the snowstorm occurred in Shanxi Province from November 10 to 12，2009 for example，this paper made a diagnostic
Taking the analysis on causes of such extreme weather events from the circulation background，vapor condition，dynamic condition and cloud images change． The results are as follows: ( 1) The snowstorm occurred when north hemisphere circulation presented high index characteristics，
and in most parts of China it was warmer; ( 2) The configuring of convergence at the lower － layer and divergence at the level of
300 hPa caused vertical rise of airflow and formed dynamic condition for the snowstorm; ( 3) The trough at tha level of 500 hPa over
Hetao area leading the cold air moving from west to east and overlaying with the cold air from east at the bottom of polar vortex，rising
airflow from lower layer together with the frontal surface，upper trough and shear line，these all were main causes of this heavy snowstorm;
( 4) There were two lower － level jets at 1 500 meters，a strong east － wind wet jet and a south － wind which provided plentiful
water vapor for the snowstorm area; ( 5) According to the cloud images of FY － 2C satellite，this heavy snowfall had close correlation
with three cloud clusters in the troposphere，the continuous formation and development of convective cloud clusters resulted in the strong snowfall lasting a long time．