Observations of cirrus clouds were made with a host of instruments on board the NASA ER-2 aircraft on April 16, 1996. These instruments provide radiometric measurements covering the visible, infrared, and millimeter-wave regions of the electromagnetic wave spectrum, as well as profiles of lidar backscatter at 1.064 &mgr;m. The cirrus clouds are classified into two groups: the modest cirrus clouds with lidar backscatter from the surface and the intense cirrus clouds without lidar surface returns. For the modest clouds, the magnitudes of brightness depressions are typically less than 5 K and are comparable at 150 GHz and 220 GHz. The depressions for the water vapor channels (e.g., 183.3¿7 GHz) are smaller. We interpret the microwave brightness depressions for the modest cirrus cases as due to reduced skin temperature from the cloud shadow, rather than ice scattering. For the intense cirrus cases, the magnitude of brightness depression at 220 GHz reach 35 K and are far greater than those at 150 GHz. After correcting for the skin temperature effect, a regression between the observed brightness depressions at 150 and 220 GHz frequencies results in a slope of 2.75. Comparison with radiative transfer modeling for solid spherical ice particles, indicates a median equivalent mass diameter of about 350 &mgr;m, and ice water path ranging up to 300 g/m2. The results illustrate the potential utility of millimeter and sub-millimeter wave radiometry for characterizing microphysical properties of thick cirrus. ¿ 1998 American Geophysical Union