The canyon geometry dynamically influences its thermal behavior during the day and throughout the year ^{[198][207][208][209][210][211]}[227,256,257,258,259,260], and changes in canyon depth have better mitigation potential for the nocturnal UHI ^[212][154][121,188]. Wide canyons are characterized by a rapid LST increase in the morning, and because of the greater heated surface area, they reach higher maximum temperatures earlier ^{[198][213][214]}[227,261,262]. At night, their thermal response is better than the high canyons. The low H/W ratios canyons have much greater net longwave radiation exiting the canyon and more significant convective cooling ^[208][213][257,261]. As a result, they cool down faster and show a greater proportion of cool area than high H/W ratio street canyons ^[167][201]. That explains why wider canyons have a smaller overall UHI than deep canyons ^[173][214][207,262] despite a greater intensity variation during daytime ^[213][215][261,263]. As the aspect ratio increases in the morning, the UHI effect becomes the urban cool island (UCI) effect because of the strong shading effect ^[97][100][132,135]. By reducing the penetration of direct sunlight ^[198][227], higher H/W allows the bottom of the canyon to remain completely shaded and thus to heat slower ^[207][256]. The maximum short-wave radiation in such a canyon occurs around noon ^[198][227]. The increase in shaded areas results in lower maximum temperatures ^[120][155], daily temperature ranges ^[167][201], the mean radiant temperatures (T_mrt) ^[98][133], and the LST ^{[107][111][216][217][218]}[142,146,264,265,266], especially walls with greater sun exposure ^[213][261]. Human thermal comfort is also improved ^[98][219][133,222]. That can be seen in the courtyards case. Their deepening can lower their air temperatures by about 2–2.5 °C ^[220][221][267,268]. The deep canyon is cooler in the afternoon than the shallow one ^[222][269]. However, the situation changes in the late afternoon. At night, the higher street aspect ratio and symmetry result in less outgoing net longwave radiation by reducing SVF, air cooling, and multiple radiation reflections inside the canyon, resulting in a thermal trap effect ^{[188][196][207]}[242,248,256]. Those increase the air temperature ^{[217][222][223]}[265,269,270], PET ^[205][254] and the UHI ^{[198][215][223]}[227,263,270] compared to shallow and open canyons ^[224][127]. Even a small change in the canyon geometry at low shape factor values can cause heat island condensation ^[120][155]. However, the H/W impact on the UHI only gains importance for buildings higher than two stories ^[146][181] or a ratio greater than 1.3–1.5 ^[97][184][132,239]. The canyon smoothness and symmetry also contribute to a slight temperature increase at night ^[96][131]. The seasonal relationship correlates with solar access variability and is more pronounced in summer than in winter, depending on the different angles of sunlight incidence ^{[207][209][210][211][225][226]}[256,258,259,260,271,272]. The temperature difference between the deep and shallow canyons drops significantly during the winter ^[207][256]. Due to the greater sunlight access, shallow canyons in winter have the highest daily T_mrt and are more comfortable than deep ones ^[227][207][119,256]. However, the increased street width does not improve thermal comfort equally in all orientations during the winter ^[204][253]. Increasing the building height and reducing the street width initially increases the minimum and average daytime and night temperatures ^[228][273]. The lower solar angle and the higher H/W ratio of the courtyard increase the reflected long-wave buildings’ radiation and reduce heat dissipation. This effect can increase with the building’s height until the H/W ratio is 5.5 ^[209][258]. Above this ratio, less and less sun reaches the canyon bottom.

