The urban climatic situation is critical in many places in Frankfurt, especially in the city centre

The Climate Plan Atlas Frankfurt from 2016 shows the need for action - the city and citizens have also already responded successfully with numerous initiatives. In particular, increasing the proportion of unsealed surfaces in the city centre represents an important lever for improving the city's climate - a potential that has not yet been fully exploited, as it has been difficult to secure the water supply for small-scale green areas in particular. With the water supply from the Frankfurt bridges, however, unsealing on a larger scale and planting with shade trees in the city will become possible.

Chapter content: The chapter provides an overview of climatic problem areas in the city of Frankfurt and the corresponding contribution of Frankfurt's bridges to solving them

It describes the factors that cause the problem zones, where they are located in the urban area and what measures can be taken to influence them.

One of the most important factors influencing the urban climate is the proportion of vegetation area in the total area of the city of Frankfurt.

The Frankfurt bridges can make an important contribution to the expansion of vegetation areas and the planting and supply of shade trees in the urban area by providing a network-like supply of water in the city.

Regarding the urban climate, however, attention must be paid to the selection and arrangement of trees in the urban area.

Shirin Kriklava - Stiftung Altes Neuland Frankfurt GNU

Frankfurt is not only dense in the city centre, but also highly built-up due to its high-rise buildings

Building density, the proportion of parking space and numerous other factors have an impact on the urban climate in city locations.

In Frankfurt, there are hardly any vegetation areas between the Alter Oper and the Konstablerwache - at the same time, precisely this area is particularly densely built up with high-rise buildings.

Even though the streets from Fressgasse to the beginning of the Zeil are designated as pedestrian zones, these areas to the north and south are nevertheless surrounded by heavy traffic.

The consequences for the urban climate can be seen directly in the Frankfurt Climate Atlas

The lack of unsealed and green areas - together with the dense development and the higher traffic volume - leads to increased heat stress in the inner city.

Due to the high degree of sealing and the low proportion of vegetation, there is hardly any evaporation of water, which means that there is practically no natural cooling of the air via plants and the soil. At the same time, the buildings and the traffic on the streets emit a lot of heat. In addition, the heated air can only be exchanged by the wind to a limited extent due to the dense development.

As a result, the heat remains in the city even at night - a heat island forms. Particularly during hot summer periods, this leads to thermal stress on the human body. Frankfurt's densely built-up inner city therefore needs clever ideas that will help the city and its people achieve a healthier and more pleasant climate.

The amount of vegetation in a city is an important factor for the urban climate

When comparing the proportion of vegetation with Frankfurt's climate function map, the spatial correspondence between areas with low green space availability and those with higher levels of overheating is striking. Although there are many other influences on which the characteristics of the urban climate depend, the degree of greenery in a district is one of the most decisive factors, as it has a direct effect on the air temperature.

While vegetated areas in the outer districts of Frankfurt make up more than half of the urban area in some cases, there is hardly any urban greenery in the area of the city centre. This can be quite problematic for the local climate, especially in the summer months.

Hessisches Landesamt für Bodenmanagement und Geoinformation; Stadtvermessungsamt Frankfurt am Main

The total proportion of vegetation area in Frankfurt is 40% - the urban green is just not evenly distributed over the city area

Frankfurt's urban area, with its roughly 250 million m2 of surface area, consists of almost 40 % so-called "vegetation area" and only another 40 % or so of settlement area (the rest is mainly roads and other sealed surfaces).

Frankfurt's Parks Department maintains 200,000 trees and cares for around 17 million square metres of urban green and open spaces - and all this with just 1.3% of Frankfurt's city budget (which, after all, amounts to almost €4 billion a year). A remarkable achievement, especially as around one fifth of this meagre budget goes on waste disposal.

So actually Frankfurt is already quite extensively greened and maintained. But the feeling of a pleasant urban climate is not evenly distributed in the city.

The city of Frankfurt is already working intensively on "blue-green" solutions

The city of Frankfurt is intensively engaged in projects and actions that promote the city's natural water balance (blue) and thus enable significantly more planting in the urban area (green):

For example, there is an action process called "Frankfurt Green City", which was launched in 2014 and in which many activities are organised, or another initiative of the city, called "Frankfurt frischt auf", where the already existing funding programme for facade greening, roof greening and front and backyard greening is being expanded and further developed.

Citizens as well as public authorities are committed and successfully active in order to further improve the city climate

The Frankfurt Office of Real Estate oversees rainwater collection projects and building greening at municipal properties, and even Frankfurt's transport associations have started greening their bus stops. Advertising pillars are being given the "Frankfurt hood", a disc top with sedum planting, which serve primarily as stepping stone biotopes for insects.

