Rainfall. – According to homosociety.com, the distribution of the rains is closely related to the relief, but their regime clearly marks the continental and maritime influences, and even more the particular characteristics of the Mediterranean climate. All depressions correspond to pluviometric minima, of which the most notable is in Colmar in Alsace and the largest is in the Paris Basin (see map on p. 883). The drought of Limagne and the Rhône corridor as far as the Saone plain should also be noted. The coasts are generally rainier than the interior, especially when they have a certain relief: the flat coast of the Landes receives considerable rainfall only where it is dominated by the Pyrenees; indeed, it is precisely there that the strongest average totals in France are noted. The hills of Brittany and Normandy, which do not exceed 400 m., they are enough to condense precipitations higher than 1200 mm.; and in the Massif Central the highest points are not those which receive the most abundant rainfall. The Limousin, directly exposed to the West winds, receives the same amount of water on the Millevaches plateau as the Auvergne. A particularly rainy location is the edge of the Cévennes towards the Aigoual, where the Atlantic rains and the Mediterranean rains fall at the same time and where the strongest precipitations that have been collected in a single day were found (Joyeuse, 797 mm. on October 9, 1927). The Mediterranean coast does not suffer at all from the drought that is usually attributed to it. Marseille receives as much water as Paris receives; Nice receives much more (750 mm.); only, the rains fall there in the form of showers, rare in every season and very rare in summer. The dominant rainfall regime in France is intermediate between the oceanic regime and the continental regime. The curve of the monthly averages in Paris has no accentuated minimum and has two weak highs in summer and winter. Only on the coasts can we see the true oceanic regime, with its very strong peak in autumn (Brest 30%); while the continental regime, with the maximum in summer, begins to take shape in the east of the Parisian Basin, and appears very clear in Alsace and in the Saone basin. In the Aquitaine Basin the regime is absolutely oceanic on the coasts and tends towards the Mediterranean regime towards the interior, without, however, presenting a real summer drought at any point. In Toulouse, the maximum is in spring (May, June) and the driest months are July and December. The Mediterranean regime dominates the entire coast of Languedoc and Provence, with highs in autumn and spring and with a very pronounced drought in summer. Marseille in three months (June-August) receives just 9% of the annual total; and often July and August pass, without a drop of water falling. This regime also reigns in the Rhone valley as far as Montélimar, in the valleys of the Alps as far as Sisteron on the Durance, and is also sensitive to the neighboring peaks.
Winds and atmospheric pressure. – Winds and variations in atmospheric pressure better than any other phenomenon explain all the characters of the thermal regime and of the rainfall regime. France is subject to the influence of cyclones with a Mediterranean trajectory and the influence of those with a more northern trajectory; and the latter make themselves felt there especially when they pass through England and the North Sea, causing the great rains which fall on the Armorican Massif and the Paris Basin, swept by the W and SW winds, which they carry with them. big cloud coverings. To them we owe the absolute dominance of the winds of the western quadrant throughout the north of France. The winds of E. are felt, either by the advance of Atlantic cyclones, or by the establishment of an anticyclone over central Europe. In the first case they are short-lived; coming from SE. the air is generally dry and warm, and in the summer the temperature rises rapidly together with the absolute humidity, making it possible to spend a few painful hours in Paris and sometimes even on the coast. The E. anticyclonic wind lasts longer, and is hot in summer and cold in winter. The cold shocks that occur in the eastern regions (Alsace and the Saone valley up to Lyon), which can cause the thermometer to drop down to −10 ° and even to −15 °, always depend on an anticyclonic regime; they, rarer in Paris, are all the more sensitive to it. The Aquitaine Basin is not directly subject, like the north of France, to the influence of Atlantic depressions, which explains the relatively continental character of its climate; gl ‘ Oceanic influences are limited to the coast and Toulouse has relatively cold winters, given its latitude. However, it often happens that a satellite cyclone accompanies, in the south, an Atlantic depression, whose trajectory passes through England; in this case the southern and eastern part of Aquitaine is subject to winds from the E. and SE., which are made particularly dry and violent by the influence of the Pyrenees and the south of the Massif Central.autan of Toulouse and the Pays Castrais. The cyclone, advancing, passes over Aquitaine itself, and usually reaches the Mediterranean; then the winds leap to the West and large showers of water often occur, accompanied by storms. This succession of events is frequent in spring, and explains the maximum rainfall of this season.
On the Mediterranean coast of France, the prevailing winds are light breezes with a northern component, which account for the purity of the sky and the rarity of the rains. The stability of the atmosphere is disturbed only in spring and autumn by the passage of cyclones, which come, one from the Atlantic via Aquitaine, the others from Morocco via the coasts of Spain. These depressions are mostly directed towards the Gulf of Genoa, and their passage causes a recall of air from the north, resulting in a wind fast enough to give an impression of cold and sometimes even violent enough to obstruct communications: this wind, which sweeps the clouds and dries up the plains, is the mistral. On the coasts of Provence and Languedoc it generally rains due to a SE wind, known under the name of marin, humid and warm.
