Over the past decade millimeter-wave radar systems have become an invaluable tool for characterizing cloud morphology and microphysics. Millimeter-wave radars are highly sensitive to small hydrometeors and they are compact enough to operate from small mobile platforms including light aircraft and unmanned aircraft. Most cloud radar systems have been developed at 35 GHz and 95 GHz corresponding to local minima in the atmospheric absorption spectra. 35 GHz radars have primarily been used in ground-based applications because atmospheric absorption and extinction by precipitation is lower compared to 95 GHz. 95 GHz radars are more popular for airborne applications where antenna size is restricted.

This paper describes the progression of 95 GHz airborne cloud radars developed in the United States. Critical features of the Airborne Cloud Radar (ACR) , the prototype for the NASA CloudSat radar, and a recently completed high altitude solid-state radar are discussed.