type in sky under textures.
to change the sky go to
map
-map properties
environment map (clskyname)
type in any of the following
space
night
moring
dusk
desert
2desert
cliff
city
black
alien1
alien2
alien3
neb2
neb6
neb7
xen10
xen8
xen9
if you get hl model viewer then you can view all of these in the valve pak0.pak file.