The average annual temperature is 2.9 °C (based on the period 1960-1990), and there are an average of 171 days of frost and 176 days with snow cover. Only the months of July and August are generally entirely frost-free. Annual precipitation is 1,814 litres per square metre, the average annual humidity is 88 per cent and there are 306 days of mist or fog. The summit of the Brocken is one of the windiest places in Europe. The highest measured wind speed was 263 km/h (in 1984). The Brocken has a natural timber line.