A one-dimensional (1-D) transient cooling tower model is presented that can be used to simulate the start-up process of natural draft dry cooling towers (NDDCTs). The model simulates the behaviour of a NDDCT following a step increase in the heat exchanger temperature. The start-up process is analysed in two successive stages. In the first stage, the dominant mechanism is natural convection operating through generation and propagation of hot plumes rising from the heat exchanger surface. An understanding of different phases of plume development based on scaled analysis helps to predict the air flow development in this first stage. In the second stage, the air flow is driven by the draft caused by the difference in the inside and outside densities. The cooling tower system air flow development in the second stage is simulated through a quasi-steady state solution of the well-known draft equation, The simulation is repeated for three different input temperatures. The results show that the higher the input temperature, the higher is the inlet air velocity and shorter the start-up process. The results are validated against data from the commissioning tests of the University of Queensland natural draft cooling tower Gatton test rig. This study aims to help fill the knowledge gap in understanding the NDDCT start-up process. This understanding is important to the employment of NDDCTs in future thermal power plants operating in dispatchable mode. (C) 2018 Elsevier Ltd. All rights reserved.