Contenu du sommaire : A propos de la séquence neigeuse de décembre 1990 : Une approche des évènements extremes

Five important climatic disasters relating to heavy snowfall affected the south of France prior to December 1990 : at the end of December 1970, the 20 and 21 December 1978, the 11 and 12 January 1981, the 26 November 1982 and between the 28 January and the 4 February 1986. Their atmospheric origin and location were strongly influenced by the Mediterranean region. The severe impact of such snowfall, increased by its high density, led to cuts in power supplies, telephone links and landward transport systems, as well as damage to property ; as a result, plans for dealing with such emergencies were put into operation (Plan ORSEC).

Exceptionally heavy falls of snow affected part of the Rhône-Alpes region during the month of December 1990. These snowfalls were unusual due to their intensity and, above all, to their location, affecting essentially areas of low altitude. Typically this situation was caused by the arrival of warm, humid air from the south confronting a mass of cold, northern air, blocking depressions in advance of the Alps. A difference in temperature of only a few degrees produced a memorable result.

Despite the existence of an apparently normal meteorological situation, both in terms of the season and isobaric pattern, snowfall occur ed in lowland areas. Initially the snow was wet and damp. Snowfall was spread over five days, covering in fact two distinct periods. The first, and longest with the heaviest falls, concerned the 8th, 9th and 10th December. The day of the 11th was a transition to the second, shorter period covering the 12th and 13 th December. These events were the result of different forms of interference between the aerological factor chiefly represented by two Polar Mobile Highs, and the topographical conditions of the Rhône corridor.

Studies at the level of the commune in the Loire Department provide information on the depth and duration of snowfall. In adition, they offer details of the socio-economic consequences of this event. A number of critical issues are then examined in relation to the development fo these events : the heavy and sticky snow on the 9 December damaged principally electricity and telephone lines at medium and low altitudes, whereas the accumulation of snow and the formation of drifts, especially on the 13th, paralysed in particular road traffic and damaged buildings in mountainous areas, notably the Pilat.

The period of heavy snowfalls which affected the Rhône-Alpes region from the 9 December to 14 December 1990 seriously disrupted regional economic activity. Assessment of the socio-economic consequences shows that disruption and damage were concentrated in the Departments of Ain and Isère, where the problems were seen as a real economic crisis. However, the inhabitants of the region saw the constraints they underwent in different ways, rendering subjective attempts to assess the real extent the phenomenon.

The inclusive period between October 1988 and September 1990 has left its imprint on the recent meteorological history of the whole of Western Europe. All French regions were affected by a strong rainfall deficiency. Nevertheless, the conceptual perception of drought (i.e. its sociological approach) indicates important regional differences. In this context the middle Rhône-Saône corridor (between Mâcon to the north and Montélimar in the south), an extended area of climatic transition, illustrates this variety of perception. It is thus shown that crude statistics are inedaquate to realistically assess the conceptual approach to drought.

The 1989-90 winter rainfall deficiency was connected with an unusually long-lived high pressure pattern. Below the anticyclonic cover were observed above normal diurnal temperatures but a strong rainfall deficiency, while the northern and southern borders of the high pressure mass had the advantage of copious rainfall. The immense «European » high does not have a «local » origin, that is to say a thermic continental one, and was not an inert airmass; instead the high pressure was continuously supplied by Polar Mobile Highs (PMH), that is to say thin layered and migratory cold airmasses, originating from the arctic basin. The PMH (a total of 25 out of 39 days) move, either along the western side of Greenland (17 for this «American » path), or along the eastern side (8 for this « Scandinavian » path). Their meeting over Europe and blocking by the PMH which have denser air due to their more direct trajectories provokes an agglutination high pressure cells over Europe.

The winter of 1989-90 in the department of Savoie was marked by two periods of extremely high winds. Their dates, speeds and directions are compared with the general characteristics of such winds using long- series data from observations made at Voglans (Chambéry). Although periods of high winds occur virtually each year, seldom is the damage as great as in December 1989 and February 1990. The maximum instantaneous speeds reached record values - particularly at high altitude - but this is insufficient to account for the strength of the winds' impact. Both the intensity and the duration of the squalls, together with the structure of the forest and ground conditions - as in the case of fallen trees - influenced the situation. The cost of the damage which occurred in the west of the department in December and essentially in the east in February will amount to a minimum of 100 millions francs, to which must be added economic and ecological effects which are more difficult to estimate. The manner in which insurance policies cover such rises means that the victims of damage are unlikely to be fully reimbursed.

Generations of Japanese people have shown their ability to adapt to the constraints of their physical environment and, as far as possible, to natural disasters. This adaptation, making use of favourable elements of the environment, cannot be disassociated from economic choices such as rice culture which, with rapid urbanisation, is creating new problems. Japanese people consider, therefore, the nature of their archipelago more beneficial than evil.

Soviet authorities wanted a major development of the Volga river despite the adverse consequences for the environment. The bureaucratic management of falls, a specifically Soviet characteristic, is explained by the subordinate role accorded to society by the state. Fortunately the consequences of this situation are not irreversible.