Statistics for more than 1000 soundings distributed globally are presented. Accurate vertical profiles of refractivity are consistently obtained from approximately 30 km altitude to approximately 7 km altitude. Below 7 km, where multi-path effects and other sources of error are large, there are increasing difficulties in obtaining accurate profiles of refractivity. These difficulties are not thought to be fundamental to the technique, and efforts are underway to address them.
Refractivity is a function of both temperature and water vapor.
Under ideal conditions, accurate soundings have been obtained down to 1 km, and under these conditions it has been possible to calculate the atmospheric water vapor pressure from the refractivity and an independent estimate of temperature.