Huygens Atmospheric Structure Instrument (HASI) for the first time performed an in situ measurement of the thermal structure in Titan's atmosphere with a vertical resolution sufficient to analyze the planetary boundary layer (PBL). The vertical potential temperature profile reveals the presence of a weakly convective PBL, with a surface layer thickness of 10 m and an outer layer with a depth of 300 m. With a mean eddy diffusivity of only ~7.4 ¿ 10-3 m2 s-1, the turbulence in the PBL is weak. The turbulent heat flux in the surface layer was upward but tiny (~0.02 W m-2), indicating that the ground surface was marginally warmer than the air. The surface heat flux is too small to cause a diurnal variation of the PBL except in the lowest few meters, so the observed profile may be a nearly steady state feature within this season at the landing site. In the surface layer the mean wind speed is likely to be less than 0.1 m s-1. Given the tiny surface heat flux, the buoyant production of turbulence is very weak. The PBL structure reveals that the weather condition at the time and place of the Huygens landing resembles a calm, overcast day on Earth but is clearly different from the PBL typical for the polar night.