Major factors in the climate are the two abutting bodies of water and the extensive continental areas to the north and south. The Atlantic influence penetrates southward across the low peaks of the Western Pyrenees, as far south as Pamplona, Spain, tempering somewhat the differences of climate between the northern and southern slopes. This is not the case in the rest of the chain, especially the Central Pyrenees. The contrast in humidity between the French and Spanish sides is remarkable. To the north the oceanic influence, meeting no obstacles on the French plains of the Aquitaine, penetrates eastward and goes a little beyond the north–south watershed of the French rivers flowing into the Mediterranean. To the east the levanters, winds from the east and southeast, carry damp air from the Mediterranean, some of which falls as precipitation over the southeastern part of the eastern spurs. As a result, these regions are humid, while to the northeast the French depression of the Roussillon acquires Mediterranean characteristics.

South of the Central Pyrenees the valley of the Ebro—which runs in a general northwest–southeast direction and is blocked by the southwest–northeast-trending Catalonian ranges near the eastern coast of Spain—acts as a “little continent.” Hence, its climate is one of great thermal contrasts that are exaggerated by the generally high altitude of the Iberian Peninsula, but it is Mediterranean and unlike anything known in other European countries. Thus, the variegated climatic pattern of the Pyrenees ranges from the limpid, sunny atmosphere of the continental zone to the mild mists of the northwest and includes all transition stages in between.

Plant and animal life

Forms of life in the Pyrenees have some remarkable characteristics that cannot be explained merely by the influences of climate and soil. The historical vicissitudes of the chain and its isolation at the southwestern limit of the main European peninsula, far from the centres of dispersion and variation of the various species (including humans), have influenced the structure and character of its population.

In the northwest–southeast direction, the vegetation shows a marked and gradually decreasing oceanic influence; the contrary is the case with the Mediterranean influence from southeast to northwest. The exposure of the mountain surfaces and the conditions of local climate caused by mountain relief create special localized enclaves of all kinds. The most characteristic feature of the oceanic influence is the predominance of broad-leaved deciduous trees in the forests of the lower levels and the medium-height mountains, while the Mediterranean influence, represented by evergreen broad-leaved trees, not only is dominant in hot surroundings but also bears drought conditions better.

The variety of altitudinal vegetation shows itself in levels. From the medium-height mountain upward, the broad-leaved woods at about 5,200 feet are replaced by needled conifers that require less water. The subalpine level, sometimes as low as 6,500 feet but usually above 7,800 feet, gives way to the more sparsely covered pastures of the alpine level. This altitudinal scheme pertains in the vegetation east of the Orhy and Anie peaks. The oceanic influence, however, with its greater rainfall gives the west of the chain a different pattern. Broad-leaved deciduous beeches may be found as high as 5,850 feet, with some mix of the subalpine conifers, and there the high pastures are more resistant to damp and permanent snow. Overall the landscape is more like that of the high mountains of western Europe.

The Mediterranean influence expands through the entire valley of the Ebro, but it acquires marked signs of a more variable continental climate in the Central Pyrenees. There, great quantities of mountain pines, which are more drought-resistant, take the place of deciduous trees in the higher, colder, and drier parts of the medium and higher levels of the southern slopes.

Some groups among the fauna, such as the cave-dwelling animals and frogs and toads, represent a migratory wave that came from ancient Tyrrhenia—associated with Corsica and Sardinia—and displaced certain native European species, relegating them to the Cantabrian Mountains. The Pyrenean fauna is rich today in larger herbivores as well as in the variety and abundance of predators. Some species, such as the wolf, lynx, and brown bear, have disappeared or had their numbers severely reduced in the northern Pyrenees, although the marmot has been successfully reintroduced. The southern Pyrenees, however, represent one of the last important reserves for wild European fauna driven out of sectors more heavily populated by humans. The present distribution and differentiation of large, warm-blooded animals is undoubtedly connected with the climate and the landscape, but the central-European origin of Pyrenean fauna is clear; for example, of the two species of desman (a semiaquatic member of the mole family), one inhabits the Pyrenees and the other southwestern Russia.

Similar comments may be made as to the origin of all cold-blooded animals as well as of the vegetation. Basic differentiations exist among the latter. Pyrenean flora of tropical origin differentiated without any ancient European competition as the new chain replaced the old Hercynian; flora of Arctic origin, brought southward during relatively recent ice ages, are represented by two different branches of orophiles, or plants adapted to mountain life, from central Europe and from Siberia. Other orophiles have long been differentiated, but they are of Mediterranean origin and are dominant in the drier, sunnier parts of the southern slopes. An Atlantic group of flora predominates in the Western Pyrenees.