The relationship between the canyon geometry and the wind environment is quite complex. It affects both the wind speed ^[98][133] and the nature of the canyon air circulation ^[47][135][47,170] and can cause mechanical turbulence ^[176][124]. The importance of geometry becomes apparent when one intends to use wind cooling ^[223][270]. In open canyons, the convection cooling quality is more significant than in narrow and closed canyons ^[98][133]. However, this relationship can be modulated. When the wind directions are parallel to the canyon, it is possible to increase the wind speed by increasing the H/W ratio ^[98][229][133,274]. In this scenario even a deeper street canyon allows weaker wind loads to penetrate to the pedestrian level ^[230][275]. Where the predominant wind directions are transverse to the canyon, great depth is not suitable for pedestrian level ventilation ^[208][230][257,275]. The wind speed reduction in this configuration is caused by the formation of vortices between buildings ^[198][227] and the increased air displacement above the roofs ^[207][256]. In this case, the high canyon smoothness and symmetry work unfavorably ^[96][131]. Increasing the asymmetry may be helpful ^[193][245]. The step-down canyons always have a higher temperature under high and low wind speeds than step-up canyons ^[231][276]. In addition, reducing the buildings’ heights at intersections to two floors may increase the canyons’ wind speeds. At the same time, this may reduce temperatures by 0.2 °C ^[201][250]. Care should be taken in modulating the wind environment to keep the canyon temperature down. In hot conditions of warmer climate variety, the H/W reduction can lead to canyon convection heating and warm air flowing from outside the urban unit ^[98][223][133,270].

The canyon geometry is not affected only by the aspect ratio itself ^[185][240]. The canyon space also has important thermal geometric features, mainly its symmetry. There can be various openings, recesses, protrusions, and the building’s facade elements, e.g., balconies or arcades ^[96][98][131,133]. Opinions on the effect of spacing between buildings along the street are divided. Some studies confirm their significant impact on temperature ^[225][271], others are negligible ^[156][190]. This is probably related to the prevailing wind patterns and street orientation. In studies conducted in China, wider spacing between buildings along the street resulted in poorer thermal comfort for pedestrians ^[216][264]. In studies conducted in the USA, the ground surface temperature decreased with increasing mean distances between buildings ^[156][190]. It is possible that several canyon configuration parameters may dominate the effect of spacing on the temperature. The canyon’s length does not significantly affect the thermal comfort at the level of pedestrians [47], but its elongation results in a more evenly distributed temperature during the day ^[232][277]. The effect of urban canyon width on the UHI effect may be multiplied by the simultaneous increase in the percentage of road coverage ^[119][135][154,170]. Wider streets also result in more significant traffic, which may intensify the UHI effect caused by increased heat and exhaust emissions in the canyon ^[135][170].

2.4.2. Street Orientation

The street orientation affects the duration and intensity of solar radiation introduced into city canyons ^[120][168][155,202]. Therefore, its importance in modulating the canyon temperature is mainly influenced by the radiation intensity ^[219][222]. When a day is sunny, the orientation determines the air temperature and soil temperature. The orientation effect is neglected when the day is cloudy ^[233][234][120,278]. That relationship also occurs with regard to the seasons ^[97][132]. Canyon orientation is a parameter that produces intra-city thermal anomalies ^[120][235][155,226]. It affects thermal comfort ^{[152][174][216]}[186,208,264], including size and duration of pedestrian-level thermal peaks ^[205][254]. Moreover, it shows a strong linear correlation with the temperature of the ground surface ^[174][208], walls ^[213][261], and the intensity of AT or UHI ^[152][186]. Street orientation determines the average ground radiation intensity stronger than the average facade radiation intensity ^[234][278]. Depending on the direction of the prevailing winds, the canyon’s orientation may also increase or decrease its speed ^[168][202] and increase the humidity [48], which affects the way the temperature is percepted.

By ingerention in the shadow areas, the canyon’s area density differentiates the daily air temperature between streets oriented in different configurations ^[222][269]. The orientation influence weakens when a canyon’s built-up area density and the H/W ratio decrease ^[174][208]. In a narrow canyon, orientation is not correlated or is slightly correlated with canyon air temperature, both in summer and winter ^[97][132]. However, it contributes to lower wind speeds ^[180][235], which positively concerns PET in the winter ^[97][132]. In addition, increasing the direction variation in the narrow street may result in less radiation infiltration into the canyon, irrespective of the street main direction ^[207][256]. In a wide canyon, orientation entails significant variations in wall surface temperatures during all seasons ^[97][132].

