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Foggy and hazy weather occurred frequently and worsened air quality dramatically during recent yearsInvestigating the causes and improving the prediction for fog and haze are essential to guide the public and protect public healthThis paper utilizes the observational data from 62 nationallevel automatic weather stations in Liaoning and the reanalysis data from NCEP to analyze the generalcirculation background, forming conditions and persistency causes for the fog and haze process occurred from 7 to 14 November 2015 in LiaoningResults indicate: (1) The general circulation, including an upperlevel westsouthwest flow, a lowlevel warm ridge, a reversed surface trough, and a weakened surface pressure, provided favorable synoptic conditions for the fog and haze event(2) Temperature inversion was the critical cause for this persistent fog and haze eventThe inversion had different structures for foggy and hazy stages, with a lower top and thinner layer for intense fog but a higher top, a thicker layer and even multiple simultaneous inversion layers for haze〖JP2〗(3) Water vapor content was the key factor for the conversion between fog and hazeA 95% relative humidity near the surface favors fog genesis, while a 60% to 70% is optimal for hazeThe specific humidity increased when fog turns into haze, whilst it decreased with a reversed conversion(4) The weak rising motion near the surface and the weak sinking at middleupper layers dynamically intensified this foghaze event(5) The characteristics of meteorological elements differ substantially before and after the fog and haze, which provides references for predicting fog and haze