Why does humid air speed up the corrosion of sodium metal?

The reactivity of sodium metal with air is directly linked to the relative humidity, or the water-vapor content of the surrounding atmosphere.

Humid air accelerates the corrosion of sodium because the metal is highly reactive with water. When exposed to moisture, sodium reacts to form a film of sodium hydroxide. This process is part of the broader chemical degradation of the metal when it is not stored in an inert environment, such as kerosene or naphtha. Once the sodium hydroxide film forms, it can further react by absorbing carbon dioxide from the air to produce sodium bicarbonate.12