and December the wettest months. Snowfall is usually confined to the period November to February, and snow seldom lies on the ground for more than a day. Less rain falls from April to August although no month receives less than . Fog is common during summer due to the cooling effect of the sea on mild southerly airflows. Because of the islands' latitude, on clear winter nights the northern lights can sometimes be seen in the sky, while in summer there is almost perpetual daylight, a state of affairs known locally as the "simmer dim". Annual bright sunshine averages 1110 hours, and overcast days are common. Prehistory Due to the practice, dating to at least the early Neolithic, of building in stone on virtually treeless islands, Shetland is extremely rich in physical remains of the prehistoric eras and there are over 5,000 archaeological sites all told. A midden site at West Voe on the south coast of Mainland, dated to 4320–4030 BC, has provided the first evidence of Mesolithic human activity in Shetland. The same site provides dates for early Neolithic activity and finds at Scord of Brouster in Walls have been dated to 3400 BC. "Shetland knives" are stone tools that date from this period made from felsite from Northmavine. Pottery shards found at the important site of Jarlshof also indicate that there was Neolithic activity there although the main settlement dates from the Bronze Age. This includes a smithy, a cluster of wheelhouses and a later broch. The site has provided evidence of habitation during various phases right up until Viking times. Heel-shaped cairns, are a style of chambered cairn unique to Shetland, with a particularly large example in Vementry. Numerous brochs were erected during the Iron Age. In addition to Mousa there are significant ruins at Clickimin, Culswick, Old Scatness and West Burrafirth, although their origin and purpose is a matter of some controversy. The later Iron Age inhabitants of the Northern Isles were probably Pictish, although the historical record is sparse. Hunter (2000) states in relation to King Bridei I of the Picts in the sixth century AD: "As for Shetland, Orkney, Skye and the Western Isles, their inhabitants, most of whom appear to have been Pictish in culture and speech at this time, are likely to have regarded Bridei as a fairly distant presence". In 2011, the collective site, "The Crucible of Iron Age Shetland",