The heavy fog occurring on September 30, 2010 in the north of Ningxia is analyzed. The results show that the heavy fog is related tornnot only the cooling and wind conditions, but also the changes in the previous 500 -700 hPa stable stratification and the physical field (such asrnTground - T850 and θse850 - θse500 ) , the previous accumulation of moisture conditions is also he main cause to the heavy fog.%针对2010年9月30目出现在宁夏北部地区的一次大雾天气进行了综合分析,结果表明,此次大雾天气不仅要考虑冷却条件、风的条件,更要考虑前期500～700 hPa维持较稳定层结及与稳定度有关的物理量场(如T地面 - T850值、θse850-θse500值等)变化对大雾天气的指示意义；另外,前期湿度条件的积累也是影响大雾生成的主要原因.
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