Icing is a significant aviation hazard, and icing conditions continue to be difficult to precisely forecast or locate in real time. An in-situ sensor, which can be flown coupled with a radiosonde, is needed which can both measure supercooled liquid water content in clouds as well as characterize the droplet sizes. This sensor will support the calibration and validation of remote-sensing methods used to detect icing conditions, and can also be used on its own to support operational meteorology applications. Anasphere, Inc. proposes to develop a sizing supercooled liquid water content (SSLWC) sonde which will meet this need. It will be based on proven vibrating-wire technology which has been used for total water content measurements, but with an altogether different physical implementation that will enable droplet sizing. Phase I will involve the aerodynamic design of the SSLWC sonde, icing tunnel tests demonstrating key elements of its function, and a live flight test to gather information on the sonde's aerodynamic characteristics. Phase II will involve further tunnel tests, laboratory calibration development, design for manufacturability, and flight tests in icing conditions.