Contenu du sommaire : Le climat de la France

The diversity of French relief is a source of difficulty in the search for representative climatic boundaries. Dynamic analysis of climatic fields provides a qualified answer to this question, outside of the scope of traditional phytoclimatic systems. The significance of the limits can be estimated by the intensity of the opposition between the systems of atmospheric circulation which determine them. Correlation analysis of the floristic fluctuations of the vegetation enables this force to be measured with precision (dynamic phytoclimatology), without being unduly affected by French phytogeographical uncertainties. Finally, climatic limits are defined, in the following order, by the intersections between dynamic, climatic and vegetation fields.

The diversity of French climate, resulting from the effect of factors operating at numerous different scales, masks the unicity (at the scale analysed) of the aerologi- cal component and therefore the relative functional simplicity of the dynamics of climate. The key meteorological factor of middle latitudes is the Polar Mobile High : it originates from the arctic basin and arrives on French territory either directly by passing to the east of Greenland, or more frequently from the North American area after crossing the North Atlantic Ocean. Interaction between this mobile factor and the specific geographical conditions of France, particularly the relief, dictates the permanent patterns of low-level circulation and the spatial distribution of rain-making conditions. These patterns are further modified by particular seasonal conditions : they gradually change from a winter situation characterised by a blocking of the zonal movement of Polar Mobile Highs, to a summer situation during which the southern part of the country is affected by air subsidence from the tropical margins (subsidence from the North Tropical High Belt).

ABSTRACT This paper presents the results of a study showing the importance of wind in the climate of northern France. This work is based on the observations for 1979 to 1988 made by the French meteorological service's seven synoptic stations in the Nord, Pas-de-Calais, Aisne and Somme "départements" together with the data from 1 7 new automatic stations. An approximate wind speed map at 10 metres above ground level has been drawn and the wind energy resources estimated.

The unique character of the Paris urban area's climate is shown by the presence of a strong heat island, a decrease in both the intensity of solar radiation and the number of days of frost and fog, and an increase in the length of insolation. The rainfall regime has been modified and local winds appear.

The general pattern of relief and proximity to Mediterrean, oceanic and semi-continental climatic influences explain the specific rainfall characteristics of the Massif Central : abundant rainfall only in the Cevennes and western upland areas : rainfall regimes which demonstrate considerable variability in their nature and over time.

Rainfall regimes are a good indicator of the remarkable inter-mixture of climatic influences which characterise the eastern central part of the Massif Central and are related to contrasts of altitude and aspect. The regimes with a summer or late spring maximum and a winter minimum, strongly marked in the basins, are sharply justaposed with those having an autumnal maximum and summer minimum in the Vivarais ; this latter feature extends far to the north on the Rhône side. Analysis of the quintilies, as well as that of a number of noteworthy periods of rainfall, makes it possible to indicate the importance and extent of "cévenol" type rainfall. It also shows, throughout the area, the regularity of the May rainfall regime, contrasting with the great variability of the autumn months.

This article analyses the various meteorological regimes which have influenced the region of Rhône-Alpes during the period 1970-89, and their effect on the weather and especially the rainfall. In terms of the annual average, disturbed systems affected the area for a total of 206.3 days (56.3 °/o of the time), anticyclonic systems for 126.3 days (34.7 %) and periods characterised by a flat low pressure field for 32.7 days (9 %). In the northern Alps and the Jura, westerly and north-westerly weather types give most rainfall, whereas in the south, and particularly the south-east of the Massif Central, it is those originating from the south-west and south. In Lyon rainfall coming from the south-west alone represents 38 % of the total.

In spite of interest in the tragedy caused by the media, the exact cause of the storm remains ill-defined. Initially, it was thought to be an exceptional and unpredictable event ; however, a full explanation of the storm's mechanism is possible, taking into account the particularities of the site and, above all, the aerological conditions. Thus the presence of an aerological dam, consisting of a polar mobile anticyclone on the northern extremity of the Alps played an important part in the development of the rain-making processes. This key factor, as well as previous events, prove that the storm of 7.14.87 has to be considered as an infrequent but none the less predictable phenomenon.

The study of dry spells gives a better characterization of dryness than the monthly means of precipitation. Their length is longer in the Mediterranean area than in other parts of France, and during summer than during the other months in the Mediterranean area. Data of five stations spread along the coast have been investigated. They display differences between the eastern and the western parts. Dry spells are longer during summer in the east and reach a maximum in Toulon. A model based on a shifted negative binomial distribution which fits well most of the observed data allows us to estimate the probability of occurrence of a dry spell according to its length and the minimal length of a spell to its recurrence.

South-west France features both a specific climate and a variety of climates. The climatic environment and associated processes make this climate different from that of mediterranean area and more notherly oceanic regions. The action of numerous geographical factors operating at various scales on the different circulation patterns leads to marked internal diversity.

Study of the secular evolution of French climate in terms of rainfall and temperature is undertaken using synthetic indices showing both national and regional characteristics. Despite very important annual differences, on a secular scale rainfall levels appear stationary. Temperatures vary in a generally regular manner, with an eight-year cycle being detected. However, on average they have continued to increase, in line with world trends, excepting a break between 1961 and 1972. This development is related principally to summer temperatures, whereas winter temperatures indicate a stable secular trend.