Abstract:While calculating vapor pressure using relative humidity and temperature of L band radar sounding,the water saturation vapor pressure formula must be used,since the relative humidity is based on water surface.There may be an error in using the ice saturation vapor pressure formula.The comparison of vapor pressure bias between water and ice saturation vapor pressure formulas is discussed.Taking -45 ℃ and 0 ℃ as a threshold,vapor pressure values are analyzed when using the ice and water saturation vapor pressure formulas,respectively.Results show that the absolute deviation produced by ice and water saturation vapor pressure formulas is not very great, but the relative deviation may be up to 50%,which has climatic effects on upper troposphere for actual water vapor pressure.