That being said, there are currently (2021) about a dozen citizen's initiatives that are either protecting existing green space from destruction or successfully launching the planting of flower beds, "urban gardening", parking lot greening and more with energy and momentum.

The unsealing and greening plan, which can be implemented with the help of Frankfurt Bridges, fits perfectly into this trend of the city and its citizen activities.

Experts on the Topic

Stadt Frankfurt, Umweltamt — Frankfurt Green City — Umweltforum Rhein-Main — Regionalverband Frankfurt Rhein Main — Klima Kommunen Hessen — Hessisches Landesamt für Naturschutz, Umwelt und Geologie — Deutscher Wetterdienst — Frankfurt University of Applied Sciences - Fachbereich für Architektur, Bauingenieurwesen, Geomatik

The bridges criss-cross parts of the city like a net and can thus send water to widely distributed green areas

Because even if there are rainy summers like 2021: Once plants have been planted, their survival must be ensured in the long term - otherwise the effort will be great after only one dry period, when most of the greenery dies, has to be disposed of and then replanted.

The water system of the Frankfurt bridges enables unsealing and greening as well as the planting of new trees on a large scale

Stiftung Altes Neuland Frankfurt GNU / Google Satellite

But not only the new greenery can be watered with it: Existing urban greenery can also be vitalised and protected from withering away.

More than 40,000 square metres of land can be unsealed and greened, and around 1,000 additional trees can be planted.

This is because even undeveloped but sealed open spaces, such as Frankfurt's Hauptwache, urgently need additional shading in the hot summer months. Trees that thin out in winter and let the sun through are best suited for this purpose.

Despite all the zeal to plant as many trees as possible in the city, it is important to know, especially when it comes to urban greenery: Not all trees are created equal

Not all trees are the same, and not all greenery is the same: When selecting urban greenery, a number of things must be taken into account with regard to the local climate - otherwise the effort is in vain or even counterproductive. Although practically all trees provide welcome cooling on hot days through shading and transpiration, if they are arranged too densely, especially large-crowned street trees whose canopy closes over the middle of the street, there is an increased risk of air pollutants accumulating in such avenues.

Another problem is posed by reactive trace gases such as isoprene, which are emitted by some tree species. This can lead to the formation of ozone in combination with car exhaust gases during high solar radiation. Elevated ozone concentrations in the human habitat are considered hazardous to health because they can cause respiratory diseases. Therefore, the selection of trees should always take into account health-relevant contexts.

If trees are planted in the course of unsealing, very specific parameters must be taken into account

In the city, the most important aim of planting trees is to cool the climate by casting shade - because their contribution to cooling through evaporation takes place in the canopy, and is therefore usually much higher than the walking height of pedestrians.

In order to maximise shade, trees should be chosen that can form as large a canopy as possible - if the site allows. The size of the tree crown is more important for the cooling effect of the shade than the density of the tree crown. The selection of the tree species is an important factor for the cooling effect from this aspect alone.

Trees with very dense canopies should only be planted free-standing or at large distances from each other anyway. Often, however, the exact opposite happens: In the city, trees are often planted as tree avenues along busy streets in urban canyons. There, they often represent an obstacle to air circulation due to the "tunnel effect", which is exacerbated by dense tree crowns.

Besides the choice of the right tree species, the respective location is therefore also of great importance. Isolated positions or arrangements at distances of 20 m or more are good for the tree and also for its surroundings.

Densely planted trees generally have poorer filtering properties than loosely planted trees, through which the wind can blow more easily.

Experts on the Topic

Universität Würzburg - Institut für Geographie und Geologie - Bereich Klimatologie — Helmholtz-Zentrum München - Institut für Biochemische Pflanzenpathologie - Abteilung Experimentelle Umweltsimulation — Karlsruher Institut für Technologie - Institut für Meteorologie und Klimaforschung - Atmosphärische Umweltforschung (IMK-IFU) - AG Pflanzen-Ökophysiologie

Conclusion: Frankfurt's bridges can help to positively influence the urban climate through unsealing

Frankfurt's inner city is identified in the Climate Plan Atlas as an urban climate problem zone. This is due to the high volume of traffic, high building density and, above all, the lower proportion of green spaces and higher degree of sealing, which results in increased heat stress in summer.

The Frankfurt bridges provide a remedy for this urban climate-relevant factor with the help of their irrigation network: Inner city areas can be unsealed and greened.

Trees in the city centre, in particular, can have a cooling effect through the shade they cast, but they must not be planted in too dense an arrangement so that air exchange is guaranteed.

Verbesserung des Stadtklimas

Auswirkungen der Brücken

Measures next to the bridges

The CO2 balance sheet of the bridges

Urban Climate - Global Climate: Old Uncharted Territory Worldwide

The goal: to arm the city against drought and floods

The green metropolis of the future

Vitalization of the urban green