Urban climate concept

The technical development of the urban climate concept by Geo-Net Umweltconsulting GmbH from Hanover was supported by an internal administrative project group under the leadership of the urban development department. The (interim) results of the urban climate concept were presented and discussed with other employees from the Jena city administration at various working meetings and in two workshops. The project was carried out in the period 2021-2024 with an upstream award procedure for the planning services in the second half of 2020.

The project was funded proportionately (40%) by the Free State of Thuringia as part of the "Klima Invest" funding program. The funding is aimed at achieving the objectives of the Thuringian Climate Act, in particular the reduction of greenhouse gas emissions and adaptation to the unavoidable consequences of climate change in Thuringia.

The project is divided into the following project modules.

1. urban climate modeling and climate analysis

Urban overheating continues to increase in Jena, particularly as a result of global climate change. Progressive inner-city densification due to high pressure of use on limited space and the loss of green areas can further intensify this development. In the interests of climate-resilient urban development, it is necessary to assess the areas in terms of their climate characteristics or climate function. This evaluation then forms the basis and argumentation aid for the assessment of structural developments, for the development of optimization proposals and for securing climatic compensation areas.

In a first step, a three-dimensional, high-resolution urban climate model was carried out for Jena using FITNAH-3D, which depicts the current land use and green structure as well as reflecting various land use and climate change scenarios. The approximately 195 km² study area was modeled in a horizontal grid of 10 m over 24 h (time step: 10 seconds). This formed the basis for the qualitative and quantitative assessment of the climate-ecological process of cold air and the overheating situation or the human bioclimatic stress situation for day and night.

The urban climate analysis was carried out for three scenarios. The two future scenarios refer to the year 2035.

In the next step, individual maps of key urban climate parameters were created from the three model calculations. These include the ground-level temperature and wind field, the cold air volume flow, the cold air production rate (night) and the perceived temperature (day), in each case for the current situation and the two future scenarios. This individual climatic information was then combined in six synthetic climate analysis maps (one for each scenario) for the day (14:00) and the night (04:00).

2. assessment and planning information maps

In this project module, the present climate analysis was evaluated. This evaluation is based on the VDI guideline 3785 sheet 1. With the help of evaluation categories, the identification of favorable and unfavorable factors of a location as well as the targeted derivation of the need for action and planning is made possible. The urban area is divided into the effective area (settlement/traffic areas) and the compensation area (green and open spaces) and assigned to a corresponding evaluation category (very favorable to very unfavorable bioclimatic situation or very high to very low bioclimatic significance) using an algorithm (the so-called z-transformation). The individual areas of the city are thus evaluated in comparison to each other, as no universally valid and legally standardized limit values can be used as a basis for evaluation. This assessment is carried out for the day and night situation in all three scenarios. As a result, six assessment maps are available for the urban area.

The assessment maps ultimately form the basis of the climatic planning information map. The planning information map is based on the results of the future scenario P2 with climate change signal and additional urban development and thus takes into account the effects of the predicted climate change in the sense of a precautionary strategy. As an abstracting map, it combines all the preceding climate analysis representations. It identifies and analyzes compensation and stress areas, provides recommendations for planning from an urban climatological perspective and thus serves as an important basis for the consideration of climatic concerns in preparatory and binding urban land-use planning. The aim is to maintain the quality of life within the meaning of Section 1 (5) BauGB and to maintain healthy living and working conditions within the meaning of Section 34 BauGB in the city while at the same time developing the location of a regional center in line with demand.

In addition to the changes resulting from the predicted climate change, the planning information map also focuses on the planned building developments and assesses their expected urban climate impact on the area itself and on the adjacent neighborhood. For 69 development areas (preliminary draft for the FNP update, as of 04.11.2022), an individual expert assessment of the effects on the climate as a protected resource was carried out as part of the urban climate concept; the results for 55 of these areas were summarized in a fact sheet . For all development areas, the experts confirmed that the construction of the areas is possible without any significant impact on the local climate, provided that the construction method is adapted to the climate. In accordance with the site assessment, planning recommendations are made for the structural implementation, which can support a climate-adapted construction method on the respective site and are to be detailed at the subsequent planning levels.

