Eddy covariance is a micrometeorological technique that allows non-invasive measurements of the exchanges of CO2 and energy between the atmosphere and a several hectare area of forest, shrubland, or grassland.  Trace gases such as CO2 are transported between forests and the atmosphere in up- and down-drafts referred to as eddies.  These eddies move across the landscape such that a fixed point above a site during daytime  periodically "sees"  updrafts that have been warmed, humidified and depleted of CO2 by the canopy and downdrafts with the opposite characteristics.  Fast response sensors at the top of the tower (for example, a sonic anemometer) can resolve the vertical velocity of these motions, and the concentrations of CO2 carried.  The covariance of vertical wind and concentration (the time-average of the product of wind velocity and the deviation in CO2 concentration) is then calculated to provide a direct measure of flux.