The purpose of this study is to analyze the influence of the ambient temperature and humidity on the performance of a counterflow wet cooling tower according to the second law, exergy analysis, of thermodynamics. First, the properties of water and air flow through the tower were predicted and validated by the experiment. Exergy analysis then has been carried out for investigating the cooling tower performance with various inlet air conditions, relative humidity and dry bulb temperature, while the water side condition is kept constant. According to the analysis in this paper, the similar result in terms of required dry air flow rate, exergy change of water and that of air, exergy destruction and second law efficiency were obtained for the various inlet air conditions. The exergy change of water Delta x(w) is higher than that of air Delta X-air, since Delta x(w) is the available energy of water to supply to air throughout the tower while Delta x(air), is the available energy of air to recover or utilize that supplied by water. It reveals that Delta x(air) is dominated by the exergy change of air due to evaporative heat transfer. In addition, it,gives a clearer explanation of the cooling tower performance and gives clear trends for optimization. (c) 2008 Elsevier Ltd. All rights reserved.