Climate. – The variety of climates in France depends on the geographical situation facing the ocean and the Mediterranean, on the extension of the territory up to the Rhine, that is to say up to the borders of Central Europe, and on the relief of the soil.
Temperature. – According to Naturegnosis, the layout of the isotherms clearly indicates the division between oceanic and continental influences. In winter, they orient themselves from N. to S. and also follow the contours of the coast of the Armorican Massif. Brest has the same January average as Biarritz and Marseille, while Strasbourg is 8 degrees colder. On the coasts of the Channel and the Atlantic, spring is late and summers are temperate: in Brest the average in July is 4 ° lower than in Strasbourg. In conclusion, the annual hike is 12 ° stronger in Alsace than in Brittany. Annual isotherms indicate some influence of the oceanic climate, as they are generally oriented from the NW. to SE .; but their most notable feature is the huddling on the coasts of Provence and Languedoc, marking a very notable advantage of the Mediterranean coast. The hottest places in France are Nice and its surroundings, with more than 8 ° in January and more than 15 ° of annual average, and the Roussillon plain, with 7 ° in January and 14 °, 5 of annual average. The relief of the soil is the cause of local variations in the distribution of temperatures. The Massif Central, being almost entirely above 500 m., Has an annual average of less than 6 °, a long-lasting winter, similar to those of Alsace, and summers as cool as those of Brittany. The same can be said of the southern Vosges. Closed basins such as Limagne have a more continental regime: winters colder than the altitude would entail, with temperature inversions. These characteristics of the mountain climate are accentuated in the Alps.
Types of climate. – In summary, we can distinguish some types of climate in France, the limits of which are often indeterminate. The oceanic climate, which could be called Breton, is characterized by a weak temperature range, mild winter, late spring, relatively cool summer, frequent rains, abundant in all seasons, but with a maximum in autumn. It is limited to a rather narrow fringe in S., which widens to N., to cover almost the entire Armorican basin, and narrows again north of the Seine.
The transitional climate, which could be called Parisian, is characterized by a greater temperature range, by more frequent frosts and also by cold bursts, very rare, however, in winter, and by a regime of rains that remain the same throughout. the year. It extends almost to the entire Parisian Basin. Then, it passes insensibly to a climate hinting at the continental type, which is found in Lorraine, Alsace and the Saone Basin, with harsh winters, very frequent snowfalls, which on the heights of the côtesof the Meuse remain on the ground for over a month. Summers are hot there, with stormy rains; and the sky is more easily clear in winter, when it is not veiled by mists, as happens not infrequently in the valleys. It is also possible to distinguish an aquitanic climate, existing almost throughout the Aquitaine Basin, except for the coastal fringe: it is characterized by summers that are already hot enough, early and rainy springs, winters less mild than in the Mediterranean region and not free from snowfalls. and from cold shocks.
The Mediterranean climate prevails, with its typical characteristics (warm winters, rainy springs, hot and very dry summers), throughout the Languedoc plain and in Roussillon, in Lower Provence and in the Rhone valley up to Montélimar.
The influence of the relief also allows us to distinguish a real typical climatic region corresponding to the Massif Central, rainier than the neighboring plains, colder during the winter, with a blanket of snow that covers the ground for a long time, but above all fresher. during the summer.
The Alps are even more to themselves: the variety of climate conditions is determined by a relief full of contrasts, which has altitudes that reach up to the area of perpetual snow. The originality of the French Alps depends on the orientation of their main axis, stretched from N. to S., instead of from E. to W., as in Switzerland and Austria. Since the highest altitudes are in the N., there is a remarkable contrast between the Savoy and Dauphiné Alps and the Provence Alps: the former are much colder, with more rain and snow, and the winter isotherm of 5 ° there is about 1300 (Mont Blanc), that is 500 m. lower than in the Maritime Alps; in summer, the difference tends to disappear in the higher parts, but it is always very strong in the valleys, going up to Digne the isotherm of 20 °. Precipitation in the Savoy and Dauphiné Prealps averages between 1 and 2.50 meters, while in Provence it rarely exceeds 800 mm. Their regime is also different in the north and south, contrasting the maximum in summer with the minimum in the hot season. Everything points to two different domains of the mountain climate, separated almost by a line that passes through the hills of Rousset and Luz-de-la-Croix-Haute in Vercors, then through the Massif of Devoluy and Col Bayard. Due to its climate, the high Durance (Ubaye) is already within the circle of Mediterranean influences. The same contrast can be seen in the Pyrenees between the eastern part, up to the sources of the Ariège, and the central and western parts: a contrast so strong that it made Strabo think that the chain was oriented not from E. to O. but from S. to N.