The World\Dnc.db file in common.mpq specifies the day-night cycle. Fortunately, the folks at Blizzard were kind enough to label every data field in this file! It is most likely not of any use anymore. It hasn't changed in any way from 1.0 or even 0.*. The lights and therefore skies are defined in .DBCs.
This looked like info for outdoor lighting with respect to the day-night cycle. The colors for the different light types are self-explanatory. The XYZ coordinates specify a directional light source.
Offset Type Description 0x00 uint32 nFields - Number of fields 0x04 uint32 nColumns - Number of columns
Next, nFields * 8 bytes define the rest of the file structure:
Offset Type Description 0x00 uint32 Always 0x53, 'S' for string 0x04 uint32 Offset to field name (which is zero-terminated)
After the header come the actual records, each being nFields * 8 bytes in size. Every field value has this structure:
The file contains 24 records, one for each whole hour (Hour = 0 to 23, Minute = 0).
Offset Type Description 0x00 uint32 Always 0x46, 'F' for float 0x04 float Field value