This study aims to validate and improve the universal evaporation duct(UED)model through a further analysis of the stability function(Ψ).A large number of hydrometeorological observations obtained from a tower platform near Xisha Island of the South China Sea are employed,together with the latest variations inΨfunction.Applicability of differentΨfunctions for specific sea areas and stratification conditions is investigated based on three objective criteria.The results show that,under unstable conditions,Ψfunction of Fairall et al.(1996)(i.e.,Fairall96,similar for abbreviations of other function names)in general offers the best performance.However,strictly speaking,this holds true only for the stability(represented by bulk Richardson number R_(iB))range-2.6