3. instrumental implementation

With the aim of anchoring the knowledge gained and the recommendations for action based on it from the planning information map in planning law and transferring them to implementation, the urban climate concept dealt with instrumental implementation. In addition to the formal planning instruments of preparatory and binding urban land-use planning, the issuance of local building regulations or the provision of incentives and support programs for citizens were also taken into consideration and corresponding proposals for Jena were presented.

The "Urban Climate Concept for the City of Jena" was confirmed by the City Council on 26.02.2025 as a basis for action for sustainable climate-adapted urban development for the city administration. In future, relevant decisions for planning and projects within the Jena city area will be made with regard to the results of the urban climate concept. The results of the climatic planning information map and the contents of the individual area profiles for the development areas will be integrated into the land use plan via a climatic supplementary map. In addition, the available findings, including the information and planning recommendations from the fact sheets, are to be taken into account as early as possible in the binding urban land-use planning (development plans).

The implementation of climate adaptation measures in unplanned inner areas - i.e. in existing areas without a binding development plan - is also of great importance in Jena. A large proportion of the climatically polluted areas are existing, but often privately owned and therefore outside the municipal sphere of influence. With the goal of healthy living and working conditions - today and in the future - appropriate framework conditions for development should be specified, particularly for existing areas with climatic pollution. The city council resolution on the urban climate concept therefore formulated the task of examining the possibilities for anchoring climate adaptation measures via local guidelines or building regulations in Jena and implementing them where possible.

Due to anthropogenic influence, modified climatic conditions prevail in a city, which tend to become more pronounced as the number of inhabitants or city size increases. Reasons for this include, for example, the high degree of sealing, which is offset by a low proportion of vegetation and natural surfaces.

The increase in surface area due to buildings (impairment of the flow due to increased roughness, multiple reflections from the buildings) and emissions from traffic, industry and households (anthropogenic heat flow) should be mentioned here. Compared to the largely natural or near-natural, undeveloped surroundings, these effects lead to higher temperatures and bioclimatic stress in summer. The phenomenon of overheating is particularly noticeable at night and is referred to as an urban heat island.

In contrast to air hygiene or noise pollution, for example, there are as yet no normative regulations on guideline or even limit values for thermal pollution. Thermal comfort and human health (well-being, performance, illness and mortality rates) are used for the assessment. The parameters of wind speed, air and radiation temperature and humidity are of central importance here. In urban and regional climatology, various human biometeorological parameters have become established in recent decades, which can be used to determine the effects of pollution on human well-being.

The Physiological Equivalent Temperature (PET) takes into account the perceived temperature, which is influenced by air temperature, humidity, wind speed and radiation. Taking into account the human heat balance, various physiological stress levels can be derived, ranging from extreme cold stress to extreme heat stress. This index is used in the urban climate concept to assess the daytime situation (14:00) in outdoor areas.

While high temperatures during the day tend to lead to stress in workplaces, high night-time temperatures in particular pose a major challenge in private living spaces. The body can only recover from the heat stress of the day under favorable thermal conditions. The Federal Environment Agency recommends a bedroom temperature of 17-18 °C for a restful night's sleep. Achieving these indoor temperatures is highly dependent on the building (summer heat protection through insulation, shading, floor plan orientation, active/passive cooling, etc.). As part of the urban climate concept, the night-time temperature at 4 a.m. at a height of 2 m above ground level is used as the basis for evaluating the night-time situation.

→ Further details in the report, section 2.1 "Thermal comfort and human health"

This is a weather situation determined by local and regional influences with weak wind currents and unhindered radiation conditions, which is characterized by pronounced diurnal variations in air temperature, humidity and radiation.

Associated with this autochthonous weather situation is the occurrence of a low-wind radiation night, in which the nocturnal radiation creates significant temperature differences in the urban area due to the lack of cloud cover. This situation results in the highest thermal loads within the city and the local climatic characteristics are particularly pronounced. These include the heat island effect and the self-developing cold air dynamics, which are not influenced by any superimposed flow. This means that the cold air flow paths can be identified in an autochthonous weather situation.

In Jena, the long-term average over the last 30 years has been around 32 low-wind radiation nights per year, although there are seasonal differences in the distribution. In the three summer months of June, July and August, around 14 low-wind radiation nights were observed - this corresponds to around 15 % of all nights in this period.

