The humidity is the amount of evaporated water in the air which is not visible and is given as relative (0 – 100%) and absolute (value in grams related to one m³ of air)  humidity.

As soon as the saturation point is exceeded, the evaporated water becomes visible as steam. Evaporation takes places at all temperatures, but its happens slower and in smaller quantities. For this reason, the drying process in “fresh air” is not a faster process than the evaporation of the water at temperatures above 100°C.

As the water absorption capacity of air does not grown linearly with increasing temperature, the values of relative and absolute humidity have become established. Both values always consider the temperature. For example, 100% humidity at 30 °C corresponds to 30.35g water/m³ of air or at 70°C it corresponds to  198.10g water/m³ of air.

These values are important, for example, for the design of a used sand cooler or a dryer. Both need air in order to remove the evaporated water properly; the amount needed for sufficient cooling or drying is determined on the basis of these values. Devices for measuring humidity can be calibrated, however, their use in the harsh conditions of a foundry is not possible as the sensors are quickly destroyed due to the high amount of dust (s. Humidity sensor).

Further references:

Moisture content
Moisture measurement
Moisture of a material
Sand moisture
Sand cooler