How can I implement weather on a map?

See Using Map Layers. This used to be very difficult, but it is now very easy! You can use layer 2 for weather effects like rain, snow, and fog.

If you want alpha transparency (semi-transparent pixels) for fog effects and the like, put this in your map's on-load script, replacing the # with the map layer you have the effect on and changing the second number to something that looks nice:

 
...

set opacity (lookup slice (sl:map layer#), 50)
...

Between layers, animated maptiles, and changing the layer opacity, you can get some really nice-looking weather effects!