给定温度下盐类的临界相对湿度指的是在盐类开始吸收潮解时环境的相对湿度。当环境的相对湿度小于临界相对湿度时，盐类不会从环境中吸收水汽；只有环境的相对湿度大于临界相对湿度时，盐类才会吸收水汽。

表1：30°C时一些常见的用作肥料的盐类的临界相对湿度

The critical relative humidity (CRH) of a salt is defined as the relative humidity of the surrounding atmosphere (at a certain temperature) at which the material begins to absorb moisture from the atmosphere and below which it will not absorb atmospheric moisture.