This paper presents a study of up to six years of ground measurements of Global Horizontal Irradiation (GHI), the total solar radiation arriving on a horizontal surface, collected by 12 automatic weather stations throughout Qatar. The year-to-year and monthly variations of daily GHI are calculated for each station; the average found is 5.80 kWh/m(2)/day, a total of 2116 kWh/m(2)/year. Spatially, the monthly averages through the country vary with a relative standard deviation of up to 4% with respect to the country averages; in general, higher GHI is found in the south and north regions of Qatar, except at the northern tip of the country, and lower values are found mainly in the central latitudes, close to the industrial areas. Frequency distributions of daily GHI present two peaks, which are found to correspond to two distinct periods of the year: one from March to September, with most frequent GM values around 7 kWh/m(2)/day, and one from October to February, centred around 4.5 kWh/m(2)/day. A similar study can be advisable for other areas with comparable climate or topographies, since these two solar seasons might also be present and, if found, should be considered in the design and implementation of photovoltaic-based systems. Finally, the monthly clearness index (ratio of ground-level GHI to top-of-the-atmosphere GHI) is presented for each location and for Qatar as a whole from ground measurements and compared to longer-term satellite-based values. The annual average of clearness index for Qatar from ground measurements is 0.64, indicative of clear-sky conditions most of the year. GHI is the fuel of photovoltaic systems, so its study is fundamental for such applications, and also provides a reference for the derivation of solar radiation data from models or satellite observations. (C) 2015 Elsevier Ltd. All rights reserved.