The N–S direction provides the shortest period of solar radiation in canyons ^[171][205], and the shading percentages on horizontal surfaces vary very little over the year. Hence, shading in the streets along the N–S axis is much more favorable than on the streets along the E–W axis, both in summer and winter ^[226][236][272,279]. Although the mean T_mrt is similar in the N–S and E–W canyons ^[227][119], the number of T_mrt hours exceeds the threshold values for moderate and robust heat stress. Therefore, the stress may be higher in the N–S canyons than in the E–W canyons ^{[227][97][236]}[119,132,279]. In the N–S canyon, the opposite facades are equally shaded throughout the year, but the hourly temperature distribution of these walls varies. During the day, the western wall temperature rises faster. Still, the eastern wall, which has a higher solar load, shows greater amplitudes of daily temperatures and higher maximum temperatures ^[213][261]. The daily temperature amplitude of the east and west walls can be greater than that of the canyon air ^[213][261]. This relationship is visible above the fourth floor and on the ground floor (radiation from the ground) ^[237][280]. The lowest T_mrt can be observed in the shaded areas of the east-facing walls ^[238][281]. The temperature of these walls rises until 10 a.m. and then drops [48]. The subsequent increase in T_mrt starts on the west-facing wall around 3 p.m., but there is no heat stress until 5 p.m. From 8 p.m., there is a T_mrt decrease ^[239][282]. The spatial N–S canyon air distribution and the WE and E walls become much more uniform at nighttime ^[213][261]. The best thermal comfort conditions for streets along the N–S axis are found for medium and high H/W ratio values (0.8–3.0) ^{[226][236][240]}[272,279,283]. This way, it reduces the daily temperature amplitude and equalizes the differences in the diurnal temperature range (DTR) and maximum temperature of canyon walls ^[213][261]. For this reason, both the W and E canyon facades have more favorable conditions than the southern facades ^[236][279]. Increasing the H/W ratio significantly reduces cooling loads and increases heating loads in the canyon ^[241][284]. Increasing the spacing between buildings along the N–S street may lower T_mrt and improve pedestrians’ thermal comfort ^[216][264]. Moreover, the shortening of N–S streets contributes to arcaded streets cooling ^[240][283].

The E–W direction provides the most extended duration of direct sunlight in the canyon and courtyard ^[226][236][272,279], and it is considered a warmer thermal configuration ^[222][236][269,279]. Due to the northern exposure of the canyon wall, the E–W orientation is exposed to sunlight from early morning. The result is the earliest maximum surface temperature ^[208][257]. Depending on the season, high T_mrt only occurs for few hours, from 11 a.m. until approximately 3–5 p.m. ^[239][282], when it reaches its highest level. After this time, due to the solar radiation blocking, the absorbed heat is relatively quickly released into the surrounding environment ^[48][239][48,282]. However, it increases AT ^[208][226][257,272]; hence, thermal stress may be felt almost until 8 p.m. ^[239][282]. The increased H/W ratio for the E–W streets has no significant effect on the shading percentages ^[222][236][269,279]; therefore, it does not lower PET levels ^[219][222] and has little effect on daytime air temperatures ^[222][269]. On the E–W axis, the low H/W ratios (0.6–3.0) can therefore represent PET outside the comfort zone for most of the day ^[219][222]. With the H/W ratio above 3.0, it is possible to achieve a satisfactory daily thermal comfort level on the E–W streets ^[227][219][119,222]. A low H/W ratio (<0.6) is the most favorable in terms of solar gain in summer and winter for southern facades. As a result of short-wave and long-wave reflections, radiation emitted from the south-facing canyon walls can have an exceptionally high T_mrt ^[238][281], especially on low floors ^[208][257]. The T_mrt difference between the north and south walls can be 20 °C ^[227][119]. Increasing the H/W ratio on E–W streets reduces cooling loads and significantly increases heating loads ^[236][241][279,284]. Increasing the spacing between buildings along the E–W-oriented street increases cooling loads ^[241][284], and narrowing the spacing reduces T_mrt and improves PET ^[216][264]. Increasing the canyon’s length in the E–W direction extends the duration of solar radiation. It increases the street temperature ^[201][242][250,285], but in the case of arcaded streets, it improves their cooling ^[240][283].

