Les types de temps à l'origine des crues dans le nord-ouest du massif central (1953-1962)

Abstract

The north-western part of the Central Massif enjoys a special position in the general atmospheric circulation pattern, a it is affected by moist oceanic air from the west, by polar air from Scandinavia and by warm air from the Mediterranean. Flood conditions due to rainfall associated with these circulations vary in extent and development. It is interesting to study the formation of these various types of weather and to relate them to the isohyetal charts for the showers and to some of the flood water levels in the distributaries in the Limousin region. The "western" type of weather centres about an anticyclone over the Azores or Southern Europe and a depression extending from Ireland to the Baltic. It brings abundant rainfall, with contrasting peak and minimum values (see Table 1). The flood conditions a causes normally occur in autumn and winter and are usually pronounced everywhere (see Table 2). The "northern" type of weather is caused by a ridge of high pressure extending from north to south over the Atlantic and a depression extending from Scandinavia to the 'Western Mediterranean and forming a centre of disturbances and southerly polar air streams. A further anticyclone sometimes also extends over Central Europe. Rainfall is of a more unpredictable type (sharp showers, thunderstorms, etc.) and may occur at any time of the year, though less abundantly than in the previous case. Floods are more local in extent and depend more on the state of soil saturation than in the previous case (see Tables 3 and 4). The "southern" type of weather results from inflows of moist warm air masses from the 'Western Mediterranean which, in meeting the cooler or cold air masses centred over Northern Europe, cause heavy rainfall and major floods over the whole Central Massif. A typical case of this occured in the Limousin region in october 1960 (see Tables 5 and (6). A knowledge of the meteorological origins of these types of weather is extremely valuable. Isohyetal charts can be of considerable use in this connection by enabling the recurrence of disastrous conditions to be predicted and appropriate counter-measures taken in good time.