Skybox Lighting enters the map through every toolsskybox-textured brush, representing direct sun/moonlight and diffuse skylight. Its basic properties are controlled by three entities used in combination (four in some games):
light_environmentdefines the direction, color and intensity of the direct sunlight, and the color and intensity of the diffuse skylight.
shadow_controldefines the color, direction, and attenuation distance of the dynamic shadows that are created by
light_environment. Dynamic shadows will be buggy and unconfigurable without this entity.
env_sunplaces a glowing sprite in the skybox to represent the Sun's apparent position in the sky.
env_cascade_lightcasts harsh, real-time shadows onto the map, usually copying settings from the
To make sense of it all, just make sure you have only one of each of these entities in your map, and that the Pitch and Yaw <angles> are set the same for all three of them. (Pitch is equivalent to the Sun's angle of elevation from the ground, and Yaw is its compass bearing.)
These lighting settings - direction, color, brightness, etc are fairly specific to the actual image used for the 2D skybox. The Sky List article suggests some settings for official game skyboxes. The
worldspawn entity defines which skybox to use. Also note that fog will look wrong if
env_fog_controller's settings don't correspond to the particular skybox in use.
env_sun, but not
shadow_control. The Pitch override also cannot be switched off and its rotation is measured counter-clockwise from the horizontal (so straight down is -90°) whereas the <angles> Pitch's rotation is clockwise (so straight down is +90°). The workaround for this is to make sure your Pitch override merely repeats the <angles> value; for example, if your
env_sun<angles> are "45 -60 0", set the Pitch override to "-45".