Improvements in radiosonde humidity profiles using RS80/RS90 radiosondes of Vaisala

Accurate measurements of relative humidity in the free atmosphere are still lacking, especially for temperatures below-30°C. Despite these problems, precise humidity measurements are needed for climate and weather research as well as for operational weather forecasting. In this paper we open new variants for exact humidity measurements using the commercially manufactured radiosondes RS80/RS90 of Vaisala. At first we give some remarks to the physical nature and the definition of relative humidity for readers who are not familiar with humidity measuring. The differences of three common vapor pressure formulas will be discussed. Second the state of the art of the widely-used RS80 radiosondes summarizes the problems which still exist. After that follows our proposal to introduce so-called standardized frequencies using a new H-Humicap polymer sensor of Vaisala which allow exact relative humidity measurements at the fixed calibration points 0% and 100% within the theoretical accuracy ±0.05% with respect to water in the pure phase. The major finding in connection with the standardized frequencies is that the quotient of two frequencies' differences (derived from polymer capacities) in its numerical value is equal to the relative humidity with respect to water, that is the water vapor partial pressure ratio. One gets this ratio as follows: Heated frequency (is equivalent to 0% rel. humidity) minus measuring frequency divided by heated frequency (is equivalent to 0% rel. humidity) minus measuring frequency of 100% rel. humidity (at normal pressure and room temperatur). The essential conclusion is the application of these standardized frequencies using heated frequencies during the radiosonde ascent from the ground up to a pressure of about 100 hPa and over a temperature range of e.g. +30°C up to -70°C. Lastly four examples of RS80/RS90 twin flights are presented showing the cloudless, the cloudy and the icing (only for RS80) case. In addition the fourth example shows the backscattering profile of a LIDAR-equipment. These twin fligths clearly show that better humidity data can be measured with the H-Humicap than with the A-Humicap using the standardized frequency method.