This is a pair of sky Environment Settings for use with EEP-enabled viewers such as the latest Firestorm or Second Life viewers. For more information on EEP, see the Second Life Wiki at http://wiki.secondlife.com/wiki/Environmental_Enhancement_Project.
These are suitable for use on your avatar for your own custom, portable environment, or for deployment on a parcel or region you own. They can also be incorporated into a daycycle if desired, but these are static settings that do not change on their own.
The first one is meant to simulate a *really* dark environment, such as a cave. If there are no local lights, your screen will appear pitch black, or very close to it!
The second one is meant to simulate a moonless, cloudy night with some amount of atmospheric scattering of light; it is very murky. Great for a city at night!