An algorithm for the extraction of hourly temperature distributions from their monthly average values is presented. The algorithm is applicable to broad regions (country level) and is especially useful for the approximate estimation of heating degree-days. This last quantity, which is indispensable for the evaluation of space heating projects, can be calculated for various base temperatures and for cases where heating is either perpetually applied or during selected hours of a day only. The algorithm is analysed and compared with previously suggested ones, while a case study is additionally included to demonstrate its applicability. As an immediate consequence of comprehensive tables with temperature data and a map which are also embodied in the paper, approximate hourly temperature data are extractable for all over Greece, the country which is referred to in the case study.