Cold air is produced at night over natural or near-natural surfaces by the radiation of heat. Areas with low vegetation (e.g. grassland, arable land, lawns and meadows, but also fallow land) are particularly productive of cold air at night. Due to their volume, forests generate large quantities of cold air. This is generated above the canopy and then sinks into the trunk space due to its weight. The largest proportion of cold air is generated in the areas surrounding the city. The intensity of the cold air flow depends on the size of the catchment area, the slope inclination, the width of the valleys and the absence of obstacles. How far the cold air can penetrate into the built-up area depends on the size of the settlement, the structure and the building density and also on the anthropogenic release of heat and the amount of cold air flowing in. In addition to buildings, trees can also act as an obstacle to air flow at ground level.

Compared to many other cities, the city of Jena benefits from pronounced cold air dynamics. This is due to the striking topography and the varied relief as well as the productive cold air formation areas, which are located in the immediate vicinity of the city. The city is ventilated during radiant nights mainly from the extensive areas from the side valleys of the Saale and the southern Saale valley itself. This promotes good ventilation of the Jena settlement area and thus has a positive influence on the urban climate.

Cold air production rate, cold air thickness and cold air volume flow are relative variables that will remain unchanged even in a warmer atmosphere in the future due to climate change (if they are not modified by land use changes such as large-scale commercial areas or settlement expansions). As a consequence, the same also applies to the central elements derived from these variables, such as cold-air flow paths. It goes without saying that cold air tends to be warmer under climate change than at present and can therefore contribute less to reducing night-time heat pollution.

Urban climate analyses can be based on various methodological analysis procedures. In Jena, a so-called numerical urban climate modeling was used. The model calculations for Jena were carried out using theFITNAH-3D climate model. The horizontal resolution of the modeling is 10 m, i.e. Jena is divided into a grid with a grid size of 10 m x 10 m (corresponds to approx. 4.5 million grid cells). For each grid cell, information such as terrain height, land use, building outlines and heights, trees and the proportion of sealed surfaces are determined and used as input data for the model calculation. With an area of approx. 458 km², the selected study area extends far beyond the area of the city of Jena (115 km²) in order to take into account the differences in altitude and land use in the surrounding area that influence Jena's urban climate.

In accordance with the city of technology, the modeling is based on a summery high-pressure weather situation with cloudless skies and a very weakly superimposed wind (→ autochthonous weather situation, daily maximum temperature above 25 °C).

→ Further details in the report, section 5.1 "Numerical modeling"

In the Jena urban climate concept, two future scenarios were developed in addition to the current situation. This was done with the aim of presenting and ultimately evaluating the effects of the predicted climate change separately from the effects of future building development.

As part of the updating of the land use plan of the city of Jena, an environmental assessment must be carried out for the construction development areas. The significant effects on the urban climate and the specific effects on the area itself (climate/air protection) that are to be expected as a result of the planned development are to be assessed. In order to separate the effects of the planned building developments from the effects of the predicted climate change, a climate change signal was defined for the model calculations of the future, which depicts the future once with and once without urban development in two scenarios. No climate adaptation measures were modeled as part of the urban climate concept.

→ Further details in the report, section 5.1.2 "Scenario development"

All model calculations for the Jena Urban Climate Concept are based on autochthonous weather conditions (low-wind, high-pressure summer weather conditions without cloud cover) in accordance with VDI Guideline 3787 Sheet 9. Typically, an autochthonous summer day leads to situations that cause the highest thermal loads in parts of the city over the course of the year due to the high insolation and the low large-scale (autochthonous) air exchange. Even if this is a special meteorological situation, such weather conditions occur regularly in Jena and several times each summer. In the summer months of June, July and August, there are more than 15 nights in Jena that are autochthonous, and 32 nights in the entire year at the nearest station, Erfurt-Weimar.

A daily cycle was modeled for each scenario, starting at 21:00 and ending at 14:00 on the following day at the time of the sun's highest point. The model start temperature for the actual situation is 20 °C at 9 pm. This was determined based on the evaluation of a 30-year time series from the DWD measuring station Jena Observatory (1991-2020) for the average summer days (daily maximum temperature > 25 °C) in the summer months of June, July and August at 21:00. For the future scenarios P1 and P2, the starting temperature was increased by 2 Kelvin and the soil moisture was lowered below the wilting point in both model calculations.

