What is the difference between ‘roof’ and ‘canopy’? And would you describe the sky as the roof of the earth or the canopy of the earth? (Sky meaning everything up to the ozone layer.)
Typically, a canopy is used to provide shade while a roof offers more solid protection; canopies filter and roofs protect. You often hear ‘canopy’ used to describe the sky-ward protection you experience in a forest or jungle–basically only shade and maybe a little protection from rain. A cave would offer roof-like protection–shade, protection from rain, and protection from big, hard, nasty things that fall from the sky.
These pseudo definitions could be applied to earth’s atmosphere in a similar way. Ask, “Does the atmosphere provide a shade-like protection against things from outer space, or does it offer more solid protection?” I think for most of the big, hard, nasty things that fall from outer space that our atmosphere is only a canopy, but if you are specifically talking about types of radiation that our atmosphere completely reflects or absorbs you could claim that our atmosphere (or sky) is a roof.