The shading intensity of diagonal streets is similar in all their rotation configurations. It is between the N–S and E–W axis streets’ shading intensity ^[236][279]. SE–NW street canyons provide more shade, higher wind speed ^[216][264], and better pedestrian thermal comfort conditions than other scenarios ^[216][264]. The H/W ratio impact on the number of sunlight hours suddenly decreases for street orientation angles of less than 30°. In comparison, when the street orientation angle exceeds 60°, the H/W ratio impact increases rapidly. The H/W ratio increase in the NW–SE canyons considerably reduces their daily temperature amplitude ^[222][269]. Diagonal streets with a H/W ratio between 1.5 and 3.0 can provide satisfactory thermal comfort conditions for most of the day ^[242][285], similar to streets on the N–S axis ^[219][236][222,279]. For H/W ratios higher than 1.3, diagonal canyons are similar to the E–W axis streets in terms of solar access to building facades in winter ^[236][279]. A closer look at the facade’s thermal dependencies in diagonal configurations shows that the maximum temperatures for the NE facade appear in the early morning. They are lower than the SW facades, where the maximum temperatures appear around noon. The NE facade minimum temperatures in the late afternoon are higher than SW facades, where the minimum temperature is measured in the early morning ^[162][196]. The SSW facade has higher maximum temperatures than the NNE elements. The most significant differences observed between these directions are visible in the afternoon and the lowest in the early morning ^[237][280].

2.4.3. Canopy Properties

The trees and free-standing building elements’ presence in the canyon modifies its shadow patterns. During the day, this directly leads to a reduction in LST and PET ^[188][205][242,254]. At night, it increases its temperature by obstructing the long-wave radiation exit from the canyon ^[173][243][207,286]. This type additionally intensively reduces the wind speed at the pedestrian level ^[214][243][262,286]. The temperature level changes in street canyons associated with the trees’ presence depend on the tree crown-cover size ^[244][245][287,288], their planting density ^[193][245], their height ^[100][135], and solar exposure ^[220][267]. Small greenery elements in the canyon will only affect the local thermal conditions ^[242][285]. Due to more effective radiation shading and more substantial turbulent transport, tall trees have a more beneficial effect on the canyon temperature than small trees ^[100][135].

The city canyons’ geometry significantly modifies the trees’ thermal behavior in alleviating the street microclimate ^[243][244][286,287]. Due to changes in sun exposure, canyon shallowing and widening increase the trees’ cooling effect in the canyon. ^[216][244][264,287]. Research shows that high-tree-coverage streets can achieve better cooling quality when the H/W aspect ratio exceeds 0.67 ^[240][283]. For H/W aspect ratio = 1.0, increasing tree cover or reducing the spacing between trees may result in better tree shading and cooling during the day and a night temperature increase ^[242][285]. For H/W aspect ratio = 1.2 small and low-crown trees can generate a lower wind speed at the pedestrian level ^[243][286]. When the H/W aspect ratio = 2.0, both small-crowned and large-crowned trees may have the most significant air temperature cooling ability in daytime ^[225][243][271,286] and heating ability at night in the canyon ^[243][286]. When the H/W aspect ratio ≥ 3.0, the shadow cast by building walls can dominate the trees’ shading. Their influence on temperature reduction becomes less critical ^[243][286]. Dredging a canyon without increasing the number of trees reduces the cooling effect because the evapotranspiration cooling energy has to handle a larger air volume ^[174][220][208,267]. Adding trees in this situation could lower the temperature by as much as 4.5 °C on a summer afternoon ^[220][225][267,271]. For the high H/W aspect ratio, the diffusive planting of small-crowned trees favors street ventilation. It is a more preferable solution for summer cooling in these canyons than the higher density of trees and large-crowned trees ^[243][286]. This relationship is inversely proportional to the low H/W aspect ratio ^[100][135]. A large-canyon tree area in the canyon with a high H/W aspect ratio may adversely affect the tall building thermal load. Street albedo, enhanced by the color of tree leaves, can multiply the radiation reflections reaching the upper reaches of the canyon ^[188][242].

2.5. Design Parameters of Neighborhood Cluster Scale

As the scale increases, the relationship complexity between the elements that make up urban areas increases. At this hierarchy level, the high synergy that characterizes the ecosystem is noticeable. Allelopathic compounds, both antagonistic and non-antagonistic, can be found here. Understanding the urban unit as an interconnected network of abiotic and biotic objects allows reusearchers to grasp how the areas of coverage, density, and spatial configuration affect its temperatures.

