I want to know if there is a way to have different skymaps for different parts of the level, these would be connected by 'tunnels' so you could not see more than one sky at a time. is this even possible in hl2?
look up the left 4 dead 2 storm effects, they change the skybox for the huge rainstorm. Thats the closest thing I can think of for what you want to do
okay I found a prefab to setup multiple skyboxes on the same map, its usually used for mini game maps but can actually have different skyboxes for different areas as well. http://gamebanana.com/css/prefabs/5660
Put this in canyon and have the rain come on and off at random times. Make the rain flow harder though. Valid tactic... WAIT FOR RAIN... RUSH NOW!