The condensation on glasses during rainy weather occurs due to the temperature difference between the lenses and the surrounding humid air. When warm, moist air comes into contact with colder lenses, the water vapor in the air transforms into liquid droplets, causing fogging and obstructing vision.

This phenomenon is similar to the condensation that forms on a bathroom mirror during a hot shower. The warm, moist air collides with the cold glass, causing the water vapor to condense into a liquid film.

The key to preventing fogging is to reduce the temperature difference between the glasses and the surrounding air. One way to achieve this is by warming the lenses.

In the 1960s, NASA developed a special spray for astronauts to prevent visor fogging in space. This spray allows water to adhere better to the glass, forming a thin, transparent layer instead of disruptive droplets. Such sprays are available for civilian use.