Based on FY-2 infrared images and the reanalysis data for six hours of NCEP(1°×1°),the heavy dust storm process occurred on 13 April 2007 in the middle west part of Hexi corridor is analyzed,particularly the cloud head of the descending jet was tracked and the gravitational instability,conditional symmetric instability and conditional instability are diagnosed in detail.

The results show that the strengthening subtropical high pressure, the momentum transmission of jet stream in the middle-lower troposphere, a violent downdraft from upper air and a sudden intensification of the outer pressure gradient force of the tyhoon are the important factors due to the strong outer w.

The results show that a warm high ridge at the middle level and transformed surface cold high over the plain provide a good background condition;a cooling process produced by net surface radiation fluxes is the trigger and strengthening mechanism for the fog event;the subsidence flow at the middle and low tro.