Statistical analyses of Hot Days Changes in Iran

Hot days are a state of temperature and an important climatic phenomenon. Long term changes (trends) of this phenomenon are evidence and consequence of temperature-climatic change. In current research it had been tried to investigate long term changes of hot days of Iran using day time temperature database during 1961-2008 as well as using statistical methods. Accordingly based on 90th percentile the hot day index for every pixels of database and for every day has been estimated. This value used as hot day threshold. Then the maximum temperatures of every day have been comparing with its threshold. Every day of every given pixel which has gotten a temperature equal or more than threshold is defined as a hot day. The result of this result showed that the mean of number of hot days is about 39 days. The months in cold seasons and in April have experienced the most days with hot temperatures. This value has experienced positive trends in about half of the country. While sixth of the country has negative trends. The area with positive trends is continuum in space in compare with area with negative trends. The mean of temperature of hot days also investigated. In more than half of the country the trends of temperature of hot days was positive. Meanwhile in third of the country there are negative trends. The trends in both, number of hot days and mean temperature of same days is following the structure of mountains structures. The correlation of trends in number days and mean temperature of hot days is positive temporary and spatially. The events of hot days in Iran have shown a 3-4 years cycle.