The selected starting conditions deliberately represent a thermally stressful situation, at least for parts of the city, but not an extreme event. The aim of the urban climate concept is to differentiate this stress situation spatially and to counteract this stress through sustainable urban development and appropriate climate adaptation measures.

→ Further details in the report, section 5.1.5 "Framework and boundary conditions"

In Jena, the future urban climate in 2035 was calculated with a very strong climate change signal as part of a precautionary strategy. Based on the RCP 8.5 ("continue as before scenario" with regard to the course of the absolute greenhouse gas concentration in the atmosphere), a temperature increase of 2 Kelvin for the near future up to 2035 is included in the calculation model. Furthermore, a reduction in soil moisture (from 60 % to 30 %) was assumed for green and open spaces. Use classes with low vegetation can then no longer evaporate, so that in the model the incident short-wave energy is converted directly into sensible heat, which contributes to an increase in the ground-level air temperature, particularly during the day. Trees are excluded from this.

The FITNAH-3D climate model was fed with the corresponding input data for each scenario (actual situation, future scenarios P1 and P2). The input data was recorded in July 2021 and reflects the state of knowledge at that time. The following information was assigned to the model for each 10m grid cell

• Terrain / orography (digital terrain model)
• Land use / degree of sealing (model use classes: Building, undeveloped sealed, track surface, non-natural soil, sand/gravel, open land/lawn, tree over sealed, tree over non-natural soil, tree over lawn, water)
• Structural height (for buildings, low vegetation and trees)

As the urban climate concept was not only intended to depict the current situation, it was also necessary to develop climatic and urban structure-related framework conditions for the future scenarios P1 and P2. All three model calculations are based on the same digital terrain model.

The actual situation takes into account the climatic and structural condition of the city at the time of data collection (July 2021) based on the available geodata. With the aim of being as up-to-date as possible, legally binding development plans and construction projects currently being implemented or to be implemented in the near future were also integrated into the calculation model of the actual situation (→ see Table 9 in the report).

Scenario P1 "Climate change" (without urban development) contains the same urban structure as the actual situation and is intended to show the expected effects as a result of climate change. When looking into the future, however, there is inevitably a certain degree of uncertainty as to how much climate change will actually occur. In Jena, the future urban climate in 2035 was calculated with a strong climate change signal as part of a precautionary strategy. Based on the RCP 8.5 ("continue as before scenario" in relation to the course of the absolute greenhouse gas concentration in the atmosphere), a temperature increase of 2 Kelvin for the near future up to 2035 is included in the calculation model. Furthermore, a reduction in soil moisture (from 60 to 30 %) was assumed for green and open spaces - this leads to a lack of evaporative cooling of these areas during the day. Trees are excluded from this.

In Scenario P2 "Climate change with urban development", the climate change signal of +2 Kelvin already used in Scenario P1 and the assumption of dryness of green and open spaces were continued. Scenario P2 also includes the planned urban development. Here, 69 potential development areas for residential, commercial, special construction and traffic as well as allotment gardens were assumed to be built on (based on the preliminary draft of the urban land use plan dated 04.11.2022). The planned development areas are based, among other things, on the "Jena 2035 residential development area concept", the "Jena 2035 workplace and commercial area development concept" and the "Garden development concept" (2024 update). The "mixed pixel approach" was mainly used to model the FNP development areas. Specific urban development planning could only be integrated into the model calculation for P2 for three development areas.

→ Further details in the report, section 4.4 "Future climate change" and section 5.1.4 "Preparation of model input data"

At the time of modeling (July 2021), no concrete urban development plans were available for a large number of the legally binding development plans and the majority of the FNP development areas. In order to transfer these areas into the calculation model, the so-called "mixed pixel approach" was therefore applied. For the areas to be developed in the future (residential, commercial, special construction), areas were selected from the existing urban area which, as so-called "area twins", most closely correspond to the development status of the future. The percentage distribution of surface cover (buildings, sealed surfaces, lawns, trees, etc.) was recorded for these area twins and transferred to the development areas.

