Abstract
Aridity scales have not been used very frequentely in Portugal, which is a region of transition between the Atlantic and Mediterranean areas. In this study, carried out over a period of 12 years (1960-1971), using 101 meteorological stations, two of the most common aridity scales were charted: the Gaussen Xerothermic Scale (fig. 1) and the Emberger PIuviothermic Quotient (fig. 2: the Emberger Climograme; fig. 3: the Emberger Bioclimatic levels).
A quantitative comparison was decided as the two maps seemed to show a marked visual resemblance where the Tagus River formed a non – rigid limit between the Northern-most Atlantic – and the Southern – most Mediterranean.
The first step was determining the correlation rate which was strong and negative, -0.73 (fig. 4) for the sations group. If the stations which are furtherest from the regression line are removed, the correlation rate is increased to -0.81. The stations in this situation correspond to two types of climatic extremes: the Northwest and mountains regions in the Atlantic area, and the South and Southeast in the more Mediterranean areas. This proves that either or the two scales can be used indifferently for other intermediate regions.
In a holistic study of the two maps, the groups of stations were separated on the basis of the taxonomic tree (fig. 5) and according to the degrees of similarity. Only three of the possible levels of similarity were kept and charted. These three maps (fig. 6 A, B, C) have allowed for the sucessive individualization of the Atlantic regions and the more or less Mediterranean type regions and, finally, the main arid sub-areas in the Southern part of the country.