Why do swimming goggles hold fog underwater?

Have you ever stirred something like fine sand into a cup of water? The sand will spread with the water creating a cloudy mixture. When you stop stirring or shaking the cup of water, the sand will eventually slow down and fall back to the bottom of the cup.

Though we can’t see it directly with our eyes, and it isn’t exactly the same process from a physics standpoint, air is capable of hosting another material, like water, in this way. When water is heated, individual molecules “bounce” out of the surface and become a gas that we call water vapor, or steam. That’s what rises out of a hot bath, or a boiling pot of water. This process is called evaporation. The energy and motion of the air and the water vapor keep the water molecules bouncing around, suspended in the air like sand grains in agitated water.

When that hot water vapor and the air it’s held within cools down, it bounces around less, and eventually lands and sticks to a nearby surface. This is what fog on a cold window is, and the dew on blades of grass during a cold morning.

Back to goggles – the warm air in the room surrounding a pool gets trapped in the small space enclosed by the goggles when you put them on. Sweat from your skin and any water already inside the goggles add to the mix and become heated by your skin. This causes some water to evaporate and fill the small air space inside your goggles with water vapor.

And then you jump into the pool…

When surrounded by the relatively colder pool water, the air inside your goggles also cools down. When this happens, the air is capable of hosting less water vapor, the vapor cools down and stops bouncing around so much, then many tiny droplets come to rest, land, and stay on the inner surface of the goggle, creating foggy goggles.