||A tethered balloon system capable of making microphysical and radiative measurements in clouds is described and examples of measurements in boundary-layer stratus clouds in the Arctic and at the South Pole are presented. A 43 m3 helium-filled balloon lofts an instrument package that is powered by two copper conductors in the tether. The instrument package can support several instruments, including but not limited to, a cloud particle imager, forward scattering spectrometer probe, temperature, pressure, humidity and wind sensors, ice nuclei filters and a 4-π radiometer that measures actinic flux at 500 and 800 nm. The balloon can stay aloft for an extended period of time (in excess of 24 hours) and conduct vertical profiles up to about 1 to 2 km, contingent upon payload weight, wind speed and surface elevation. Examples of measurements in mixed-phase clouds at Ny-Ålesund, Svalbard (79° N Latitude) and at the South Pole are discussed. The stratus clouds at Ny-Ålesund ranged in temperature from 0 to −10°C and were mostly mixed-phase with heavily rimed ice particles, even when cloud top temperatures were warmer than −5°C. Conversely, mixed-phase clouds at the South Pole contained regions with only water drops at temperatures as cold as −32°C and were often composed of pristine ice crystals. The radiative properties of mixed-phase clouds are a critical component of radiative transfer in Polar regions, which in turn is a lynch pin for climate change on a global scale.