Abstract: The growth and evolution of supercooled water are always a concern in the cloud physics and weather modification science. Based on the hypothesis proposed by Hobbs in 1974, a westerly trough in Hebei Province on 21 September, 2012 is analyzed by using the airplane observation data, satellite monitoring data, radar monitoring data, and other regular observation data. Results show that (1) a wide and thick area of supercooled water exists in front of the trough where the supercooled water is abundant, the concentration and root mean cube diameter of the forward scattering spectrometer probe (FSSP) are relatively of high value, and the cold cloud is thick without dry layers and warm cloud; (2)as the trough draws near, the cloud near the trough develops rapidly, the concentration and depth of the area of the supercooled water increases significantly, the cloud becomes more thick with dry layers, the liquid water content (LWC) in cold and warm cloud is abundant, meanwhile the concentration of 2DC and 2DP is of high value, and the mesoscale structures move quickly; (3) in the rear of the trough, the rootmeancube diameter of the droplets and the amount of the supercooled water are remarkably small, and the height and width of the supercooled water decreases a lot, although the cloud concentration is still high, the cold cloud is very thin with a dry layer between warm cloud, and the warm cloud has several layers with high value concentration of 2DC and 2DP. The precipitation is produced by the warm rain process. Usually the peak of the LWC stays over the temperature inversion layer. The results based on the analysis of the particle measurement system (PMS) data are well consistent with those based on the satellite and radar monitoring data.