Originally Posted by

**ldenk** Hi!

I don't understand the following:

F = 1/tf * V

F... vector quantity, represents the friction

tf ... friction timescale

V ... velocity of an air parcel V= (u,v,w), also called wind velocity

In the Article "Ray tracing volume densities" Kajiya and Von Herzen (1984) writes:

"Frictional effects are approximated by a simple relation yielding an exponential decay of wind velocities with time"

(the wind velocities are described by a simplified equation of motion and also

change at every time step)

Why is this a exponential decay ?!

hope you can help me

ldenk