The purpose of this research is to calibrate snow water equivalent (SWE) estimates from passive microwave sensors. Using USDA National Resources Conservation Service snowpack telemetry (SNOTEL) data as ground truth, the western US SWE estimates from the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) and the US Defense Meteorological Satellite Program Special Sensor Microwave/ Imager (SSM/I) are improved using a regression technique. The 1982 - 1998 time period is divided into calibration and validation periods, where the calibration period is used to develop the regression and the validation period is used to test the calibration. Once the western US SWE estimates are calibrated, the calibration is extended over the Northern Hemisphere using a regression coefficient between the SWE estimates and percent vegetation longevity and cover from the AVHRR sensor. The calibrated SWE estimates are then used to study the seasonal and inter-annual variability of SWE in the Upper Colorado River basin and the Columbia River basin.