2.5.1. Coverage Area

Covering with elements of the urbanized environment is one of the indicators of the intensity of development, showing a close relationship with the surface temperatures ^[246][247][289,290]. Impermeable surfaces in the neighborhood unit represented by buildings and roads have constant high LST and UHI ^[126][137][161,172], and modulation of the coverage parameter strongly influences their variability ^{[248][249][250][251]}[291,292,293,294]. The increase of paved surface coverage leads to LST and UHI increases ^[252][295]. Increasing the built-up area coverage by 1% may cause a rise of the UHI effect intensity up to 1.7% ^[135][170]. On the other hand, a 1% reduction may cause a decrease in the night-time near-surface air temperature by 0.1 °C ^[249][292]. Urbanization degree is proportional to the distance from the city center ^[153][187] and size-dependent urban area ^[137][172]. It is also inversely proportional to the greenery area ^[250][253][293,296]. Therefore, it seems logical that old cities are usually characterized by the highest concentration of impervious surfaces ^[101][136]. Large-scale architecture and infrastructure and a high percentage of commercial and industrial areas ^[190][135][122,170] have a high UHI correlation. It is worth mentioning that the modernistic districts from the 1950s–1960s are better in terms of the number of porous surfaces than the currently built districts and those from earlier centuries ^[179][234]. This is related to the Athens Charter implementation.

The building cover ratio in a given area significantly impacts the intensity of AT, LST, UHI, and PET ^[107][254][142,297]. As it increases, the degree of pedestrian-level ventilation decreases ^[255][298], and the LST and UHI intensity increase ^[256][299]. In China, with a 10% increase in building land cover within a 500 m radius, a daily T_max increase of approximately 3.4 °C was noted ^[152][186]. The correlation of building cover with temperature can be positive and negative ^[256][299] and may similarly affect the nighttime and daytime UHI ^[145][170][180,204]. These relations are distinguished by the type of land cover, building materials’ thermal properties, and exposure of the urbanized environment to solar radiation ^[126][161]. Increasing the building cover degree may reduce the maximum T_mrt during the daytime by reducing the canyon’s solar exposure ^[257][125]. A stronger positive correlation between building cover and temperature is noticed at night. That correlation results from the heat release after dusk from the housing estate thermal mass ^[251][258][294,300] and the anthropogenic heat trapped in the canyons ^[108][143]. The time at which the highest correlation is achieved depends on the temperature level on a given day and occurs later in the nights after hot days compared to the nights after cool days ^[183][238]. The influence of land cover on air temperature strongly correlates with built-up area density ^[94][129]. In a dense context, the percentage of built-up coverage has a greater impact on the increase in UHI than its density itself ^[201][250]. In this context, reducing buildings’ land cover would increase solar exposure for buildings and paved surfaces and thus affects the radiation and temperature increase ^[259][301]. However, paradoxically reducing the temperature in densely built-up areas is possible with a building cover increase ^[156][190]. There is, however, a limitation. The adjustable LST range decreases when the built-up area exceeds a certain critical threshold. This means the ability to mitigate high temperatures by adjusting the built-up area is also limited ^[147][182]. This ability is evidenced by the fact that, in some specific cases, increasing the land cover index in high-density regions may increase the temperature and UHI ^[94][129]. A possible reason for this is the reduced convection cooling at the pedestrian level ^[255][298]. In such a situation, it is necessary to reduce the coverage density to mitigate the UHI ^[94][129]. These dependencies may explain the lack of correlation between the building cover percentage and the UHI intensity in some studies ^[202][251].

The road coverage degree strongly correlates with the AT, UHI, and PET intensity ^[152][186]. Its increase within the city leads to a rise in LST ^[147][182][182,237] and enhances the UHI effect ^[151][185]. Due to the increased thermal mass, the duration of high air temperature is also extended ^[212][121], making the city’s microclimatic conditions more stable throughout the day. Unfortunately, this also contributes to the formation of the nighttime UHI. An increase in the road surface ratio causes a temperature increase ^[170][204]. That is especially true when the roads are highly congested—this increases the anthropogenic heat in the area ^[150][184].

