Heat in the city: urban heat island effect
The quality of life in cities is particularly affected by heat. Even today, measurements by the German Weather Service show that urban heat islands are caused by the difference in air temperature between a densely populated city and the surrounding area alone, averaging up to 10 Kelvin in large urban centres and up to 4 Kelvin in smaller cities. The often-asked question of weather or climate can be clearly illustrated using the example of the urban heat island effect.
What is the urban heat island effect?
The urban heat island effect (UHI) describes the phenomenon that urban areas have higher temperatures than the surrounding rural areas. This effect is caused by the specific development and structure of cities and by human activities in these areas.
The causes of the urban heat island effect are varied and complex. In urban areas, materials such as asphalt and concrete store the sun's heat during the day and slowly release it again at night, which leads to a continuous rise in temperature. The lack of vegetation in cities also contributes to the heat island effect, as plants can regulate the ambient temperature through evaporation and transpiration. High buildings also hinder the natural circulation of air and make it difficult to dissipate heat. Other sources of heat are vehicles, industrial plants and air conditioning systems, which also release heat into the environment.
The urban heat island effect has numerous impacts on the urban climate and the daily lives of residents. On the one hand, there is a risk of heat-related health problems such as heat stroke and dehydration, especially for vulnerable population groups such as the elderly and children. On the other hand, energy consumption in cities increases as air conditioning systems and fans are used more frequently to cool indoor spaces. This leads to a rebound effect through higher energy costs and greenhouse gas emissions.
How can the urban heat island effect be reduced?
To reduce the urban heat island effect, cities can take several measures. One important strategy is to increase urban green spaces. Parks, gardens and green roofs contribute to cooling the environment through evaporation and shading. Vegetation also helps to improve air quality by absorbing carbon dioxide and producing oxygen. The use of reflective materials on roofs and road surfaces can also help to reduce heat absorption. Materials with a high reflectivity reflect more sunlight and absorb less heat, thereby lowering surface temperatures. Thoughtful urban planning that allows for good air circulation can also help to reduce the heat island effect. Wide streets and open spaces promote air circulation and help to dissipate stored heat. In addition, measures to reduce sources of waste heat, such as the promotion of low-emission vehicles and the introduction of energy-efficient technologies, can reduce urban temperatures.
Is there any interesting research on the subject?
The recommendations for action of the National Urban Development Policy programme, which makes the challenges for the urban climate and the necessary adaptation strategies a current focus topic, are interesting in this context. The C³ City Climate Canopies project, funded by the ZukunftBau research initiative, also deals with the worsening climate situation and the question of whether and how the urban climate can be improved and influenced by selective structural measures.
The project introduces the idea of developing City Climate Canopies (C³), which create added urban, social and climatic value—on a small scale, but with the greatest possible impact. Specifically, these are lightweight, climate-adaptive and flexible membrane canopies that counteract heat islands through their reflective effect, provide shade and maintain and create urban meeting and utilisation spaces.