Urban climate
The urban climate is defined by the World Meteorological Organization (WMO) as a local climate that has changed compared to the surrounding area . Very dense buildings and a lack of vegetation as well as the emission of air pollutants and waste heat can lead to a higher average temperature and pollutant concentration in cities as well as to lower humidity and wind speeds than prevail in the rural surroundings. Urban climates can cause damage to health (increased mortality and disease) and changes in flora and fauna .
Climatic factors
The urban climate is shaped by various climate factors that can be divided into two categories:
- natural factors
- anthropogenic factors
The natural factors include the geographical location, the relief , the altitude and the proportion of the still existing near-natural surfaces within the urban area.
The anthropogenic factors essentially include the type and density of buildings, the heat storage capacity of the building materials and the degree of sealing of the soil. They influence the radiation and heat balance in cities and industrial conurbations. From an air hygiene point of view, there is also the type and number of emitters (industry, households, motor vehicles) in the urban area that influence the urban climate near and far from the ground through exhaust gases, dust and waste heat.
Through their interaction, these natural and anthropogenic factors determine the respective characteristics of the urban climate. This means that there can be no uniform urban climate.
Urban heat island
.png)
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 significantly warmer compared to the surrounding areas.
The geometry of the development increases the surface area on which solar radiation is absorbed. This leads to the building structure heating up , particularly in periods of good summer weather with little exchange . In contrast to undeveloped areas, built-up areas act like a heat store . The soil in undeveloped areas does not heat up as much due to the shading by the vegetation and its evaporation capacity . During nights of radiation, natural soil with little coverage can give off its thermal energy again through thermal radiation . The air cools down faster at night over undeveloped meadow areas .
In contrast to this, built-up areas heat up much more strongly due to solar radiation: the angle of incidence of the sun is more direct due to the vertical facades ( horizon elevation ), and there is usually no vegetation that provides shade and can cool the air with evaporation. The stone used heats up faster. But it is also a better heat store and gives off its heat more slowly at night. The surrounding air can no longer cool down. The nocturnal radiation of heat is also partially prevented by the narrowing of the horizon in street canyons. Multiple reflections occur on the house walls.
The air circulation and the inflow or infiltration of cooler air from the surrounding area or from larger green areas is also restricted by the development. If emitters are built in so-called cold air corridors (traffic and industrial facilities), the cold air that now flows in more slowly is enriched with pollutants . The increased particle concentration in the urban air in turn dampens the radiation of heat.
Another factor that leads to the warming of inner cities is the sealing of large areas . Rainwater therefore runs off quickly and is not available for evaporation . Since evaporation consumes heat, this effect also leads to less cooling (or, conversely, heating) of cities.
With the infrared recording process, surface temperatures can be determined and displayed in color.
Cooling factors
The main cooling effect is the air flow from rivers and streams . Cold air outflows, especially from a cold air generation area, can also be significant if the slope is at least 1 to 2 degrees (approx. 1 to 3 m gradient per 100 m). On the other hand, underground cavities such as sewer networks , underground trains or underpasses ensure further cooling. These do not give off as much heat as would be the case with the mass of the earth, and they also cool down faster in the wind . This can be seen well with bridges , on which black ice forms more quickly in the cold . Curiously, a dense and high surface area is also an advantage. While stores and gives them the heat longer, but is it also a shadow spender. In addition to the shade, evaporation through trees absorbs a lot of energy and thus has a cooling effect on their surroundings.
Urban rainfall
Due to the characteristics of the heat island, the relative humidity in cities is lower than in the surrounding area. Nevertheless, it can be observed that heavy rain and thunderstorms often last twice as long and give off more precipitation. The reason for this is a 3–5 times higher concentration of condensation nuclei. These result from emissions from industry and car traffic. The influence of traffic is so strong that two maxima can be observed in the weekly course of precipitation. With the increased traffic on Tuesday to Wednesday after the weekend, there is increased rainfall and then again on Friday to Saturday.
