In this paper the mean gradient of the generalized potential refractive index, denoted by M, calculated from the meteorological data observed at Pan-Chiao rawinsonde station and the vertical echo power measured with the Chung-Li VHF radar are studied. It is found that the ratio of wet term to dry term in M varies from 20 dB at 1 km to −10 dB at 10 km, where 0 dB is occurred at 8 km. This indicates that the magnitude of M is dominated by the contribution of humidity below 8 km. The correlation between echo power and M2 is good. The slope of the regression line relating the echo power and M2 is found to change for different weather conditions. Furthermore, the echo power profiles during the period of typhoon passage are analyzed. The variation of tropopause height and the maximum radar detectable height during the typhoon passage will be studied.