Its all to do with this:
Water vapour (actually the OH- bonds) absorb a LOT of emmissions in the IR band, but Visible light is attenuated more. Thus, in fog, you cannot see the runway due to fog due to severe attenuation, however, the runway is still visible at short ranges using IR.
In cloud, the proportion of Water per m^3 is much higher than in fog, thus much higher attenuation and thus far shorter range.
Outside of cloud, there is no attenuation, every object by its inherent nature emits IR (yes, even you) and this makes FLIR very useful in picking out CBs in the dark. Unfortunately, CBs emit little visible light other than the flash of lightning. Otherwise they would be glowing in the dark.
I use my NVGs more enroute than I do at the pointy end; they are a fantastic aid with the Radar for picking out distant cells. If you are interested, NVGs work in the Near-IR spectrum.
Radar IS attenuated by water vapour, this is why you get shadows behind really nasty cells and this is why you can penetrate one really nasty wall to find another, even nastier one just behind it!