the relative vorticity) and the vertical component of the vorticity of Earth (equal to the Coriolis parameter).

absorptivity The ratio of the amount of radiant energy absorbed by a given substance to the total amount incident upon it, at a given wavelength. Absorption may occur at the surface if the substance is opaque, or during the transit of the radiation through a translucent substance.

adiabatic Describes a process in which the thermodynamic state of a system changes without the transfer of heat or mass across the boundaries of the system.

advection A process by which an atmospheric property is transported from one location to another by the motion of the air. A distinction is often made between horizontal and vertical advection.

advection fog A fog that is formed by the horizontal advection of moist air over a surface that is cooler than the air, and the consequent cooling of the air near the surface to its dew-point temperature.