Contenu du sommaire : Le climat urbain

The pattern of air temperatures in the city of Lyon shows an urban heat island which is particularly pronounced in the eastern part of the central area. This thermal distribution relates the topography, the density of the built-up area, the location of each weather stations and the synoptic situation with respect to the Mobile Polar Highs (MPH) paths.

Over a period of less than 48 hours (25th-27th June 1996), Bucharest was subjected to the effects of a double storm which had lasting effects on the landscape of the Romanian capital. This meteorological event demonstrates the importance played by extensive concreted surfaces in urban areas within the context of a pattern of atmospheric circulation already modified by the relief. The resulting lows were difficult to forecast, while the environmental contrast between an overheated urban zone and an adjacent humid rural area accentuated the meteorological instability.

Over recent years, several towns have suffered the consequences of flash floods. This paper studies the effect of urbanization, first on the increase in the frequency and importance of floods and, second, on their consequences. The exemple of the Yzeron, a small river which crosses the western part of Lyon, is presented as a case study.

Anemometric observations carried out for twenty years at the top of one of the highest hills of Saint-Etienne show the dependence of the wind rose on the orientation of the three large divergent valleys which characterize the site of this agglomeration. They also show the regular existence of daily alternating breezes from these valleys to the Pilat mountain. Moreover, they demonstrate that the southerly storm winds which rush down from the Pilat are a major climatic hazard for the town. However Saint Etienne is affected by periods of major atmos-pheric pollution when the atmosphere is stable.

Intensive field work was carried out in the Chambéry valley in order to estimate topographic and urban effects on surface airflows. This work also highlighted the influence exerted by the city on local air temperatures, especially in anticyclonic weather (clear skies) ; the spatial analysis of data showed an urban heat island whose intensity and extent depend on the nycthemeral cycle and meteorological conditions.

In the urban basin of Annecy heat islands are often numerous and important. Their daily, seasonal and spatial variations are analysed in order to improve understanding of wind movements in the Annecy basin.

What policy can respond efficiently to the regular and rapid increase of traffic in urban areas ? Should the city be adapted to the car or, alternatively, should the automobile be banned from central areas and public transport systematicaly favoured ? The author shows the ineffectiveness or irrealism of these strategies and advocates the highter control and more efficient use of the car. He also proposes a system of spatial planning on a wide scale to optimise trip problems in relation to transport modes which complement rather than complete with each other.

The analysis of winter and summer peaks of air pollution for 1997 in the urban area of Saint- Etienne shows the strong relationship between local climatological conditions and levels of air pollution. The vertical temperature gradient in the lower layers of the atmosphere appears to be an essential feature. This temporal relationship needs to be completed by a spatial analysis of air quality distribution.

There are relatively few studies of urban climate in tropical areas and they rarely refer to aspects related to human comfort and hearth. Given the topographic/ geographic diversity of the few low- latitude cities where urban climate studies have been made, it is not possible at this stage of knowledge to draw any genera-lizations. But urban agglomerations cause measurable changes in the atmosphere immediately adjacent to them. Temperatures are increased, horizontal winds often slowed and humidity fields altered. All these characteristics may affect human comfort yet through the design of certain elements of the urban structure it is possible to modify in a predetermined way various features of the urban climate so as to produce a more pleasant environment.

Over a long period Lyon has been associated with fog and hazy skies. While this reputation is not entirely false, in reality such a view appears to lack objectivity. Lyon is not the Yog capital1. Since the beginning of the century, there has been increasingly less fog along the banks of the Saône and the Rhône. This situation is explained essentialy by the rise in temperatures in the city centre, creating an urban heat island, increased atmospheric movement and better control of industrial discharges into the atmosphere.

The recent trend of French temperature seems, at first, confirm the scheme of the "global warming". But increasing of temperatures, which is not observed in every station, can be attributed (the true contributions being unknown) to topography and to the urban greenhouse effect, namely to the nocturnal temperatures. But the influence of the town is far to explain the whole : the aerological dynamics commands the warm southern airstreams and the strengthened and long lasting anticyclonic character which increases the effect of the aerological urban stratification on temperatures.

Analysis of urban climate has became a key of climatology. With the diversification of measures, it is best and best understood and explained. However, if its control by the "natural" mechanisms seems to be clear, its influence on the global climate is still questionable. The extension of urbanized area increases the hazard for the man-inhabitant.