Gabriel G. Katul,  - Nicholas School of the Environment, Box 80328, Duke University, Durham, North Carolina 27708, USA, Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina 27708, USA

 Dan Li - Department of Earth and Environment, Boston University, Boston, Massachusetts 02215, USA

Heping Liu - epartment of Civil and Environmental Engineering, Post Office Box 642910, Washington State University, Pullman, Washington 99164-2910, USA 

Expressions for the ratio of the turbulent Schmidt (Sct) to Prandtl (Prt) numbers are derived for an idealized atmospheric surface layer flow over water surfaces. Conditions where the ansatz Sct/Prt≈1are then discussed. It is shown that Sct/Prt≈1 is consistent with the active role of temperature in turbulence generation or destruction even when perfect similarity between turbulent transport of heat and water vapor is absent.