Land surface microwave emissivities are important geophysical parameters for atmospheric, hydrological, and biospheric studies. Using combined data of satellite Special Sensor Microwave/Imager (SSM/I) observations and the ground measurements of the Atmospheric Radiation Measurement (ARM) Program over the Southern Great Plains (SGP) site during October, 1995, this study estimates the land surface microwave emittance values based on microwave radiative transfer model. Because cloud properties are available for the ARM observations, the emittance values are retrieved for both clear and cloudy sky conditions. Large variations of the emissivities (standard deviations are about 0.035) are found: a great part of the variations are produced by diurnal cycles and precipitation. Early morning emissivities are much lower than those of other times (differences are about 0.06), while the values of polarization difference among the data from different times have similar magnitude, which is likely the results of dew effects. Ground observations of dewpoint and temperature difference between air and skin support the idea of dew effects. Although emittance values are significantly negatively correlated to those of soil moisture, soil moisture can only explain about 25-50% variances of the emissivities. The correlation coefficients between soil moisture and emittance are considerably high when pentad values are considered.