Flux footprint

The wind would bring in air generated somewhere upwind and brought upwards to a large degree due to turbulent exchange.

In the adjacent image, the darker the red color, the more contribution that is coming from the surface area a certain distance away for the instrument.

In such case, the footprint is the area of cumulative contribution to flux measurement computed from analytical solutions of the diffusion equation.

[1][2] Three main factors affecting the size and shape of flux footprint are: Increase in measurement height, decrease in surface roughness, and change in atmospheric stability from unstable to stable would lead to an increase in size of the footprint and move peak contribution away from the instrument.

The top figure shows relative contribution of the land surface area to the flux for two different measurement heights at near-neutral stability.