The mixed pixel approach offers an opportunity to examine and evaluate the planned building developments in terms of their urban climate compatibility. However, it must be taken into account that this model approach can only depict future development to a limited extent. On the one hand, a random distribution of the model use classes (as pixels in a 10m grid) is applied to the area. This does not correspond to the actual building structure of the area twin, but only depicts the percentage distribution of surface coverage. There is no modeling of contiguous building structures, e.g. buildings of a certain size and orientation, which may act as an obstacle to flow, or contiguous green structures, which could then be more effective for cooling, for example. Furthermore, this methodology does not apply a climate-adapted construction method, but only the adoption of existing building structures or their percentage area shares.

→ Further details in the report, section 5.1.3 "Mixed pixel approach"

Buildings are included in the calculation model as a 3D building model (LoD2) with their floor area, height and standardized roof shapes (e.g. saddle, hip, mansard, pent or tent roofs). The buildings are embedded in the digital surface model of the city. Above-ground buildings are modeled without roof superstructures and without texture (materials, colors, etc.) of the facades and roof surfaces. Green roofs and façades are therefore not included in the model.

→ Further details in the report, section 5.1.4 "Preparation of the model input data"

By definition, models are incomplete representations of reality. They therefore do not claim to represent the system to be modeled comprehensively, but merely aim to represent it sufficiently well. Model applications are always associated with certain uncertainties - uncertainties in the selected model on the one hand and uncertainties in the input data on the other.

In the urban climate concept, all input data in the analysis was converted into a regular grid in which one grid point represents the main use on an area of 100 m². This is the highest resolution that can currently be modeled for cities the size of Jena. However, small-scale structures cannot be fully taken into account even at this high resolution. There are often several different usage structures (e.g. sealed surface, lawn, tree) in one and the same grid cell. In the input data and therefore also in the model, however, only the use that takes up the largest proportion of the area in the grid cell is taken into account. It is therefore possible that a street with small-crowned individual trees is not recognized as an avenue, or at least not consistently, and shading effects are therefore not taken into account. However, the affected sub-areas are generally small with rather local effects, so that no relevant effects are to be expected in the overall urban perspective or in the basic conclusions. Nevertheless, in specific cases of application/doubt, it is always advisable to take a critical, localized look at the underlying model input data.

Another source of uncertainty is the depth of information of the data considered in the model. For example, although the buildings are integrated into the model in their cubature as a "block model" (with exact location and dimensions with averaged roof height), the heat exchange with the surroundings is only represented via the building volume. Building materials, surface albedo, roof and façade greening and the proportion of windows in the buildings are not taken into account in the model.

In future scenario P2 "Climate change and urban development", the so-called "mixed pixel approach" was used to model the FNP development areas. This methodical procedure helps to simulate the future structural development of the city of Jena and to evaluate the associated climatic effects, but also involves uncertainties with regard to the results. Further explanations can be found under "What is the mixed pixel approach?".

→ Further explanations in the report, section 5.3 "Uncertainties and challenges"

For each grid cell, the urban climate concept provides model results for the night-time air temperature, the cold air production rate and the cold air flow field as well as the heat load during the day. With the exception of the cold air volume flow (flow over the entire lower air layer), the results apply to the area near the ground where people are present and consider the times 04:00 for the night situation (maximum cooling) and 14:00 for the day situation (maximum radiation).

Determining the night-time air temperatures near the ground makes it possible to identify overheated urban areas (so-called urban heat islands) and to estimate the spatial effectiveness of cold air currents. The absolute air temperature values listed are exemplary for a radiant summer weather situation. The relative differences within the city or between different land uses, on the other hand, also largely apply during other weather conditions.

Six climate analysis maps were created as part of the urban climate concept. There is one for the daytime situation and one for the night-time situation for all three scenarios (current state, future P1 and future P2). The climate analysis maps summarize the essential statements of the meteorological parameters (model output variables). For the nighttime situation at 04:00, the cold air process is specified by merging the near-surface wind field, cold air production rate, cold air volume flow and near-surface air temperature in one map. The climate analysis map for the daytime situation at 14:00 is based on the PET.

Climate analysis maps belong to the so-called factual level - this means that the climatic facts are shown at grid cell level (10 m x 10 m) and no assessment is made, for example, of the extent of the thermal load in the effective area (settlement and traffic areas) or the value of the compensation area (green and open spaces).

