Intense urbanization contributes to dramatic changes in built-up areas: changes in urban densities, land use, and land cover. As a result of these changes, very high temperatures are recorded in the urban environment. These high temperatures are related to controlled factors (such as urban design) and uncontrolled factors (such as meteorological parameters and environmental conditions) [1
]. It has long been acknowledged that cities face multiple problems related to the variety of microclimates that prevail within the various urban open spaces. In general, the parameters of the urban climate are the result of human interventions, with the most influential of these being how cities have been designed, developed, and built and the activities they host [4
The environmental, or bioclimatic, approach to urban design is based on an understanding of the characteristics of the urban microclimate [8
]. The microclimate refers to the climatic conditions prevailing in the cities, mainly as a result of the form of the urban fabric. Specific forms of urban fabric are determined by land-use planning and urban design and, of course, by the activities of people within this physical form of urban development [4
]. Therefore, the microclimate is impacted by human interventions in the built environment, which often leads to unintended climatic conditions, localized within small enclaves of the urban fabric [5
The urban microclimate describes the climatic conditions prevailing within any urban locality, such as a neighborhood, a square, etc. The climatic conditions prevailing within this locality usually differ significantly from the climatic conditions of the wider region. Both morphology and the activities within cities can cause changes in the atmospheric and surface properties of the affected areas, disturbances in the thermal equilibrium, the airflows, the inflow of solar radiation, etc. [7
]. It is also worth noting that significant differences in air temperature, wind speed, solar radiation, etc., are often observed between the different localities within the urban fabric, even when there is a very short distance between them. This is due to the different spatial configurations of urban morphology including orientation, height to width ratio, and other factors such as the presence of vegetation [1
Urban morphology largely determines airflow (ventilation), the inflow of solar radiation, shading, etc. Therefore, it is responsible for the variation in climatic conditions across different localities within the city [7
]. Characterized as either a “lose” or a “compact” urban form, a locality’s either “scattered” or “cohesive” building complex affects the spatial distribution of the built-up areas, which in turn, causes the commensurate variation in airflow, solar radiation, etc. [17
]. The ratio “envelope surface-to-volume” (S/V) and the ratio “building height-to-width outdoor space” (H/W) are two basic indicators of the spatial distribution of the built-up area, both of which directly affect the microclimate. The S/V factor expresses the degree of coherence of the building volume. In contrast, H/W expresses the degree of “openness”, i.e., the relationship of the building size to the surrounding outdoor spaces, which can be described by the sky view factor (SVF). SVF is a measure of the solid angle of view of the sky as seen from an urban space [16
]. It is directly related to the phenomenon of urban heat island (UHI) [7
] and temperature variations across different urban environments. If the value of this factor is equal to 1, it means that there is a complete view of the sky. Consequently, temperatures tend to be in line with predicted meteorological values, while if the value is 0, the sky view is blocked, and the temperatures are affected by the urban environment [16
The compact urban form is characterized by a small S/V ratio, a large H/W ratio, reduced sky openings (SVF), and large amounts of land occupied by built-up areas. It is structured in cohesive building blocks that ”protect” roads and open spaces. As a result, compact urban forms are presenting difficulties in exploiting the winter sun due to increased shadows and the different orientations of each side of the building blocks. In addition, high urban densities help to shade the surfaces of urban canyons and reduce solar radiation absorption, trapping long-wave radiation emitted from the ground and building surfaces at night, thus preventing cooling. In contrast, loose urban forms are characterized by a large S/V and small H/W ratio, less ground covered by buildings, wider outdoor spaces, and a lower density of a building volume [1
The H/W ratio directly affects the urban microclimate as it affects the inflow of solar radiation into the urban canyon, wind flows, and the trapping of radiation emitted by materials. Additionally, the building volumes, the shading, and the form of the openings of the buildings affect the configuration of the microclimate [15
]. Different H/W ratios have different effects on individual environmental parameters. For example, a high H/W ratio may mean low to moderate winter solar gains, moderate or high exposure to the summer sun, high trapping of sunlight with reflections, high obstruction of air movement, and the low to moderate available area for tree planting [18
]. On the other hand, a low ratio means large solar gains, low exposure to the summer sun, low to moderate trapping of sunlight with reflections, low to moderate obstruction of air movement, and large available area for tree planting [18
]. Therefore, it is not easy to achieve an optimal H/W rate that corresponds to all the case-by-case parameters.
The phenomenon of UHI is directly related to the climate prevailing within urban areas as it significantly affects the formation of the urban microclimate [22
]. This phenomenon is reflected in the microclimatic changes caused by anthropogenic interventions in the urban environment [23
]. These changes can be attributed to multiple factors such as urban morphology, discarded heat, properties of building materials, and insufficient presence of free spaces, green spaces, and water surfaces [4
]. Regarding the relationship between urban geometry and the phenomenon of UHI, according to Emmanuel and Johansson [27
] and Oke [19
], the intensity of the UHI phenomenon increases as the H/W ratio increases.
