A method for determining the relative humidity of the air entering the cooling heat exchanger of an air conditioning system as well as an apparatus for implementing the method. The cooling capacity of the system is varied and, if occurring, the transition between a condition where moisture is condensing in the heat exchanger and a condition where moisture is not condensing in the heat exchanger. At this transition, the temperatures of the exiting air and the entering air are measured. From the measured exiting air temperature, state point of the saturated exiting air can be determined. From the exiting air state point and the entering air state point, the entering air relative humidity can be determined. If no condensation occurs even at maximum cooling capacity, the maximum that the relative humidity of the entering air can be determined. If condensation continues even at minimum cooling capacity, the minimum that the relative humidity of the entering air can be determined.