The climate analysis maps for the night show the ground-level air temperature as absolute values for the impact area. For the green and open spaces, the model results of the cold air volume flow are shown in graduated area colors. Green areas with an above-average cold air production rate (> 14.7 m³/m²h) are also marked with a black dot hatching. Furthermore, the flow field close to the ground or the direction of flow of the cold air from a wind speed of 0.2 m/s, which is considered to be climatically effective, is visualized with an arrow signature. For better readability, the wind field was aggregated to 100 m. These wind arrows (size and density) show the extensive cold air outflow that is so important for Jena. Due to the spatial-topographical situation in Jena, these areal slope outflows play a decisive role in night-time cooling. Particularly on the slopes of the side valleys of the Saale, large quantities of cold air are produced at night, which, following the slope of the terrain, supply the adjacent settlement areas with cold air. In addition to the extensive cold air outflow, the linear cold air conductors are an important component of the cold air processes in Jena. They connect cold air-producing compensation areas (green and open spaces) and effective areas (settlement and traffic areas) with each other and usually have high cold air volume flows.

The climate analysis maps for the day are based on the Physiologically Equivalent Temperature (PET). The PET is shown here for both the effective area and the compensation area. Further explanations of the PET can be found under "Are there guideline or limit values for human thermal exposure?".

→ Further explanations in the report, section 7.1 "Climate analysis maps"

The model results available at grid level (as absolute values) allow a detailed representation of the most important climate-ecological processes in the city. However, an assessment and the planning statements derived from it (e.g. on the degree of thermal stress within the effective area and the bioclimatic significance of certain areas in the compensation area) must refer to clearly definable spatial urban climatic functional/usage units in the urban area. For this purpose, the so-called "basic geometry" was used for the entire urban area. Urban planning and spatial usage units were summarized and assigned to one of 20 usage categories (e.g. watercourse, cemetery, park and green area, residential area, care facility, etc.). The climatic information of the grid cells within the usage unit was summarized and spatially averaged so that an average value for each climate parameter (night-time air temperature, PET, cold air production rate, etc.) is finally available for the respective usage unit. This value then forms the basis for the urban climate assessment in the next step.

→ Further details in the report, section 8.2 "Geometric basis"

The evaluation maps are based on the climate analysis maps. They summarize the individual urban climate parameters (ground-level temperature and wind field, cold air volume flow and cold air production rate at night as well as perceived temperature during the day) for the day and night situation in the various scenarios (current state, future P1 and P2). A total of six evaluation maps are available.

In contrast to the climate analysis maps (10 m grid), the assessment maps provide a summarized assessment of areas with similar building structures and uses (based on the so-called basic geometry). As there are no legal limit values for the thermal load on the urban population, the bioclimatic load in the effective area (settlement and traffic areas) or the bioclimatic significance of the compensation area (green and open spaces) is assessed via the deviation of certain climate parameters from the average conditions in the area under investigation (z-transformation). This means that the individual areas are evaluated in relation to each other. This shows which settlement areas have a rather favourable or rather unfavourable bioclimatic situation and which green spaces have a rather high or rather low bioclimatic significance.

Night situation

The assessment of the bioclimatic significance of green and open spaces at night (very low to very high) is based on their function for the cold air balance. An evaluation algorithm is used to take into account cold air production, volume flow, wind speed and the location in relation to polluted settlement areas. The assessment of the settlement area (very favorable to very unfavorable) is based on the night-time overheating and is determined by the ground-level air temperatures at 04:00. How much cooling actually takes place inside the buildings depends to a large extent on the structural standards (thermal insulation, possibility of cross-ventilation, etc.). In the evaluation maps for the night, the uninhabited residential areas (e.g. businesses, daycare centers, schools, etc.) as well as streets and squares are not evaluated (greyed out), as the focus of the considerations is on healthy sleeping conditions at night. The respective information for the grayed-out areas is nevertheless available in the GIS data for the project and is included in the evaluation of the planning information map.