In addition to the above, more than a quarter of compact urban areas are generally covered by a network of roads, the design of which has a significant effect on the urban climate. Urban roads differ geometrically, as they are defined not only by the height/width (H/W) and length/width (L/W) ratios but also by the orientation of their axes. These parameters directly affect the absorption and emission of solar radiation, as well as the ventilation of the urban environment, resulting in significant temperature variations [29
]. Different climatic conditions are observed on the roads with different orientations (East-West or North-South). The E-W roads are the least thermally comfortable roads, as they shade in winter and “sunbathe” in summer. Conversely, the N-S roads are considered to be thermally comfortable as they are exposed to the sun’s rays in winter, while in summer, they are sufficiently shaded in the morning and evening hours. Finally, roads with a northeast/southwest (NE–SW) and a northwest/southeast (NW- SE) orientation lie somewhere in between these two poles of thermal comfort, being partially sunny in summer and partially shaded in winter [31
Apart from the urban form, which mainly concerns the built environment, green and tree-planted areas that affect the urban climate usually have a positive impact on thermal comfort. More specifically, urban greenery affects the microclimate by absorbing a high percentage of solar radiation, reducing air temperature through transpiration, reducing air speed near the ground, etc. [34
]. Tree planting, in particular, has a significant impact on the reduction of urban temperatures, and it is an effective strategy for reducing the effect of the UHI phenomenon, improving the air quality, and creating conditions that cause a reduction in temperature [36
]. Bowler et al. [39
] pointed out that data from different studies suggested that, on average, an urban park would be around 1 °C cooler than a non-green site. However, this phenomenon, often referred to as “urban oasis”, is also affected by other parameters such as local climatic conditions, the degree of tree cover, and tree types. [39
Finally, special mention should be made of wind and urban air ventilation issues as they are important factors in improving the microclimate and achieving thermal comfort in outdoor urban areas [16
]. Givoni [4
] stated that the flow of wind in the urban environment changes more than any other climatic element. The wind speed in cities changes significantly over relatively short distances and within relatively short time intervals, creating gusts of wind, which are perceived as sharp increases in speed [7
]. Modern compact urban environments, whose form consists of accumulated buildings of different heights, experience a phenomenon in which the tallest buildings break the flow of strong winds and often divert them to the axis of the roads, resulting in their ventilation [43
The present paper examines the microclimatic conditions within a typical urban compact form, focusing on an area within the historical and commercial center of the city of Thessaloniki. Further, it evaluates the contribution of urban morphology to and the effect of open spaces and greenery on such compact forms, examining how the specific parameters of urban morphology contribute to the formation of specific microclimatic conditions.
The findings of the present study indicate a strong relationship between urban morphology and the microclimate, as defined by the local climatic conditions prevailing within the urban environment, even at a very local scale. Geometrical characteristics of the urban form, such as building height, street width, etc., directly and variously affect the ventilation and insolation of the urban fabric, thereby significantly impacting the formation of microclimatic conditions. In particular, within the compact urban forms, it is easy to detect differences in climatic conditions between areas that lie very short distances from each other. These variations are due to small differences in urban morphology, for example, the availability of open and green spaces or the properties of building materials, etc. In the present research, the emphasis was placed on very compact forms within the city center, characterized by the high coherence of building blocks and the great soil sealing. The findings of the simulation developed in the field study show that this particular urban form can, in some cases, have a positive effect on the microclimate in the summer period due to outdoor protection areas and road shading, whereas, during the winter, this form presents obstacles exploiting the winter sun and increased shading.
Comparisons were drawn between the two, adjacent sections of the study area. These differed in terms of the urban canyon’s width, the availability of open spaces, green spaces, and tree planting, and there were indications that these disparities lead to disparate effects on the microclimate. This was evidenced by the significant variations in values for variables recorded in areas that lay a very short distance from each other, within the same urban space. For example, the simulations have shown that while the type of urban fabric is equally compact in both sections, a small change in the H/W ratio of the urban canyon can cause an unimpeded flow of hot air (if the ratio is small), thereby increasing the temperature inside the urban space, or it can act as a barrier (in case of a bigger ratio) to the access of hot air to the interior of the urban fabric, thereby creating a cooling effect.
Another significant finding of the paper concerns how the design of roads and urban canyons, the geometry and orientation of which greatly affect the ventilation of urban space, solar radiation, shading, etc., affects the overall indoor and outdoor environment. According to the season, the effect of the orientation of the canyons on the microclimate can differ, therefore choosing a suitable design to achieve optimum microclimate conditions in all seasons is a difficult task. For example, wind and temperature simulations showed that urban canyons perpendicular to the wind flow showed a significantly improved atmospheric temperature, compared to canyons whose orientation was parallel to the wind flow. It should be emphasized here that the simulations generated from the present study concern the summer season. As mentioned in the introduction, the morphology of the urban space can have different results in different seasons.
