confluence

The rate at which adjacent flow is converging along an axis oriented normal to the flow at the point in question. It is the opposite of diffluence. In natural coordinates the confluence may be measured by

where V is the speed of the wind, the n axis is oriented 90° clockwise from the direction of the wind vector, V_n is the wind component in the n direction, and ψ is the wind direction, measured in degrees clockwise from the reference direction.