flux density

1. The time rate of transport of some quantity in a particular direction crossing a plane surface of unit area, in the atmosphere usually energy (W m^-2; vector), mass (kg m^-2 s^-1; vector, transport of water vapor or other chemical species), or momentum (kg m^-1 s^-2; second-degree tensor). In radiation, the radiant energy per unit time crossing a plane surface of unit area. The term flux density is often abbreviated to flux in the fields of turbulence and boundary layers.

2. The dynamic flux density of a quantity is the flow of that quantity per unity area per unit time, where often the word dynamic is assumed, it is not explicitly stated. Flux densities are also expressed in the kinematic form, by dividing dynamic fluxes by the air density. The advantage of a kinematic flux density is that it has units that are more easily measured by a conventional meteorological instrument. The units are usually a velocity (m s^-1) times a temperature (K), specific humidity [kg of water (kg of air)^-1], or wind speed (m s^-1).

See also flux, irradiance, emittance.

Kowalski, A. S., 2024: The flux you say?: Comments on “The Integrated Carbon Observation System in Europe.” Bull. Amer. Meteor. Soc., 104, 949–952, https://doi.org/10.1175/BAMS-D-22-0274.1.

Monson, R., and D. Baldocchi, 2014: Terrestrial Biosphere–Atmosphere Fluxes. Cambridge University Press, 487 pp.

Monteith, J., and M. Unsworth, 2010: Principles of Environmental Physics. 3rd ed. Academic Press, 418 pp.

Papale, D., and Coauthors, 2024: Standards and open access are the ICOS pillars: Reply to “Comments on ‘The Integrated Carbon Observation System in Europe’.” Bull. Amer. Meteor. Soc., 104, 953–955, https://doi.org/10.1175/BAMS-D-23-0216.1.

Term edited 12 April 2024.