Urban morphology affects the ventilation of urban space significantly. This is a complex process and one of the most important factors in shaping the microclimate. In general, the compactly built-up space is responsible for the reduction of the wind speed. However, high H/W ratios obstruct its movement, creating local gusts. Given that the ventilation factor is the most changeable climatic factor within the urban space, it was examined in detail in this work. The findings show that the H/W ratio, along with the orientation of the urban ravine and the through openings in the building blocks, is the factor mainly responsible for the urban space’s ventilation. Within the study area, in cases where airflows were perpendicular to the urban canyon, the high H/W ratio and the lack of through openings were responsible for the urban environment’s lack of ventilation. At the same time, the tall buildings at the corners of the building blocks appeared to create strong local gusts of wind.
It is well known and widely acknowledged that urban greenery and tree planting within the urban areas affect the urban climate, mainly having a positive impact on thermal comfort. In addition to lowering air temperature, urban greenery can also reduce other factors which harm the microclimate, such as pollution, and is also an effective strategy for dealing with the UHI [45
]. However, the results of the simulation show that planting’s positive effects on air temperature can only be seen in cases of dense tree cover, as sparse and spot planting cannot have significant effects. The simulations of the present case study indicated that positive effects are recorded only in cases where there is adequate tree planting on both sides of the streets. At the same time, on roads with sparse and spot planting the air temperature remains high. In addition, the simulations for section B showed that, due to the lack of dense tree cover and shading in existing parks, the latter does not necessarily function as an obstacle to the flow of hot wind, with the result that there is an invasion of intense thermal loads, especially within the built-up area.
This study has, of course, certain limitations, which, however, affect neither its main findings nor the issues raised in the discussion above. To begin with, a simulation model to be fully validated should be tested and compared to field measurements of air temperature, humidity, wind speed, etc. Sensitivity analysis and a spin-up period of more than 24 h would also provide much safer results. Significant limitations of such a study relate also to the limited availability of similar studies with field measurements in the city of Thessaloniki. Moreover, there is a lack of data reporting measurements of both air pollution and the heat released from vehicle traffic. If such studies were available, even for other compact urban cores within Greek cities with similar characteristics, they could be used to investigate other factors affecting the formation of the microclimate and to provide further validation of the reliability of our results. Our main purpose was to focus on issues of urban morphology and open and green spaces. In other words, to focus on issues, determined, to a large extent, by urban planning and design. Therefore, a further assessment of all the factors that may influence the formation of the microclimate in compact urban fabrics is critical.
Another limitation of the study is that the simulations were carried out for the summer period and an average hot day, for which we examined a typical afternoon hour. Hence, the results extracted concern this specific period. During the winter period, simulations will, of course, deviate significantly from the findings reported in the present study. Therefore, a comparative assessment of the microclimate for different time periods/seasons is also necessary. Among the critical challenges ahead is the use of such models to implement appropriate regeneration proposals for improving the microclimatic conditions in the compact inner cores of the cities. Urban morphology, in the areas which are already built-up, is difficult to change. Τhe use of simulation models enables planners and policymakers to look into various alternative solutions for implementation within different urban forms and within different seasons of the year.
This study aimed to analyze how critical elements of urban morphology, such as the street layout, the urban canyon, and the open and green spaces in a compact urban form, contribute decisively both to the creation of the microclimatic conditions and to the influence of the bioclimatic parameters. In the context of a compactly planned and built-up city center with a Mediterranean climate, our findings, based on simulations, are generally in line with findings from previous studies regarding the impact of urban morphology on microclimate. Within this field of literature, we paid particular attention to the detailed, street plan level, where we examined the microclimatic variations in elements such as air temperature, urban ventilation, and the individual’s thermal comfort.
Overall, our study highlighted that, in highly compact areas, it is important to conduct a detailed investigation of the typological relationship between the building blocks within the urban fabric and the microclimate. This would enable the evaluation of microclimatic conditions and provide insights into the necessary and appropriate urban planning and design interventions to improve these conditions. Compact areas, especially densely built-up ones, with open and green spaces comprising a low percentage of the total area, are the norm in older cities with old housing stocks. In such areas, improving the microclimate conditions due to urban morphology is a very difficult and multi-dimensional task, made even more complex when the related effects of UHI and general adaptation to climate change are considered. In these cases, it is equally important that the simulation models used are supported by field measurements of specific parameters such as air temperature, humidity, and wind speed so as to acquire more accurate and much safer results which are needed to find adequate solutions for improvements.
The current evidence base shows that to make specific recommendations for best handling of the microclimate problem in these urban forms, we need detailed investigation at the very local scale. There is no doubt that some of the most accessible measures would be nature-based. These may include increasing the various forms of urban greenery, its detailed distribution and design within the street layout, as well as on building facades and roofs, creating bright openings in building blocks for ventilation of the entire urban space, and a change of materials used to build structures, roads, and sidewalks, along with the pedestrianization of roads. Finally, as there are no “one-size-fits-all” solutions, this detailed planning and design should be continuously evaluated in order to substantially contribute to the improvement of diverse urban forms and their microclimates.