The zone of the prairie, intermediate between tropical Argentina and the sub-desert regions of western Patagonia, has a medium rainfall. It decreases gradually from north-east to south-west. There is a rainfall of 1,200 to 1,000 millimetres on the lower Parana, and only 400 to 600 millimetres on the western edge of the Pampa. The zone which lies between the 800 millimetres and 600 millimetres average is more than 270 miles in breadth. But what is most characteristic of the climate of the Pampa is the equal distribution of the rain throughout the year, and the absence of a real dry season. In this the Pampa differs from the surrounding regions, both in the south-west and the north. At Buenos Aires the six months of the (relatively) dry season yield, nevertheless, 44 per cent. of the total rainfall, and at Bahia Blanca 40 per cent. This regularity diminishes in proportion as one approaches the coast. At Rosario the six months of the dry season only yield 30 per cent. of the year's rain; at Villa Mercedes (San Luis province) 25 per cent. When one goes beyond the limits of the prairies the ratio of rain in the dry season decreases rapidly; it is only 20 per cent. at Cordoba and 18 per cent. at San Luis. At Cordoba, the curve of the rainfall indicates a typical tropical regime, with a summer maximum and a very low minimum in winter. Passing south-eastward from Cordoba, at Bellville, Villa Maria and especially Rosario, the dryness of the winter diminishes, and at the same time a secondary minimum appears in the middle of summer (January-February). At Buenos Aires, the form of the curve changes completely. The summer minimum is almost as low as the winter minimum, and most of the rainfall is in the spring (September) and the beginning of the autumn (March).[77] [77] Argentine Mesopotamia, which is a continuation of the Pampean region from the climatological point of view, is also, even in its northern part, without the rigorous dry seasons of the Chaco. Ascending the Parana, from Corrientes to Posadas, just as in passing from Cordoba to Buenos Aires, one notices that the winter minimum decreases, and a secondary maximum appears in the spring. The predominance of the spring rains, which is a characteristic of southern Brazil, is conspicuous on the middle Uruguay. On the lower part of that river the rain-system approaches that of Buenos Aires, with maxima in spring and autumn, a principal minimum in winter, and a secondary minimum in summer. These various shades of the Pampean climate are of essential importance in the history of colonization and the spread of cultivation. The belt of summer rain is the belt of maize-growing, whereas the cultivation of wheat requires spring rain and a comparatively dry summer.