Abstract:Large scale environmental vapor flux plays a very important role in th e genesis, development and dissipation of tropical cyclones. A regression method is developed to get the relationship between radiances measured by the Advanced Microwave Sounding Unit (AMSU) and water vapor fluxes. The method uses the brig htness temperatures at three channels 18, 19 and 20 of AMSU B, which are cente red around the 183.3GHz water vapor line, and the water vapor flux is derived fr om the calculation of stream function. With the regression, it is found that the brightness temperatures at the corresponding channel 183.3±7GHz, 183.3±3GHz a nd 183.3±1GHz decrease exponentially with water vapor flux on the upper, middle and lower troposphere, respectively. Then, Typhoon Matsa (0509) is illustrated to analyze the distribution and evolvement of the surrounding water vapor. By this exponential algorithm, there are great water vapor transportations as indicate d by the water vapor channel brightness temperatures. And it shows the importanc e of the environmental water vapor in the development of tropical cyclones. Addi tionally, the incoming vapor collected around the cyclone is mainly from southwe stern transportation, but the water vapor from other routes could not be ignored , especially the eastern and the southeastern.