Seasonal transitions of water vapor and cirrus clouds are examined over the western United States, extending east from the Sierra Nevada range to the Continental Divide of the Rocky Mountains. This region contains the Great Basin and Colorado Plateau, main heat and cold sources for this area. Data from the Level 3 monthly mean products, available via the Moderate Resolution Imaging Spectroradiometer (MODIS), a multi-channel sensor aboard the Terra spacecraft, were used to determine water vapor concentrations and cirrus cloud reflectance over this region.

The data was studied along with the meteorology of this region during a period between November 2000 and December 2002. Water vapor concentrations during this period were at their maximum in July of 2001 and 2002 and their minimum in January and February of 2001 and 2002, respectively. Cirrus cloud reflectance is at its maximum during April and March of 2001 and 2002, respectively. Also, reflectance minimum is generally in September of 2001 and 2002. With the increased importance on understanding the role of cirrus clouds on global climate, MODIS contributes high resolution global measurements that will expectedly enhance model verification. This data over the western United States will hopefully offer another step towards this goal.