Information on the distribution and transport of upper tropospheric humidity helps our understanding of atmospheric circulation. However, the efforts to develop a dataset that is not diurnally biased – that is, one that is based on geostationary data – has been hindered by a lack of inter-calibration of the geostationary satellite water vapor channels. Toward this end, the High-Resolution Infrared Radiation Sounder (HIRS) inter-satellite bias is determined by using collocated observations of geostationary and HIRS, which are collected monthly and used to determine biases between HIRS instruments on different satellites. The resulting HIRS inter-satellite bias then allows the inter-calibration of the water vapor channels on the geostationary satellites. This inter-calibration will allow a uniform analysis of upper tropospheric water vapor.