Between the Vosges Mountains and the Black Forest, winds coming from either direction are often deflected by these natural barriers (wind NNE-SSO). In Alsace, weather conditions are calm with warm Mediterranean air coming from "Trouée de Belfort", the average annual precipitation is low and the perceived summer temperatures can be inordinately high. Finally, behind the Vosges Mountains, Alsace wind's average is not strong and pollution does not disperse well and in winter foggy days are numerous around Strasbourg.
Consequence: Ozone pollution peaks in summer or particulate peaks in winter occurs on upper Rhine valley with some meteorological conditions (bad pollutants dispersion, temperature, sunny…).
Because of the density of environmental polluting activities cities and their agglomerations are especially affected by air pollution. In Strasbourg, particulate matter (PM10), nitrogen dioxide (NO2) and ozone (O3) cause problems. Main reason for the excessive load of these pollutants is the motorised traffic: The A4, A35 and A351 motorways enhance Strasbourg's economic status but are major pollution sources. Congested city streets also lead to breaches of pollution limits on arterial routes and in the city centre.
Industrial pollution sources in the north area do lightly affect the north of the city.
In the centre of town and along the main traffic axes the annual mean limit value for nitrogen dioxide (40 µg/m³) and particulate matter PM10 (40 µg/m³) is not exceeded on urban background station. On contrary, the daily limit value for particulate matter PM10 (50 µg/m³ in 24 hours max 35 days a year) and long term objective value for O3 (1 hour > 120 µg/m³ in 8 hours max 25 days a year) may exceed the limit.