<p>Transient natural convection along a vertical wall was investigated experimentally. To achieve a high Rayleigh number (<i>Ra</i>) and instantaneous thermal condition, a mass transfer experiment using the CuSO₄–H₂SO₄ electroplating system was adopted based on the analogy concept. <i>Ra</i>_<i>H</i> was 2.11 × 10¹³ covering laminar and transitional regimes. Time-dependent heat transfer and flow were measured through electric current and PIV (Particle Image Velocimetry) measurements, respectively. The velocity fields and Reynolds stresses were compared with the heat transfer results to analyze the relationship between heat transfer and flow behavior. In both the laminar and transitional regimes, four distinct stages were identified. Due to the high Prandtl number of the working-fluid, the time required to reach steady-state heat transfer was observed to be significantly shorter than that for the velocity field. This study deepens the understanding of the transient development of the natural convection and provides experimental background for passive safety system design in nuclear reactors.</p>