 Winds are usually weak, although sudden bursts of foehn or thunderstorms can sweep the air clean.The almost enclosed nature of the Padan basin, indeed, added to the crushing weight of road traffic, makes it prone to a high level of pollution in winter, when cold air clings to the soil.The natural potential vegetation of the Po basin is a mixed broadleaved forest of pedunculate oak, poplars, European hornbeam, alders, elderberry, elms, willows, maples, ash, and other central-European trees.