Neverwinter Nights 2 comes packaged with a few skydome effects that you can use in your areas. The skydomes are:

fx_skydome_stormy
fx_skydome_shipwreck
fx_skydome_nx2lobby
fx_skydome_generic
fx_skydome_astral
fx_skydome_fugue
fx_skydome_dream
fx_skydome_shadow

To use them, type the desired skydome effect into the skydome field in the area's properties.