Natural convection over an isolated horizontal plate in a stably stratified background condition is fundamental and challenging in industrial, atmospheric, geographical, and oceanic environmental studies, which, however, is still lack of investigation. In this study, the water tank modelling experiment with two main techniques, Thermal Image Velocimetry (TIV) and Particle Image Velocimetry (PIV), was conducted to investigate large eddy structures, the mean velocity field and heat transfer over an idealized square heated surface under the stable stratification. Shirking, growing and twisting eddy structures were identified in this study. By measuring the velocity fields at different horizontal planes with varied heights, an asymmetric flow structure over a square urban area is suggested by the PIV method to be characterized as the main diagonal inflow at lower levels and as the side outflow (perpendicular to edges) at upper levels. The mean flow speeds measured by TIV and PIV were quantitatively compared and analyzed by using cross-correlation coefficients. The low-level (2 mm or less above) velocity field measured by TIV also well presents the similar characteristics of near-surface diagonal inflow observed in the PIV measurements. Consequently, the reliability of the TIV method in this study is well verified for resolving the near-surface convective flow fields. The stable stratification is found to significantly impair the heat transfer efficiency as results indicated that the heat transfer in a stable environment is significantly weaker than that in a neutral environment by 71%. (C) 2019 Elsevier Ltd. All rights reserved.