Abstract:Based on conventional data and NCEP reanalysis data (1°×1°), 15 strong coldness processes with rain and sonw in late springs in Yunnan have been summarized. The results show that the characteristics of the processes were two trough and one ridge at 500 hPa and north higher than south at 700 hPa. The two different kinds of strong coldness processes in 14-19 March 2011 and 2-6 March 2005 have been emphatically compared and analyzed. Sichuan Yunnan shear line and Kunming quasi stationary front came southward during the processes and Novosibirsk cold air invaded Yunnan from northeast. The main cause of severe surface cooling in the first process was strong cold advection in middle low level and in the second process was strong cold advection in lower level and cold advection sink in higher level. The cold rain and snow weather had deep vertical upward movement or strong rising movement column in middle low level. The Bay of Bengal was the vapor source. Strong upper level and low level jets and their secondary circulation transported water vapor and heat. While heavy snow was falling there were big and strong water vapor flux values and intensity of water vapor convergence without South Branch Trough (SBT), but there was a water vapor flux enhanced process in middle low level with SBT. The cold layer at lower level or strong cold advection near the surface was beneficial to snowing. The convergence of warm and moist southwest airflow which was ahead of the trough or westerly jet on low level and strong cold air strengthened frontogenesis. Ahead of moving front, zero line of frontogenesis function was corresponding to 〖WTHX〗Q〖WTBZ〗 vector convergence line at 850 hPa. The greater horizontal gradient of instability energy was, the more energy accumulated, which means the bigger snowfall ranges were. The isodrosotherm had the Ω characteristic at lower level. Uplifting role of the dew point front triggered dry cold air and warm wet air convergent on wet/dry interface. The top of the Ω isodrosotherm dense zone, the dew point front and location of lower level jet, 〖WTHX〗Q〖WTBZ〗 vector convergence zone had good correlations with snow areas in Yunnan, the magnitude of surface cooling was proportional to the intensity of positive frontogenesis function at 850 hPa.〖HJ〗