lies at an elevation of above sea level. The lowest point, in the north, and on the border with the Netherlands, is at . Climate As the westernmost city in Germany (and close to the Low Countries), Aachen and the surrounding area belongs to a temperate climate zone (Cfb), with humid weather, mild winters, and warm summers. Because of its location north of the Eifel and the High Fens and its subsequent prevailing westerly weather patterns, rainfall in Aachen (on average 805 mm/year) is comparatively higher than, for example, in Bonn (with 669 mm/year). Another factor in the local weather forces of Aachen is the occurrence of Foehn winds on the southerly air currents, which results from the city's geographic location on the northern edge of the Eifel. Because the city is surrounded by hills, it suffers from inversion-related smog. Some areas of the city have become urban heat islands as a result of poor heat exchange, both because of the area's natural geography and from human activity. The city's numerous cold air corridors, which are slated to remain as free as possible from new construction, therefore play an important role in the urban climate of Aachen. The January average is , while the July average is . Precipitation is almost evenly spread throughout the year. Geology The geology of Aachen is very structurally heterogeneous. The oldest occurring rocks in the area surrounding the city originate from the Devonian period and include carboniferous sandstone, greywacke, claystone and limestone. These formations are part of the Rhenish Massif, north of the High Fens. In the Pennsylvanian subperiod of the Carboniferous geological period, these rock layers were narrowed and folded as a result of the Variscan orogeny. After this event, and over the course of the following 200 million years, this area has been continuously flattened. During the Cretaceous period, the ocean penetrated the continent from the direction of the North Sea up to the mountainous area near Aachen, bringing with it clay, sand, and chalk deposits. While the clay (which was the basis for a major pottery industry in nearby Raeren) is mostly found in the lower areas of Aachen, the hills of the Aachen Forest and the Lousberg were formed from upper Cretaceous sand and chalk deposits. More recent sedimentation is mainly located in the north and east of Aachen and was formed through tertiary and quaternary river and wind activities.