BLACK BODY RADIATION
Black body radiation is an interesting way to understand UV: our sun exhibits a classic black body radiation curve. What that means is, barring odd sections of the electromagnetic spectrum that are absorbed or reflected or in some way eliminated by the troposphere / stratosphere / Van Allen Belt, it has the classic radiation properties of any material that's just really, incredibly hot.
The black body radiation curve establishes the basic principle that, for all the visible and infrared light out there, there's not all that much ultraviolet. This is the simplest way to understand why ultraviolet cameras give so much worse images than regular cameras: there's just much less light. For this reason, UV imaging in 2015 is about where visible imaging was in the 1930s: it's black-and-white, pretty grainy, and if someone is moving basically at all, your picture will come out blurred.