Green area coverage is a significant predictor of elevated temperature for the neighboring unit scale ^[206][255], explaining up to 50% of the variability of intra-city temperatures ^[152][186]. There is a strong negative linear correlation between greenery coverage and the intensity of AT, UHI, PET ^{[260][261][262]}[214,302,303], LST ^[218][266], and buildings’ peak cooling loads ^[199][224]. There is also a positive correlation with soil permeability, urban spaces water capacity, ground surface shade percentage, and wind speed ^[133][168]. The surfaces may have a similar daily effect on the UHI ^[126][161], significantly reducing the peak air temperature and the UHI during the day ^[261][263][302,304] and at night ^[264][305]. In a study conducted in China, a 10% increase in green area coverage decreased the mean UHI by approximately 0.94 °C in a 250 m radius ^[152][186]. Green area coverage shows a positive relationship with the SVF. Increasing the exposure of green areas to direct solar compensation results in better plant conditions and greater efficiency of evaporative cooling. Due to the above, these cover types cool the space exceptionally well in summer and in warmer climate types ^[151][185].

The water object coverage percentage shows a strong negative linear correlation with the LST, AT, UHI, and PET intensity ^[152][186]. The cooling effect of water reservoirs also depends on their size, location, and wind direction ^[103][138]. For example, after reducing the lake’s surface in the built-up area of Wuhan by 130 km², the UHI intensity increased by 0.2 °C–0.4 °C ^[265][306].

2.5.2. Elements’ Density

The density of the urbanized area is an urban geometry parameter that significantly affects the minimum temperature, LST, and UHI intensity ^[183][238]. This parameter is the most complex, as increasing the built-up area’s density may have a negative ^[156][190] or a positive impact on the LST and UHI ^{[143][144][145]}[178,179,180]—depending on the climate type. Its importance is greater ^[163][197] in warmer regions than in cooler areas ^[156][190]. The urbanized area’s density is a hybrid parameter that describes the mutual synergistic relationship of other parameters, such as population density, anthropogenic heat, the vegetation ratio, building and road coverage, building height, and spatial configuration ^[149][120][118,155]. There is also a group of parameters such as the floor area ratio (FAR) related to density but primarily determining the building development volume. They significantly impact the thermal building mass and the shade patterns ^[120][155]. These parameters also negatively correlate with the LST and AT ^[266][307]. The influence of these aspects makes it challenging to infer one universal relation. The collective effect of all the elements influencing the density is the final density effect on the city temperatures ^[120][262][155,303].

The buildings and roads coverage density are mainly responsible for the density of land development. The influence of the building cover density on the ambient temperature rises with the increase of the built-up space radius ^[251][294]. The density is negatively correlated with the distance from the urban center ^[153][187]. The built-up area density parameter significantly influences the solar exposure of the housing estate ^[187][241]. As the built-up area density increases, the average SVF decreases ^[267][308]. A similar relationship occurs for the building’s volume ^[95][130], but it has a more substantial effect in already densely built-up areas ^[185][240]. The influence of these parameters on the air temperature is significant, but its distribution shows diurnal differences ^[261][302]. An increase in the buildings’ cover density and their volume can reduce the AT ^[187][263][241,304], the surface temperature of the lower canyon parts ^[117][268][152,217], and the heat stress at the pedestrian and UHI levels during the sunny hours of the day ^[148][263][183,304]. Those are easy to see in the example of squares. Their temperature can be even 18 °C higher than that of dense built-up areas ^[259][301]. During a cloudy day, densely built-up areas remain warmer, but the difference in T_mrt between dense and scarce contexts decreases because of the reduced radiation intensity ^[259][301]. The increase of built-up areas’ volume and density can reduce the wind speed and the convective heat transfer. That increases the thermal mass, thus having a more substantial impact on the radial energy absorption and lower SVF leading to the thermal trap effect ^[180][187][235,241]. That leads to the growth of wall, street, and air temperatures in canyons and causes the nighttime UHI ^[198][187][227,241]. These dependencies are appropriate for warm seasons. In colder seasons, a temperature rise associated with high built-up areas’ density may be desirable to provide better thermal comfort ^[227][186][119,123]. The density is related to the anthropogenic heat emission ^[120][155]. A higher proportion of building walls means more anthropogenic heat and significantly influences the LST and UHI ^[135][145][170,180]. Depending on the season and context, it may positively or negatively affect thermal comfort and the environment. The increase in the built-up areas’ density reduces the annual energy demand ^[197][269][249,309] but makes the heating load more sensitive to solar radiation ^[183][266][238,307]. Moreover, intensified anthropogenic heat emitted from buildings may modify the thermal response of other city geometry parameters ^[270][310].

