Urban heat island

The city climate is defined by the World Meteorological Organization (WMO ) as compared to the surrounding altered local climate. Very dense development and a lack of vegetation, and the emission of air pollutants and waste heat in cities can lead to a higher average temperature and concentration of pollutants, as well as at lower humidities and wind speeds than prevail in the surrounding rural areas. Urban climate can cause or aggravate health problems (increased mortality and morbidity ) and changes in the flora and fauna.

Climatic factors

The city climate is influenced by various environmental factors that can be divided into two categories:

• Natural factors
• Anthropogenic factors

Among the natural factors, the geographical location, the relief, the altitude and the percentage of remaining natural surfaces are counted within the city limits.

Among the anthropogenic factors mainly include the type and density of development, the heat storage capacity of building materials and the degree of sealing of the soil. Through them, the radiation and heat balance is affected in cities and industrial areas. Addition of air- hygienic point of view, the type and number of issuers (industry, household, motor vehicles) in the urban area, close to the ground floor and away from noxious gases, dust and heat affect the urban climate.

Through their interaction, these natural and anthropogenic factors determine the specific expression of the city climate. This means that no single urban climate can exist.

Urban heat island

The heat island is a typical feature of the urban climate and is caused by the interaction of several different effects. Due to the strong warming during the day and the limited cooling at night, the cities are much warmer compared to the surrounding area.

The geometry of the building increases the surface area, is adsorbed on the solar radiation. This leads especially in low-exchange, summer periods of fine weather to heating the building. In contrast to built-up areas built-up areas act as a heat store. The bottom land surface heats up due to the shading by the vegetation and the evaporation performance less sensitive to. In radiation periods can low -covered natural soil release its heat energy by radiation again. About undeveloped meadows, the air cools at night therefore faster.

In contrast, heat built-up areas more strongly to by sunlight: The angle of the sun is to cool the air through the vertical facades direct (horizon elevation ), also often lack the vegetation that provides shade and evaporation performance. The built stone heats up faster. But it is also a better heat storage, and its heat slowly at night. The air in the environment can no longer cool itself so. The nocturnal radiation of heat is prevented by the narrowing of the horizon in street canyons partially. Occur multiple reflections on the walls of houses.

The air circulation and the flow or seepage of the cooler air from the surroundings or from larger green areas is also limited by the development. If issuers are built into so-called cold air lanes ( traffic and industrial plants ), which is now slowly flowing cold air is enriched with pollutants. Increased particle levels in the urban air turn again attenuates the radiation of heat.

Another factor that leads to the heating of the inner city, is the large-scale surface sealing. Therefore, rainwater drains quickly and is not available for evaporation available. Since the evaporation consumes heat, this effect also results in a lower cooling ( or conversely a heating ) of the cities.

With the infrared absorption method surface temperatures can be determined and displayed in color.

Cooling factors

Main effect of cooling on the one hand, the flow of air flows. On the other hand provide underground cavities such as channel networks, metro or underpasses for further cooling. These do not give as much heat as would be the case with Earth's mass, and cool wind in addition faster. That's good to watch for bridges, where ice forms more quickly in cold weather. Curiously, but is also a dense, high surface building an advantage. While stores and releases it from the heat longer, but it is also a shade. In addition to shade the evaporation rate decreases by trees much energy and thus has a cooling effect on their environment from.

Urban rainfall

Through the expression of the heat island, the relative humidity is lower in urban areas compared to the surrounding area. Nevertheless, it is observed that heavy rain and thunderstorms often twice as long to stop here and give more precipitation. The reason for this is a 3-5 times higher concentration of condensation nuclei. These result from the emissions of industrial and automobile traffic. The influence of traffic is so strong that it is possible to observe two maxima in the weekly precipitation transition. So it comes with the onset of intensified after the weekend traffic on Tuesday to Wednesday to increased precipitation and then again on Friday to Saturday.

Urban wind field

The roughness of the surface is greater than in undeveloped in built-up areas. Thus, the wind speed in urban areas is on average lower than in open terrain. On buildings arise depending on the wind direction Leewirbel, which can cause small-scale strong gusts. In addition can occur at building gaps jet effects, which can spatially and temporally limited greatly increase the wind speed also. Skyscrapers that rise far above the average roof level of a city, can distract the (stronger ) wind field from higher layers of the atmosphere and provoke violent gusts and turbulence at the foot of the building under certain circumstances.

Air Hygiene

Through a variety of pollutants ( house fire, road traffic, industry) is the chemical composition of the air in cities. As a result of the lower average wind speed of air exchange in cities is also limited to air pollutants can accumulate. This can result, especially in the summer to tropospheric ozone formation of the stimulus gas.

Urban radiation balance

The radiation balance always depends on the zenith angle of the sun and the turbidity of the atmosphere. Thus, one can also explain why the city 's radiation balance is less than that of the surrounding area. Through the haze over the city the reduction of global radiation over the undeveloped surroundings can be up to 20%.

Depending on the season and weather conditions, UV radiation in the city is up to 35% less than in the surrounding areas. This is due to the absorption of UV radiation by the ground-level ozone, as well as the reflection of dust particles. At times, the UV radiation in the city be higher than in the surrounding areas, as ozone is formed only after several reduction steps from different precursor gases.

Atmospheric boundary layer of the city

In the city you will find mainly dry surfaces, it will emit air pollutants and waste heat released. For these reasons and because of the greater roughness in the city, it is necessary to further subdivide the near-surface boundary layer and that in the:

• City obstacle layer. This ranges from basic to intermediate level roof.
• Transition layer
• Actual city boundary layer.