It keeps the mask from fogging up.
When the diver puts on the mask, it traps the surrounding air. During the dive, that air is surrounded by cold water. This cools the air until it can't hold the moisture in solution. This forms water droplets on the inside of the lens. The same effect can be seen on the surface of a cold glass, as shown on the video. Even when snorkeling, it's not uncommon to have a fifteen-degree temperature difference between the water and the air, which is often enough to reach the dew point.
So, why spit on the mask?
The saliva acts as a surfactant. When the moisture leaves the air, the saliva keeps the water from get a firm grip on the lens. These droplets slide off before they can become large enough to obscure the diver's vision. While there are commercial surfactants on the market for diving goggles, spit is free and always available.
Posted 3466 day ago