Creating Skybox and Light_Env inside Hammer Source will give us the ability to create outside area with a sky and a sun/moon light.
Any area that has skybox texture will project light_env into our scene. It's very simple to do, just watch the video below and go over the notes.
1. Proper placement of brushes around your map to block off the entire level. There should be no gaps, so there is no vis leaks.
Make sure your snaps are on.
2. Use skybox texture to any brushes that you want to project a sky from.
3. Any face that a player will not see apply a 'nodraw' texture to it. This is the beginning to optimizing your map.
4. Under Map --> Map Properties you will choose a sky.
On developer.valvesoftware.com wiki you can choose which sky with you want and it will give you specific numbers you need to put in for lights.
5. After you choose the name of your SkyBox Texture name and entered it into Map Properties. It is time to insert light_environment entity.
Light_Environment can be inserted anywhere in your level. It will project a light from any surface that has a skybox texture applied.
6. Double Click on the light_env and go into the properties. In here you will use the numbers from developer.valvesoftware.com wiki and input them into the following fields.
7. Here is our rendered out environment with only a light_env. Will still need to do a lot of work with our lights, but it is a base start you want to work with.