the ability of materials to absorb moisture from the air.

Hygroscopicity is a property of wettable (hydrophilic) materials of capillary-porous structure (such as wood and grain), in whose fine capillaries moisture condenses; of water-soluble substances such as table salt, sugar, and concentrated sulfuric acid; and especially of chemical compounds that form, with water, crystalline hydrates. The amount of water absorbed by a porous material (hygroscopic wetness, or W_hyg) increases as the moisture content of the air increases, reaching a maximum when the relative humidity of the air is 100 percent. For wood, the maximum W_hyg is about 30 percent (in terms of weight), whereas for wheat it is about 35 percent. It is important to know the hygroscopicity of a material in order to determine drying and wetting processes. Hygroscopicity is also considered in long-term storage and transportation of materials, especially by sea. Hygroscopicity explains the dampening and even deliquescence of a number of salts when stored in the open air. Some hygroscopic substances, such as concentrated sulfuric acid, are used to dry the air.