So why does the atmosphere block gamma rays but not UV rays?

it has to do with the how the various levels of energy in different frequencies interact with the various molecules in the atmosphere

for example, UV rays are mostly absorbed by the ozone layer http://en.wikipedia.org/wiki/Ozone_layer

infrared rays are apparently most absorbed by water vapor

visible light is apparently low enough energy to not excite oxygen, but too energetic to excite molecular vibrational frequencies of molecules in air.

http://en.wikipedia.org/wiki/Electr...absorption_of_EMR_in_the_Earth.27s_atmosphere



since the atmosphere is not the same everywhere, there are different places that absorb different kinds of EMR. But by the time you get to the surface, a lot of it has been absorbed.