Cities are more densely built up and characterized by a larger fraction of impervious surfaces compared to the more rural surroundings. Moreover, buildings, vehicles and industrial facilities all emit heat into the urban environment. These factors lead to a changed local climate in terms of air temperature, humidity, radiation and wind, as well as air quality and noise pollution.
The urban heat island
The urban heat island effect is one characteristic of a city’s microclimate that can form in any season. The urban-rural temperature difference is especially visible during summer and at nighttime. In Swiss cities, the temperature at night can be up to 5-7°C higher than in the surrounding rural areas. During the day, the temperature difference between urban and rural site is usually smaller, and air temperatures in cities are often only slightly elevated. In terms of temperature-based heat indices, we thus see significantly more tropical nights (days with a minimum temperature of at least 20 °C) in cities than in their rural surroundings, while the number of hot days (days with a maximum temperature of at least 30 °C) is only slightly higher. In street canyons or urban areas without vegetation, however, daytime temperatures can be considerably higher than in the rural surroundings.