Why is there a layer of frost on the security door in wet weather?

It is calcium oxide formed by carbide in the air and surface substances of the security door;

Calcium oxide is an inorganic compound with chemical formula of CaO, commonly known as quicklime. Physical properties: the surface is white powder, grayish white when it is impure, pale yellow or gray when it contains impurities, and it is hygroscopic. Boil to drive away clean carbon monoxide. Lime milk is added to the hot solution and filtered to remove hydroxide precipitates, such as copper, iron and magnesium. , the filtrate is heated, and when it is close to boiling, CO is introduced to generate Ca(HCO), and the amount of Ca(HCO) is equivalent to that of added Ca(OH). When the solution boils, Ca(HCO) is decomposed into calcium carbonate precipitate and trace Fe.