An investigation on vertical variations of the mixing lengths for momentum and heat under neutral and stable conditions was conduced using the data collected from the Cooperative Atmosphere-Surface Exchange Study in 1999 (CASES99). By comparing κz with the mixing lengths under neutral conditions calculated using the observations from CASES99, the vertical layer where Monin-Obukhov similarity theory (MOST) is was identified. Here κ is the von Kármán constant and z is the height above the ground. On average, MOST is approximately valid between 0.5 m and 10 m. Above the layer, the observed mixing lengths under neutral conditions are smaller than the MOST κz and can be approximately described by Blackadar's mixing length, κz/(1+ κz/l∞), with l∞ = 15 m for up to z ~ 20 m for the mixing length for momentum, and up to the highest observation height for the mixing length for heat. Above ~ 20 m, the mixing length for momentum approaches a constant. Both MOST κz and Blackadar's formula systematically overestimate the mixing length for momentum above ~ 20 m, leading to overestimates of turbulence.
