Abstract: A cooling tower monitor for determining the performance of a cooling tower includes temperature transmitters for determining the input water temperature, the output water temperature and the surrounding temperature of the tower. A flow transmitter measures the output flow of water and a speed transmitter determines the flow of air through the tower. A relative humidity transmitter is also utilized to measure the surrounding relative humidity. The transmitters are connected to function blocks to generate a value corresponding to the performance of the cooling tower as a function of the parameters measured. A first set of function blocks utilizes the surrounding temperature and relative humidity to calculate the wet bulb temperature of the air outside the tower. This measurement is utilized in conjunction with the input and output temperatures as well as the air flow value to calculate a nominal or available water flow rate for the tower.