1. The covariance between two variables, associated with turbulent motions. For example, if the overbar represents a mean value, and the prime denotes a deviation from the mean, then

is the eddy correlation of vertical velocity w and potential temperature θ, where i is a data index and N is the total number of data points. If one of the two variables is a velocity, such as in the example here, then the eddy correlation represents a kinematic flux associated with turbulence. In this example, the correlation is a vertical kinematic heat flux (units of K m s−1), which can be transformed into a dynamic flux (W m−2) by multiplying by air density times specific heat at constant pressure. 2. A method of measuring the flux densities of mass, heat, and momentum across a plane at a point in turbulent flow. For vertical fluxes, the fluxes are found as the covariance of the fluctuations in the vertical wind velocity with local variations in concentration, heat content, or horizontal wind velocity, respectively. For conservative quantities in the atmospheric surface layer, such vertical fluxes are virtually equal to the fluxes at the surface of the earth. Measurements from towers require sensors with a speed of response that is typically no larger than about 1 s, but the required response varies with height, wind speed, and amount of buoyancy-induced mixing. Observations from eddy correlation systems on fixed-wing aircraft in the planetary boundary layer typically require a response that is about ten times faster than stationary systems.