Last night, I took the big kids to the swimming pool. When I entered the swimming pool area, I saw something that looked wavy and shimmery. It was water. Some time later, on my daughter's advice, I submerged my head while wearing goggles and looked upward. I saw something that looked wavy and shimmery. By symmetry with my previous seeing, it had to be... air.
So air is visible, even in moderate quantities. (Of course one could see air in sufficient quantities, because of how it would dim the light.)