<p>The clearness index (CI) is a key indicator of atmospheric transparency and solar energy availability. This study examines the temporal and spatial variability of the CI in seven representative cities of Iraq (Basra, Nakheb, Kut, Baghdad, Rutba, Kirkuk, and Shakhan) using POWER/NASA reanalysis monthly data from 2005 to 2024. The CI values were analyzed to examine seasonal and annual behavior, long-term trends, the relationship with elevation, and the spatial analysis. Polynomial regression model and ordinary Kriging technique were applied to better fit temporal annual variations and to make the spatial distribution of CI, respectively. The results revealed a pronounced seasonal cycle, with the highest CIs in June and the lowest in December, and an annual range of ~0.65–0.73 across the study area. Elevation exerts a positive but limited influence: highland and desert plateau regions generally exhibit clearer atmospheric conditions than low-lying urban areas. Long-term analysis indicates that the nationwide annual CI follows a cubic polynomial regression with an overall negative trend across all studied cities. There is evident regional variability: desert cities such as Rutba and Nakheb remain relatively stable, while more urbanized or humid areas, including Baghdad, Basra, and Kirkuk, exhibit stronger declines. Reduced CI lowers available solar irradiance, so policymakers should focus on air quality, dust storm control, and adaptive solar energy strategies to ensure long-term sustainability.</p>