The causes of a sudden local heavy rainfall with lightning and hail occurred in Beijing on 7 August,2015 are investigated by composite analyses of surface and conventional sounding data,Doppler weather radar data and wind profile radar data.The following results are given:The heavy rain occurred in a strong unstable stratification.The middle level trough,lower-troposphere shear line and surface wind convergence facilitated the development of convection in northwest of Hebei,which then moved southeast and caused heavy rain in the northwest of Beijing.The direct producer of heavy rainfall over central Beijing was thunderstorms locally triggered by convergence between the cold pool outflow of pre-existing thunderstorms and the warm,moist southeast airflow in the boundary layer.The terrain in the northwest of Beijing accelerated downhill cold pool outflow and lifted the thunderstorm cold pool outflow.Then,the cold westerly outflow overlapped above the warm and moist easterly flow near surface that increased the convective instability of atmosphere and contributed to the initiation of thunderstorms at the east side of the mountain.