Wiki: light_environment

light_environment - Point Entity

This entity makes the map's sky (specifically, User posted image

sky-textured brushes) emit light. The only practical way of lighting outdoor maps.

Name (targetname) - Property used to identify entities.
Pitch, Yaw, Roll (angles) - Sets the pitch (up/down), yaw (left/right) and roll (bank) respectively.
Pitch (pitch) - A negative number will give downward pitch (which is normally what you want).
Brightness (_light) - The first three 3-digit numbers are the colour (RGB) and the fourth is brightness/light intensity (e.g. 248 244 237 350).

ZHLT/VHLT-specific keyvalues:

Ambient Color (_diffuse_light) - Diffused light colour and brightness. This simulates diffuse light usually from blue sky.
Sun Spread (_spread) - Diffuses the sunlight.

Use Pitch to shift the sun (or moon) vertically (similar to daytime/hours) and Yaw to shift it horizontally (cardinal direction or obliquity)

Pitch	Result
0°	Sun at the horizon (like in sunrise/sunset) - huge shadows on floor if at least a minimal amount of -pitch were given
-45°	Sun between horizon and top (like 9 AM or 3 PM)
-90°	Sun directly overhead (like 12 Noon) - only parallel shadows on floor

Sky brushes must have the "SKY" texture on all faces or they won't reliably emit light (depends on version of HLRAD used).
The key pitch have the same function as the one in angles, although if you add a value (≠0) to pitch it will overwrite the one in angles. Roll has no effect at all.
For modern compiler, you are not advised to use multiple light_environment in a map. However, you can still use multiple light_environment for multiple colors.
This entity doesn't spawn a visible source of light in the sky. Therefore, the sun or moon must be part of the skybox as a picture for visual effects.
The angles/angle+pitch and brightness values of this entity is used to override lighting on models. If the entity has direct line of sight to the sky (i.e. a "SKY" face) in the reverse direction of this light_environment entity's angles/angle+pitch value, its lighting will be overridden with values calculated from the brightness of this light_environment entity.
- To break line of sight and have entities take lighting from ground, use a brush textured with black_HIDDEN facing down above the affected area, and SKIP on the other faces.
- To avoid this effect altogether, use light_spot with "Is Sky" keyvalue set.