Turkey lies in a critical region that is projected to be one of the most vulnerable to the impacts of climate change in the Mediterranean region. In this study, climatic zones of Turkey were classified with respect to their climatic and meteorological characteristics. The Thornthwaite precipitation efficiency index was used to identify aridity and humidity characteristics. The index values were mapped to determine climate zones and associated climate classes and to evaluate change in time and space. Two distinct periods (1950–1980 and 1981–2010) were used to assess climatic conditions and evaluate historical changes. The Thornthwaite index indicated significant spatial variations of climate parameters across Turkey with varying degrees of vulnerability. The results indicate that during the 60-year time frame, no arid zones had been experienced in Turkey. On the other hand, an increase of semi-dry and dry humid zones and a decrease of semi-dry–less humid, semi-humid and humid zones had been experienced. In this context, it is important to note that semi-arid zones have increased substantially (approximately 14%) between the two 30-year periods.