Urban wind field
The roughness of the surface is greater in built-up areas than in undeveloped areas. As a result, the wind speed in cities is on average lower than in open terrain. Depending on the wind direction, leeward eddies develop on buildings, which can cause strong gusts in small areas. In addition, nozzle effects can occur at development gaps, which can also greatly increase the wind speed for a limited space and time. High-rise buildings that protrude well above the mean roof level of a city can, under certain circumstances, deflect the (stronger) wind field from higher air layers and cause violent gusts and turbulence at the foot of the building.
Air hygiene
The chemical composition of the air in cities is changed by a large number of pollutant emissions ( house fire , road traffic, industry). Since air exchange in cities is also restricted due to the lower mean wind speed, air pollutants can accumulate. This can lead to the tropospheric formation of the irritant gas ozone , especially in summer .
Urban radiation balance
The radiation balance always depends on the zenith of the sun and the cloudiness of the atmosphere . This also explains why the radiation balance in the city is lower than that in the surrounding area. Due to the haze over the city, the reduction in global radiation compared to the undeveloped surrounding area can be up to 20%.
Depending on the season and the weather conditions, UV radiation in the city is up to 35% lower than in the surrounding area. This is due to the absorption of UV radiation by the ozone close to the ground and the reflection from dust particles . The UV radiation in the city can sometimes be higher than in the surrounding area, as ozone is only formed from various precursor gases after several reduction steps.
Atmospheric boundary layer of the city
In the city you will mainly find dry surfaces, air pollutants are emitted and waste heat is released. For these reasons and because of the greater roughness in the city, it is necessary to further subdivide the boundary layer close to the ground into:
- City obstacle layer. This extends from the ground to the middle roof level.
- Transition layer
- Actual city boundary layer.
Individual evidence
- ↑ Cold air outflows - Climate Atlas NRW ( Memento of the original from July 29, 2017 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ Strasburger: Textbook of Botany. Spektrum, Heidelberg 2008, p. 423. ISBN 3-8274-1455-5
literature
- Karsten Brandt : Microclimate - Meteorology near the ground . ISBN 3-9807827-5-1
- Claus Rink, Heyo Eckel, Ulrich Hüttemann: Urban development changes the climate . In: Deutsches Ärzteblatt , 96 (15), p. A974, Cologne 1999, ISSN 0012-1207
- City of Stuttgart , Department of Urban Development and Environment, Office for Environmental Protection, Department of Urban Climatology, in conjunction with the Department of Communication (Ed.), Series of publications by the Office for Environmental Protection - Issue 3/2010 : Climate Change - Challenge for Urban Climatology . ISSN 1438-3918
- W. Kuttler: Urban Climate, Part 1: Basics and Causes. In: Environmental Sciences and Pollutant Research - Journal for Environmental Chemistry and Ecotoxicology 16 (3), 2004, pp. 187–199, ISSN 0934-3504 .
- W. Kuttler: Urban Climate, Part 2: Phenomena and Effects. In: Environmental sciences and pollutant research - Journal for environmental chemistry and ecotoxicology 16 (4), 2004, pp. 263-274, ISSN 0934-3504 .
- A. Kratzer: Das Stadtklima , 2nd, revised edition. Braunschweig 1956
- Schmidt, Andreas (1994) On the trail of the urban climate. Earth sciences; 12, 1; 18-20; doi : 10.2312 / geosciences . 1994.12.18 .
Web links
- envi-met.com - A microclimatic urban climate model
- karstenbrandt.de - Under Delta T a lot of information about the urban climate and microclimate
- stadtklima-stuttgart.de - Information on climate, air and noise - not only for Stuttgart
- Bautschweb.de - Weather station at the Ohmoor high school, with weather data in Excel format
- stadtentwicklung.berlin.de - Complete range of maps on the urban climate of the federal capital Berlin in the Berlin Environmental Atlas
- stadtklima.de - A lot of information on the subject of urban climate - also a lot of local data from cities all over the world
- stadt-und-klimawandel.de - Urban strategies for climate change Exchange of practice and science