(Also called coefficient of diffusion.) The ratio of the flux of a conservative property through a specified surface by turbulence to the gradient of the mean property normal to the surface. In the special case of isotropic turbulence and no mean motion the Fickian diffusion equation (or Fick's equation) takes the form where represents the diffusivity, ∇2 is the Laplacian operator, and is the mean value of the property . This equation describes decreasing where the Laplacian is negative and increasing where the Laplacian is positive. The general case is more complex. In the statically stable atmosphere or ocean, the horizontal scale of turbulence is much greater than the vertical scale and turbulent diffusion in the horizontal may greatly exceed diffusion in the vertical. On the other hand, in the case of buoyancy, vertical diffusion may be greater than horizontal diffusion. See also mixing, eddy flux, turbulence length scales. Fleagle, R. G., and J. A. Businger, 1980: An Introduction to Atmospheric Physics, 2d ed., Academic Press, 64– 66. Hinze, J. O., 1975: Turbulence, 2d ed., McGraw–Hill, 48–55.

Tendency to become diffused; tendency, as of heat, to become equalized by spreading through a conducting medium.

the tendency of materials to spread out (mix with their surroundings), and measurements thereof.

Thermal diffusivity is the measure of the way heat flows through a material to the other side. It is also often expressed as the rate of change of temperature in a transient heat transfer process. The higher the thermal diffusivity of a material, the higher the rate of temperature propagation. Where:= Thermal Diffusivity= Thermal Conductivity= Specific Heat= Density According to this relation, thermal diffusivity will affect any conductive transient heat transfer process within the sample medium. Thermal diffusivity has the dimension length2 / time and is expressed in the units' mm2/s.

the temperature depedent rate that an impurity diffuses.

Measure of the extent to which very small particles are influenced by molecular collisions which cause the particles to move in a random manner across the direction of gas flow.

a measure of the rate at which diffusion occurs through a material.