Climate of the city
The average annual temperature is 9.4 °C. The average annual precipitation is 1136 mm, with almost half volume falling between May August in the form of mostly convective precipitation.. In the city of Zurich, on average 88 frost days (Tmin < 0°C) and 26 ice days (Tmax < 0°C) can be expected. There are on average 30 summer days (Tmax > 25°C) and 3 heat days (Tmax > 30°C) per year. However, these measurements are typically not representative of the entire city area, since the official measurement station for Zurich is located on the slope of the Zurichberg, about 150 m above the city centre. The recent "Masterplan Stadtklima" of the City of Zurich documents 10 - 20 heat days per year, and this number can be expected to further increase in the future. A substantial variation of temperatures across years, the Zurich city area and its surroundings underlines the importance of small-scale air temperature measurements within the city. The highest air temperature ever measured in the city of Zurich was 37,7°C, recorded in July 1947. As heat waves and heat days will occur more frequently in the future due to climate change, climatic adaptation measures will have to be given greater consideration in urban planning.␠
Topography of the city and land surfaces
The urban climate in Zurich is influenced by various factors. Cold air flows into the city from the slopes of the surrounding hills at nighttime and substantially cools down suburban areas at the foot of the hills. For example, cold air masses coming from the slope of the Uetliberg flow into the direction of the Zurich city centre. Cold air inflow is especially effective in green areas and parks. These areas are numerous and distributed over the entire city (for example Rieterpark, Platzspitz, Seepromenade, and␠␠Bäckeranlage). Small parks can be found even in the densely built-up centre. Conversely, large areas north of the city centre are covered by railway tracks. These sealed surfaces lead to additional heating and form local heat islands. The city of Zurich is surrounded by a large area of woodland in the west (Uetiberg), north (Käferberg and Hönggerberg) and east (Zurichberg and Adlisberg). Those areas are higher than the city centre and typically 2-5 degrees cooler. The lake of Zurich in the south of the city area also contributes to cooling of suburban areas in the summertime. However, the urban development of new areals, such as in the centre of Zurich, in the Limmat valley, and in the direction of Oerlikon, increases the city's susceptibility to heat waves.