Transitional Fluctuations of Thermal Boundary Layer in Horizontal Convection

•Temperature fluctuations in thermal boundary layer are experimentally investigated over 2×1010≲Ra≲1.3×1013.•The hot thermal boundary layer undergoes several transitional changes, from laminar to chaotic states.•Transitional changes of temperature fluctuations affect the mean and fluctuation temperature profiles in the hot thermal boundary layer.•For Ra≳7×1012, the temperature variance σT2(z) in the convective mixing zone above the heated plate, follows a power-law function σT2(z)∼zγσT2(z)∼zγ with γ=2/3.