From AMS Glossary
The equilibrium vapor pressure at the surface of a saturated salt solution is less than that for a similar surface of pure water at the same temperature. This effect is exhibited by all salt solutions but particularly so by lithium chloride. The dew cell hygrometer works in the following manner: The lithium chloride solution is heated until a temperature is reached at which its equilibrium vapor pressure exceeds the ambient vapor pressure. At this point the balance will shift from condensation to evaporation, which can be detected by a characteristic decrease of the conductivity of the solution. By properly regulating the heating current, the equilibrium temperature can be transformed by means of empirical equations to the ambient water vapor pressure and the dewpoint. Provision is not usually made for cooling the dew cell, and consequently it will not measure vapor pressure less than the vapor pressure over saturated lithium chloride solution at the ambient temperature. Dew cells are used for observational purposes, especially for automatic weather stations.