Daytime situation

The evaluation of the bioclimatic situation during the day for the effective area (very favorable to very unfavorable) and the compensation area (very low to very high quality of stay) is essentially based on the evaluation of the thermophysiological index PET at 14:00, which can be interpreted as the perceived temperature. The heat load outside buildings is considered here. Inhabited and uninhabited areas are of equal importance, and the street space and the quality of stay in squares and green spaces also come to the fore. The heat load is primarily dependent on shading, so that in the case of green and open spaces in particular, contrary assessments can sometimes arise between day and night. An open space cools down considerably at night, but has a high heat load on summer days (without cloud cover) if there are no trees. In contrast, the quality of stay in forests and tree-lined parks is very high even on summer days.

→ Further details in the report, section 8.3 "Evaluation maps"

In the climate analysis maps as well as in the evaluation maps for the night, the green areas with an above-average cold air production rate are marked with a black dot hatching in addition to the arrow signatures for the cold air flow paths and the extensive cold air runoff. In addition, the cold air impact area is shown in the assessment maps as a black, grid-based line hatching. This indicates how far cold air can penetrate into the existing settlement areas (residential, commercial, streets, squares, etc.). Cold air impact areas are all those areas in the effective area that reach or exceed the mean value of the cold air volume flow (corresponds to 38.54 m³/m*s in the actual situation) and the corresponding mean wind speed of 0.2 m/s.

Various priorities for action to improve the urban climate situation are assigned to the impact area - i.e. the existing settlement and traffic areas - in the planning information map (PHK). For this purpose, the results of the day and night situation from the assessment maps of the future scenario P2 (with climate change and urban development) were combined and summarized as a result in a 4-level scale. The combination of day and night was carried out for different uses with different weightings. In inhabited areas (residential, mixed use, care facilities), the assessment was carried out with a view to healthy sleeping conditions - therefore the night situation is given greater importance. In unoccupied areas (e.g. commercial areas, schools, daycare centers, institutes) and in public spaces, the assessment is primarily based on the quality of stay during the day, as people generally do not spend long periods of time there at night. For traffic areas, the assessment is based solely on the daytime situation.

The assessment according to priorities for action is intended to provide guidance on the areas where climate adaptation measures are particularly important and should be prioritized. Structural developments are understood and used as an opportunity to adapt to climate change. In the course of urban redevelopment, climate adaptation goals and measures can be taken into account at an early stage in informal planning processes or made binding via development plans. Private developers should be made aware of climate-resilient construction methods during the planning permission process. Settlement areas with a (very) high bioclimatic load in conjunction with a high population density should be converted as a priority to adapt to climate change (focus areas heat stress residential).

→ Further details in the report, section 8.4.1 "Methodology for impact area and compensation area"

For the existing green and open spaces, the planning information map is used to assess the need to maintain the urban climatic function. This is determined by the bioclimatic significance of the areas and is also carried out by intersecting day and night assessments from the assessment maps of future scenario P2 (with climate change and urban development). Due to the decisive climate function and effectiveness (cold air generation, cold air flow), the night-time situation is weighted much more heavily than the bioclimatic quality of the areas during the day. The night-time situation therefore accounts for 80% and the daytime situation for 20% of the overall assessment. As a result, there are four assessment classes for the need for conservation.

→ Further details in the report, section 8.4.1 "Methodology for impact area and compensation area"

For Jena, areal slope runoffs are just as much a part of the cold air process as the linear conductors. It is not possible to make a clear spatial distinction between cold air formation areas, extensive cold air runoff and linear conductors. Rather, it is a complex system with overlapping and overlapping functions.

Surface developments within spatially limited guideways can lead to a narrowing of the flow cross-section and an increase in roughness. In order to avoid a restriction or even the loss of the guideway function, structural developments should be carried out extremely cautiously and in a climate-adapted manner. Expert supervision of the planning is strongly recommended.

Cold air outflows following the slopes react much more robustly to a moderate level of building development due to the fact that the air is usually able to escape. Small-scale consolidation of the existing settlement structure, which takes into account climate-adapted construction methods, can generally be implemented in a way that is compatible with the urban climate.

→ Further details in the report, section 7.1.1 "Climate analysis maps - night-time situation"

The planning information map shows the development areas in accordance with the updated land use plan (status: preliminary draft 10/2022) with regard to their impact on the urban climate. For all 69 development areas, an expert assessment of the effects on the climate as a protected resource was carried out as part of the urban climate concept. The level of detail of the expert assessment and the methodological approach differ. The survey of the areas and their integration into the climate modeling took place in July 2021 and correspond quantitatively and qualitatively to the state of knowledge available at that time.

