most prominent valleys of this network are the Aarhus Valley in the south, stretching inland east–west with the Aarhus River, Brabrand Lake, Årslev Lake and Tåstrup Lake, and the Egå Valley to the north, with the stream of Egåen, Egå Engsø, the bog of Geding-Kasted Mose and Geding Lake. Most parts of the two valleys have been drained and subsequently farmed, but in the early 2000s some of the drainage was removed and parts of the wetlands were restored for environmental reasons. The valley system also includes the stream of Lyngbygård Å in the west and valleys to the south of the city, following erosion channels from the pre-quaternary. By contrast, the Aarhus River Valley and the Giber River Valley are late glacial meltwater valleys. The coastal cliffs along the Bay of Aarhus consist of shallow tertiary clay from the Eocene and Oligocene (57 to 24 million years ago). Climate Aarhus has a temperate oceanic climate (Köppen: Cfb) and the weather is constantly influenced by major weather systems from all four ordinal directions, resulting in unstable conditions throughout the year. Temperature varies a great deal across the seasons with a mild spring in April and May, warmer summer months from June to August, frequently rainy and windy autumn months in October and September and cooler winter months, often with frost and occasional snow, from December to March. The city centre experiences the same climatic effects as other larger cities with higher wind speeds, more fog, less precipitation and higher temperatures than the surrounding, open land. Western winds from the Atlantic and North Sea are dominant resulting in more precipitation in western Denmark. In addition, Jutland rises sufficiently in the centre to lift air to higher, colder altitudes contributing to increased precipitation in eastern Jutland. Combined, these factors make east and south Jutland comparatively wetter than other parts of the country. Average temperature over the year is with February being the coldest month () and August the warmest (). Temperatures in the sea can reach in June to August, but it is not uncommon for beaches to register locally. The geography in the area affects the local climate of the city with the Aarhus Bay imposing a temperate effect on the low-lying valley floor where central Aarhus is located. Brabrand Lake to the west further contributes to this effect and as a result, the valley has a comparably mild, temperate climate.