In this study, information was given about sinusoidal functions, that is to say, trigonometric functions. A trigonometric curve fitting was performed on the basis of a data set so that error sum of squares could be minimum. The ratio of air relative humidity (%) was investigated during the time period between 02.11.2012 at 00:00 PM and 06.11.2012 at 11:00 PM at Cebeci, Cankaya, Demetevler and Sincan districts of Ankara province. As a result of the analysis performed it was found that while the air humidity was lower during 09:00 AM -07: 00 PM. hours with high temperature; it was higher during 01:00 AM-07:00 AM hours with low temperature in general. It is expected that these results will recur periodically in the following periods. Therefore, a trigonometric curve can be fitted to these data.