In this study, a simple mathematical model is proposed to describe the dynamic thermal behaviour of a counterflow cooling tower. The proposed model takes into account both the sensible and latent heat cooling effects on the tower performance. A simple perturbation technique is used to solve the governing equations of the proposed model in order to get a closed form solution for the temperature variation within the cooling tower subject to the cooling tower having a very large convective heat transfer coefficient between the water droplets and the air. Under steady state conditions, a mathematical criterion is derived to obtain the proper length of the cooling tower which is sufficient to ensure a complete cooling for the hot water stream.