A detailed expert assessment was carried out for 55 development areas and the results were summarized in a profile. As a result, the experts confirmed that the areas can be built on without any significant impact on the local climate, provided that the construction method is adapted to the climate.

For 14 development areas, no in-depth expert assessment was carried out in a fact sheet. These are areas for which an environmental assessment has already been carried out as part of planning procedures or for which no environmental assessment is required under certain conditions (simplified/accelerated planning procedure, inner development, consolidation of outlying areas). In some cases, specific planning or project-related microclimatic reports are available for these areas, which provide more detailed information than is possible in this urban climate concept. However, these development areas were nevertheless integrated into the model calculations. The model results can be read directly from the climate analysis and evaluation maps and finally from the planning information map. More detailed information can be found in the overview of all FNP development areas in the final report (Annex 10.2).

→ Further details in the report, section 8.4.2 "Evaluation of the FNP development areas"

The assessment of the effects on the climate as a protected resource carried out as part of the urban climate concept is intended as the basis for the environmental assessment to determine the likely significant environmental effects that are to be expected as a result of a construction project. This involves a comparison of the current situation with the anticipated effects of implementing the project. The urban climate modeling was therefore carried out for three scenarios in order to separate the effects of climate change from the direct effects of the planned development.

An assessment matrix was developed for the urban climate assessment of the FNP development areas. This takes into account the climatic effects on the area itself (what is the bioclimatic situation likely to be like in the future?) and the climatic effects on the neighboring areas (will there be significant temperature changes?). The resulting overall expert assessment of climatic compatibility can be found in the climatic planning information map. Detailed information on the individual development areas can be found in the fact sheets.

→ Further details in the report, section 8.4.2 "Assessment of the FNP development areas"

The settlement areas with a high thermal load in Jena are also those for which heat-reducing measures should be examined and applied as a priority. Structural developments (urban redevelopment) should be understood and used as an opportunity to adapt to climate change. The climate planning information map provides basic recommendations for action and assigns priorities to the individual settlement areas for the necessity of climate adaptation measures. For the purpose of targeted implementation of these planning recommendations within the existing building stock, the spatial summary (clustering) is carried out in so-called "residential heat stress focus areas". These focus areas are particularly climatically stressed residential areas that are also characterized by an above-average population density. Here, measures to adapt to climate change and reduce heat stress are to be implemented as a priority.

The following seven focus areas are marked on the planning information map: City Center, Northern City Center, Lutherstrasse, Wenigenjena, Magdelstieg, Jena-Nord and Lobeda-Altstadt. The report describes the structural and climatic situation in conjunction with corresponding recommendations for climate adaptation measures.

→ Further details in the report, section 8.4.6 "Focus areas heat stress"

The planning information map shows areas in the city where measures to improve the thermal situation are necessary or recommended. The catalog of measures contained in the report shows 21 different options for implementation and is intended to help concretize the planning advice. The selection of specific measures from the portfolio depends on the type of area (use, building structure, etc.) and the assessments in the planning advice map or the assessment maps (e.g. bioclimatic load at night and/or during the day, significance for the cold air balance, quality of stay).

In principle, all measures are suitable for directly or indirectly reducing thermal stress for the urban population and thus contributing to achieving a healthy urban climate in Jena - if the measures are combined, the positive urban climatic effects of the individual measures are generally enhanced.

→ Further details in the report, section 8.5 "Catalog of urban climate measures"

The "Urban Climate Concept for the City of Jena" is an informal plan with a binding effect within the administration. By resolution no. 24/0130-BV of the Jena City Council of 26.02.2025, the city administration was commissioned to use the urban climate concept as a planning basis for all climate-ecological statements and planning processes. Informal plans have no direct legal effect on citizens. They serve to prepare, support or pre-coordinate the subsequent formal planning (binding urban land-use planning).

For the implementation of climate adaptation measures - particularly in existing buildings (areas in accordance with Section 34 BauGB) - the Lord Mayor was instructed to examine and develop binding guidelines for the city of Jena in accordance with a city council resolution.

→ Further details in City Council resolution no. 24/0130-BV "City climate concept for the city of Jena"