1. Classification of weather Patterns On Cloud Maps
A server dust storm is defined as: the strong-convection dusts storm weather with a wind speed of >20 m/s Km and a visibility of <200 m. Different from a common dust storm with a visibility of <1.0 km, a server dust storm may result in disaster in the area on its track. According to the analysis of 20 severe dust storms from 1977 to 1998, severe dust storms can be divided into two categories: dust storms before and behind a cold front.

1.1 The image characteristics of pre-front dust storms
In case of pre-front dust storms, there is usually a cold front cloud band stretching from northeast to southeast with a clear-cut the back boundary and several small convection cells developing from a front boundary. Then, the convection cells further develop into a squall line and invoke a severe dust storm. The cold front behind further intensifies the dust storms. The dust storms occurred in 5 May 1993 is an example (Fig. 1).


Figure 1: The dust storm in Jinchang monitored by NOAA/AVHRR in May 5,1993
Figure 1 is a NOAA-II three channel composite map. The satellite passed through Jinchang at 16:00, Jinchang is situated behind a mesoscale cloud cluster (A); a cloud band (B) corresponding the surface front is located in the northeast and the dark area (C) between cloud cluster and front cloud band is a part of the dust storm area observed by the satellite. At the moment, the cloud front had not yet arrived at Jinchang, and the severe dust storm had passed about 20 min. therefore, it can be said that the severe dust storm was caused by the mesoscale the cloud cluster, whose top temperature was as low as -550C. The cloud cluster combine with the convective cloud band into a squall line arrived at system. Before the squall line arrived at Wuwei (D) the surface temperature is relatively high (red); surface objects are very distinctive; and weather is clear and fine.

1.2 The image characteristics of post-front dust storms
The most common dust storms in China are those caused by the strong wind behind a cold front after the cold front passing through. On cloud maps, these cold front cloud bands can divide into two kinds: wide and narrow clouds bands. Cold fronts related to the wider cold clouds tend to bring about severer dust storms (Figure omitted).


Figure 2: The dust storm in southern Sinjiang monitored by NOAA/AVHRR in April 5,1999