Here is a quick tutorial about controller scripts.
With these ‘controller’ scripts you can control other scripts: Touch a light switch to turn on lights, Fly and your jet-boots start to puff smoke, turn a shower on or off, etc, etc.
This version emits snow all the time. You have to delete it to stop the snow.
-Rez a prim, and make it transparent.
-Drag the texture ‘Snowflakes‘ from your inventory into the content tab. <==IMPORTANT: If you do not do this, you’ll see strange, glowing particles
-Open the content tab, and make a new script.
-Open the new script and replace the standard script with the script below.
-Save the script, and you’ll be standing in snow within seconds.