The physical impact at work with fogging is termed condensation. Air, perhaps what we would certainly call dry air, consists of water vapor, or just water as being a gas. Plenty of water vapor bestows the feeling we refer to as humid. All air bears a quantity of water vapor, including the air within your scuba mask.
You are aware exactly how when liquid water becomes cold enough it develops into a solid, a.k.a., ice? The moment water vapor cools down, it changes into liquid water.
What does this ought to do with foggy masks? The lenses on the mask come in contact with the water on the outside, which unlike the air inside, is pretty cool, may be even in the tropics. All the water vapor molecules are continually tossing around within your mask, and as they reached the cool mask lens, it consequently cool down and revert into liquid. This process is referred to as condensing.
While your mask seems clear and smooth, it is really stuffed with teeny tiny flaws, of course dirt and dust that accumulates. The recently cooled down water droplets use all these to hold on to your own mask and thus block your sight as fog.