1.3. Air humidity, precipitation, floods

The city is generally a little humid environment because it contains too many artificial surfaces. It produces very little water vapor. Thus the city of Strasbourg is drier than the countryside for a large part of the year, from March to October (Figure 9). It can be considered as an urban draught island. This water vapor deficit is caused by the intense evapotranspiration that occurs in rural areas. On the other hand, during the cold season (N, D, J, F), the air in the city contains a little more water vapor. Town can be considered as an urban moisture island. In fact, in winter, the air in the city has a hygrometric capacity greater than that of the air in the countryside, especially at night, due to the urban heat island.

Artificial surfaces are mostly completely waterproof (roofs, roads), so they prevent water from entering the soil. We know that 90% of the water that falls on an artificial surface like concrete, runs off. When we compare the urban and rural hydrological system, on average, we see that 43% of the precipitated water flows in urban areas and reaches an artificial collector, while this is reduced to 10% in a rural area where most water is intercepted, evapotranspired or infiltrates. This is not without problems, because in case of a heavy rainfall, all the water which falls on these impermeable surfaces must be properly evacuated. If it's not the case, it gives rise to problems in the drainage networks. Moreover, as water is brought rapidly into rivers, the outflow of these increases suddenly: urbanized watersheds react more violently than natural watersheds and floods are more intense (Figure 11).