The spatial variation of the nocturnal urban boundary layer structure and the time variation of the mixing height, the nocturnal inversion top and strength after sunrise are presented for urban sites located upwind, downwind, and near the center of the heat island and for upwind nonurban sites. Observations were obtained from high resolution temperature soundings made by helicopter spirals during 35 morning experiments in St. Louis, Missouri. The mixing height (base of the nocturnal inversion) evolved in a parabolic manner after sunrise at the urban locations. A rise in the inversion top after sunrise at urban sites is believed to be due to low level convergence which caused the entire inversion layer to be lifted. Large horizontal temperature gradients are associated with the heat island effect at the surface.