Convection velocity of temperature fluctuations in a natural convection boundary layer

•For the first time, two-point temperature measurements are carried out in a turbulent natural convection boundary layer.•The two-point correlations of temperature fuctuations revealed that the thermal structures have a uniform convection speed in the first half of the thermal boundary layer and the convection velocity scales as Ra1/4, where Ra represents the Rayleigh number.•Shadowgraph measurements are in excellent agreement with the results from the two point measurements and provided a visualisation of the turbulent structures in the flow.•A physical interpretation is proposed based on the experimental observations, which is consistent with the physical model put forth by Wells and Worster JFM 609 111–137 (2008).