In exploring the suitability of natural draft dry cooling towers (NDDCTs) for dispatchable thermal power plants, the transient start-up process of NDDCTs is investigated numerically. It is an established fact that the draft caused by the density difference between the interior and outside air drives the tower flow at steady-state conditions. The study finds that, starting from cold, the tower flow goes through three stages before the steady-state is reached. In the first stage, natural convection is dominant. The stagnation of thermal plumes before pinch-off contributes to a sharp increase in the inlet air temperature and the merger of rising caps further increases the rate at which the interior is heated. As the inside air warms up, an inside-outside density difference emerges and the draft starts contributing to the flow development in addition to the plumes. This is the second stage characterized by mixed convection. The distribution of air temperature inside tower becomes horizontally uniform and vertically linear through the mixed convection stage. In the third stage, the draft finally becomes the dominant mechanism and is sufficient to explain subsequent transition to steady-state and onwards. The dimensionless number, Richardson number (Ri) is introduced to identify the boundaries between these three stages. The cold air incursion is observed as a parallel cyclical phenomenon with impact on the tower flow development. A general criterion for steady state is proposed that accounts for cold air incursion. The results are validated against the experimental data from the University of Queensland natural draft dry cooling tower Gatton test rig and compared with the results of a simplified theoretical model. A better understanding of the start-up process through the numerical investigation presented in this paper provides insights to how to reduce the NDDCT transient response and thereby improving the dispatchability of the CST system employing the NDDCT. (C) 2018 Elsevier Ltd. All rights reserved.