Urban Climate

The increasing population of the world is more and more concentrating in urban areas and megacities. Urban and industrialised areas are characterised by a strong change of the surfaces properties. Natural surfaces (meadows, forests, fields) are turned into stone or concrete surfaces. This affects the heat and water balance, water runoff, causes differences of temperature, precipitation and cloud cover. High buildings with sharp corners make the wind flow in a different way compared to rural areas.

Urban heat islands

In a city, the magnitude of emissions, immissions and additional anthropogenic heat supply is different from unsettled regions. Think about all the households, heatings, lights, industry and vehicles concentrating there. Therefore, the urban climate has to be discussed as a special climate type and we do this in a separate topic. Big cities release on a local scale much energy compared to the land around. Therefore they are called urban heat islands. However, in contrast to former assumptions the majority of scientists do not believe, that big cities cause a strong perturbation of the world's climate.

The contribution of the urban heat island effect to the last century's global temperature increase of 0.4 to 0.8 °C, is estimated to be not more than 0.05°C. In particular minimum temperatures are higher than in rural areas, leading to a smaller temperature difference between minimum and maximum daily temperature. Although the climate in the cities is not a big factor in the world's heat balance, it is very important to us, because most of the people live or work in towns and spend many hours of their day in the climate of the cities. Think also about the conditions and indoor pollution in your room!

The images above explain the special character of the city climate. Due to heatings, production processes, air conditioners and other sources heat is generated in the city itself. Additionally, major parts of a town's surface absorb more sunlight than an average landscape. The albedo (reflection of sunlight) is low as figure 4 shows. Cities also tend to store this heat, because natural cooling processes do not work. Vegetation is cooled by evaporation of water, which consumes energy. In the cities however, the ground is sealed. Water disappears quickly in the canalisation and the cooling effect is low.

Air pollution

In the urban areas diverse organic (benzene, solvents, polyaromatic hydrocarbons, ...) and inorganic compounds are emitted. While some of the organic compounds are cancerogenic and a danger for human health, they are on the other hand not a global burden for nature. Organic emissions are dominated by vegetation. The main problems of urban air pollution with a bigger climate impact are inorganic emissions: 
a) high emissions of sulphur dioxide
b) high emissions of nitrogen oxides
c) local concentrations of certain aerosol classes (globally desert dust causes most of the aerosols)
d) release of certain chemicals which are disturbing natural cycles as CFCs did.

Acidification and acid rain

Sulphur dioxide emissions came from industry and first of all power plants (burning of coal for power generation). Nowadays this issue does not make the headlines anymore in Europe. Sulphur dioxide emissions drastically decreased due to economic crisis and lower production in post-communist countries, and due to modern filter techniques. But the problems are rising in south east Asia. And long-term damages in the forests, caused by the acidification of the soils, are not yet repaired.

Acidification of the air and acid rain are the consequences of the atmsopheric sulphur oxidation process leading to sulphuric acid. Additionally from nitrogen oxides nitric acid is formed. Damages of acid rain can be seen in the forests but also on buildings and other matter in the city (images below).

City design

In former times towns grew without caring much about the environment. Nowadays much more is known about the different heat balance, about the water runoff and the disadvantages of sealed areas, also about the windflow in cities. If new quarters in a modern city are planned, such factors are taken into account. Higher buildings are sometimes tested as model in a wind channel and modern houses can be constructed with energy saving claddings. Such techniques are still expensive but may determine the cities of the future.

Visit the topic CLIMATE IN CITIES in our encyclopaedia in order to learn more about air pollution, the special conditions of the urban climate, its energy balance and developments and dangers of acid rain.

About this page:

author: Dr. Elmar Uherek - MPI for chemistry, Mainz
last published: 2004-06-11