We will develop a compact and tunable radar transceiver operating in the underutilized short-millimeter-wave frequency regime to enable high-precision global mapping of humidity inside upper tropospheric (UT) clouds for the first time. This work addresses the Focus Area of Climate Variability and Change because clouds are a leading source of uncertainty in estimating the climate sensitivity from global models, and UT humidity affects radiative feedback and cloud formation. Over three years we will build and validate a radar transceiver to enable humidity sounding inside UT clouds using the technique of Differential Absorption Radar (DAR) operating around the 183 GHz water absorption line. By capitalizing on recently-developed III-V semiconductor Schottky diode and amplifier millimeter-wave devices with state-of-the-art efficiency and power handling capabilities, our approach offers the fastest, lowest cost, and lowest risk route to realizing an active instrument capable of range-resolved water vapor absorption measurements in cirrus clouds. The transceiver will integrate all-solid-state source and detector devices inside a compact (~10x6x2 cm) module with 5% tuning bandwidth. Continuous, 1 W transmit power will be reached in two steps. First, commercially available GaN power amplifiers at 90 GHz will drive a JPL-fabricated GaAs diode frequency doubler with 20% conversion efficiency and 0.5 W output power capacity. Then 1 W will be achieved either through a chip-stack waveguide power-combining geometry or, pending commercial availability in 2016, by 183 GHz power amplifiers. For receiving, an InP low-noise amplifier and a 100 dB isolation quasioptical duplexer will achieve a noise temperature of 500 K even while transmitting. The DAR technique will be validated in ground-based measurements using a custom-built millimeter-wave radar test bench. We anticipate demonstrating DAR sensitivities with few-percent precision, thus enabling a new class of future airborne and orbital measurements for cloud and climate science.