Using the three dimensional observation test data of urban heat island(UHI) in Nanjing in summer 2010,spatial and temporal distribution characteristics of urban heat island under typical weather conditions were analyzed.The results showed that daily mean UHI intensity on sunny days was over 1 ℃ in Nanjing in summer.UHI intensity was stronger and more stable during nighttime than that in daytime,with UHI distribution displaying directional characteristic and corresponding well to urban land-use conditions.During daytime,mixed layer in urban areas developed faster and was higher than that in the suburbs.During nighttime,there existed a long lasting convective mixed layer at low altitude in the city due to the unstable atmospheric stratification and the roughness of the underlying surface,which was as thick as more than 250 m.Owing to the storage of a large amount of heat and strong turbulent transport in urban underlying surface,heat island came into being in urban boundary layer,with UHI intensity decreasing with height.Heat island in boundary layer extended up to 900 m in daytime and maintained at 300 m at nighttime.