water vapor, ice or liquid absorption band is 2-4 micron (NV region), IR bolometer band is 8-12 micron. Now if you get moisture on the lens, you will see it as constant distortion. Did a frame by frame vid of the muzzle gas from a shot, lasted at most 2 frames (300BO H110 load), lasted 30 millisec so the IR barely sees it. Visible with the eye, lasts a lot longer!
We 'see' fog from reflected visible light. I suppose IR could see 'hot' moisture. In the 70s I was flying test IR seekers against ships in the gulf, never noticed any moisture problems.