Implementing Snow and Rain in Daikatana
If you have not been successful in getting a basic map to compile and run in-game yet, then DO NOT CONTINUE. Go back to the Making Your First Map page and get that done right first. No point in reading about snow/rain if you can't see your results.
The first thing to note is that, despite the fact that snow and rain in the maps released by ION match the episode and weather of that area, you are not constrained in your own mapping. Want a bright daylight Ep2 map with snow? Then go for it, the engine will allow it.
Several of the texture directories contain a simple texture that says either SNOW or RAIN on it (e1m1 for example has a black and white SNOW texture) so that you can create your snow/rain brush and know what it is later on when looking at your map in the editor. Note though that you could really use any texture you want since it won't appear in the map; it'll just be harder to spot when you're editing. Since not all of the retail maps in Daikatana have snow/rain in them, there are a lot of texture directories that will be missing this texture anyway.
The basic concept is that you first create a brush to define the volume that will have the snow or rain effect showing in it. It is worth mentioning that if you have a room, for example, and you create a brush that goes from the ceiling to halfway above the floor and then turn it into a snow brush, you'll see snow fall from the cieling and then just stop midair. So remember to bring your snow/rain brush down to the floor.
Select the SNOW (or whatever) texture, and create a brush that defines your snow volume. Keeping this brush selected, right-click to bring up the entity selector and choose effect and then effect_snow. This will alter your brush to show just the top of your snow volume. That's it... same steps apply for rain.
There are some checkbox choices in the entity editor (bring up using the 'N' key) that can change things a bit. By default snow falls at random angles, but by checking the FALL-ST box the snow will fall straight down. Rain falls straight down by default, but using the checkboxes you can pick which angle direction it falls at (i.e. North, South, etc).