LSL Wiki : llWind

vector llWind(vector offset)

Returns the wind velocity at the current position + offset.

Wind affects avatar clothing/hair, trees, and can affect particles and flexible prims. Wind "naturally" (programatically) varies in velocity and direction. Wind does not cause friction.

Example:

vector wind = llWind(<0,0,0>);
llSay(0,"Wind speed: " + (string)llVecMag(wind));

"The wind is based on the 2D stable fluid method described in Jos Stam's article in the Siggraph 1999 proceedings. Each region is its own incompressible simulation but it trades boundary conditions with its neighbors so it is possible for one region to affect another. The consequence of this is that the largest coherent vortex that the wind can form is on the scale of a single region, however wind flowing out of one region will push that in the other so some wind structures are larger than a single sim.

It has some pseudo stable chaos magic injected that keeps it boiling, and the chaos is scaled by the sun position so that the ambient activity varies on a daily schedule. I think there is a small offset that causes the global wind average to point away from the sun around sunrise and sunset, however it has been a long time since I've looked at that code." - Andrew Linden, quoted from Link

Jos Stam's Navier-Stokes equations
Physical based fluid simulation java applet based on Jos Stam's Navier-Stokes equations

Wind used to be affected by an avatar: a fast-moving avatar moving in a large circle inside a sim over a period of time would create an effect similar to a cyclone--just without the gale force behind the wind. This effect has since been removed.

This article wasn't helpful for you? Maybe the related article at the LSL Portal is able to bring enlightenment.

Has anyone else noticed that the wind effect on particles and prims seems to lag behind the value returned by llWind? I made a wind sock (flexi cylinder) and mounted it on a pole that looks in the direction of the wind. The pole would often times turn to a new direction a second or two before the sock would react. It was as if the simulator decided the values of the wind, then had to spool up the wind to meet the values. It makes for a rather unnatural disconnection between the value of llWind and the actual wind effects.

-- LashStringfellow (2006-12-21 16:36:42)