Making it Rain or Snow

First Thing to do is build your map, then make a brush at the top covered in the nodraw texture,
just below the sky brushes and hit the "To Entity" button and tie the nodraw brush to a
func_precipatation entity

Then in the properties window you can change it to Rain or Snow, also you can change the
Density (0 - 100 %) (the higher the density the more rain/snow) of the Rain or Snow.

<-- Rain
<-- Snow

Now you would add in your sounds using the ambient_generic entity.