http://skyrim.nexusmods.com/mods/images/25666-3-1350575092.jpg
http://skyrim.nexusmods.com/mods/images/25666-2-1350575091.jpg
http://skyrim.nexusmods.com/mods/images/25666-1-1350575091.jpg

Overview:

One of nature's most beautiful light shows makes it to Skyrim. From partial rainbows to the elusive full lunar rainbow these visual effects will appear in the direction of the sun/moonlight if the following conditions are met:

• Rainbows

A weather transition from rainy to clear/pleasant and low sun position (8:30 to 10:15 AM and 4:00 to 5:45 PM).

• Lunar rainbows (Moonbows)

A weather transition from rainy to clear/pleasant and low full moon position (8:00 to 10:00 PM and 2:00 to 4:00 AM).

A rainbow's luminosity depends heavily on what weather/lighting mods you have installed. It will last anywhere from 30 to 120 real time seconds but moving too far away or getting too close will cause the illusion to dissipate prematurely until you've moved back to an appropriate position.

More info and download available here: http://skyrim.nexusmods.com/mods/25666