Looking at Spain as a whole, for the year as a whole, we can distinguish, in respect of temperature, the very different climates of the equable Biscayan and north Atlantic coasts; of the interior, with extremes of temperature always, but with a winter which varies from the traditional nine months of the higher parts of Old Castile to the short two-months winter of the lowlands of Extremadura; of the Mediterranean coast with its very short winter and rather hot summer; and of the sub tropical south and south-east, where winter, in the popular sense, hardly exists.
More important is the distinction in respect of rainfall be tween Spain of the north-west and north (roughly as far south as the line Leon-Pamplona-Huesca), with a mean annual rainfall of 24 in. and over, fairly well distributed over the year, and the rest of the country, arid Spain, with rains insufficient in quantity, or badly distributed, or, more commonly, both. There is a general decline in the mean annual rainfall from north-west (Santiago, 66 in.) to south-east (Almeria, c. io in.), but this is interrupted by the minimal rainfall of the basin of Old Castile, where Salamanca and Zamora, like Zaragoza in the Ebro basin, lie within an isohyet of 12 in., and by the orographic rainfalls, which are sometimes remarkable; thus, in the Baetic calcareous zone, in spite of a rainless summer season, the annual rainfall may be of 8o in. on the outskirts of the Serrania de Ronda.
The rains of arid Spain are spasmodic, Mediterranean rains falling in large drops for a few hours of a few days of the year. They are also irregular in amount from year to year; the rain fall records of 95 years for San Fernando, near Cadiz, for ex ample, show a mean deviation, from the mean, of 26%. Long term data are accordingly necessary for the construction of satis factory maps of rainfall, while the number of long-term stations is limited (contrast France, 31.3 in.; Portugal, 31.6 in.).
Irregularity of the seasonal rain-supply, and the entrenched courses of the rivers of the interior, prevent much use being made of their waters for the partial relief, by irrigation, of the prevailing aridity. Both of these difficulties are now being over come for the rivers of the Ebro system by the provision of immense engineering works. (See ARAGON.) On the lower courses of the rivers of the short eastern slope the relief is, by exception, highly favourable to the establishment of important irrigation systems. (See VALENCIA.) Properly conserved, arid soils, although they are inevitably exposed to the sterility resulting from sequences of years of drought, have, nevertheless, certain advantages of chemical com position, due to the smaller degree to which plant foods are lost by leaching, which enable them to maintain a standard of pro ductivity lower than that of more humid regions and requiring a greater output of labour, but more independent of artificial fertilization, especially if natural manure is supplied by pasturing.
Dry-farming, the cultivo de secano, is the third and most wide spread of the geographical institutions of arid Spain.
Subsoils include the Andalusian "black-earths" and new soils, chestnut or red in colour. An extreme type is the calvero soils, for which the Castilian name seems likely to become interna tionalized. In the calveros, which arise commonly from past or present attempts to extend cultivation beyond its proper limits, the soil cap is discontinuous and the native rock conspicuous on the surface; tufts of permanent vegetation depend on, and pro tect, the small patches of soil; in the open spaces lichens are the chief covering, with occasionally transitory grasses.
Saline soils occur locally, in the more arid parts of Old and, especially, of New Castile and of the Ebro basin, and elsewhere, but the extension of the saline area is greatly modified by human agency. In east Spain, in general, and in Andalusia, red soils, the "Mediterranean red earths," are commoner than chestnut, and in the dry south-eastern corner tawny or grey soils are commoner than red ; the precise significance of the colour is not known.
The Northern, humid area is dominated by mesophytic f orma tions of the Central European and North Atlantic type, while the rest of Spain is characterized by xerophytic formations distinctly Mediterranean in type. Increasing aridity and proximity to the African continent are reflected in marked African features in the vegetation of the south-east. Further, owing to its extraordinary variety of climates, altitudes, exposures and soils, Spain is richer in botanic species than any other European district of the same size, and endemic species are particularly numerous. These are not only characteristic of the higher parts of the Pyrenees and of the Cantabrian and Central ranges, but they also occur in great numbers in the central plateaux of the interior. Of the 5,66o vascular species which have been identified, about one-third are Central European or Alpine in character, one-quarter are endemic, one-fifth are Mediterranean and one-twelfth are African and Atlantic.