Abstract: This study deals with the spatial and temporal variation of clearness index (KT) in different meteorological stations, well distributed in Iraq, during the period between 1995 and 2015. The results indicate that the clearness index varies with the geographical location and period of the year. Monthly values of clearness index (KT) in the months of January and December show the lowest values of the index ranging between 0.38 and 0.64. Meanwhile, August shows the highest values ranging between 0.53 and 0.71. Seasonal variation of KT is well noted. The range of (0.40 to 0.58) has been measured in winter, while the range of (0.53 to 0.70) has been detected in summer season. The annual variation of KT shows a range between (0.47 - 0.64) in all stations. The standard deviation (SD) of the monthly values of KT ranged between (0.01-0.06) for all stations, while the coefficient of variation (CV) for the monthly values of KT ranged between (2-10) percent. Keywords: Clearness index, Global solar radiation, Extraterrestrial radiation, Time series, CV, SD.