Abstract: The principle of dew-point hygrometer is introduced, the influence factors in measurement of hygrometer is analyzed, the Kelvin equation is got, through calculating the influence of Kelvin effect. Furthermore the forming process and principle of Ural effect and super-cooled water are analyzed, on the base the means of reducing these influence factors is put forward.

Key words: Dew-point Hygrometer, Mirror, Kelvin effect, Ural effect, Super-cooled water