There is a particular paradox for the built-up areas’ density parameter—the total area of roofs, facades, and streets changes with an increase or a decrease of those areas ^[191][268][126,217]. That means one can increase the surface temperature by increasing or decreasing the building density. By increasing the density, rwesearchers increase the fractional area of the roofs. As roofs get hotter more easily than streets, the UHI increases as the fractional roof cover increases ^[191][126]. On the other hand, rwesearchers increase the sun exposure of road surfaces by reducing the density. Since roads are associated with a low albedo, they strongly influence UHI ^[117][192][152,244]. The solution to this situation is to manipulate the density by changing the height. In this way, it is possible to increase the shadow area without changing the fractional roof cover ^[191][126] and reduce the surface temperature as a result ^[192][244]. At a constant high density, the spatial configuration of blocks has a dominant effect on surface temperatures ^[144][179]. The increase in density for interlaced and linear block systems reduces the daily UHI ^[115][150], and the simultaneous increase in height and building density increases the frontal area index (FAI) ^[186][255][123,298] and night UHI effect ^[104][139]. This relationship can be seen in Hong Kong, where a 10% density and height reduction resulted in a 10% UHI reduction ^[262][303]. This shows how the density and configuration of construction objects on the estate are strongly correlated.

The road surface density depends on the urbanization degree of the area ^[253][296]. The LST and UHI are positively correlated with the coverage density of roads, parking lots, and sidewalks ^[271][272][311,312]. This is directly influenced by the thermophysical properties of the materials used for the construction ^[135][170]. However, the road connections density alone is insufficient to understand the relationship between city texture and the UHI effect. Other factors, such as road connection size and capacity, are also important determinants of the UHI effect intensity ^[135][170]. The areas closer to the functional city center are characterized by more significant node and communication link density, making these areas warmer than the rest ^[151][155][185,189]. The UHI effect is negatively affected by a higher transit route density. The reason may be trivial—faster vehicle traffic and the absence of congestion reduce the amount of heat, pollutants, and greenhouse gases discharged into the canyon ^[135][170]. Scattered and anisometric settlements are associated with more traffic, affecting higher emissions ^[248][273][291,313]. Therefore, increasing the built-up areas’ density can help deal with anthropogenic heat emission problems ^[135][170].

The vegetation density is determined by its height, cover type, and greenery amount. It depends on the growing season during a year, too. In addition to shade patterns, the density of the vegetation models the airflow ^[180][274][235,314]. Increasing the greenery density within a green area reduces not only the daily LST ^[136][137][171,172] but also the wind speed ^[180][274][235,314] and the convective cooling effect. Increasing the vegetation patch density reduces temperature ^[156][252][190,295], especially at night ^[115][228][150,273]. Increasing the edge density of these patches reduces the ambient LST by having a larger heatsink contact area with the paved area and increased energy flow between them ^[156][190]. The vegetation cover density is negatively correlated with the built-up areas’ density ^[120][145][155,180]. The more dense built-up area ensures greater efficiency of green roofs and green facades ^[148][183]. An increase in built-up area density increases green infrastructure’s impact on the LST changes ^[144][179]. However, after exceeding a certain density threshold, other variables related to the housing estate spatial configuration, e.g., the heights of buildings, begin to significantly impact the lowering of temperature by vegetation ^[270][310].

2.5.3. Spatial Pattern

The 2D and 3D composition of the housing estate, which consists of the mutual spatial relationship and the configuration of green areas, water objects, streets, and blocks of flats, is significantly related to the LST and UHI ^[168][174][202,208].

The spatial distribution of the impervious surface within the estate, significantly when fragmented and insulated, can increase the LST ^[186][123]. On the other hand, increasing hardened surfaces with complex shapes sometimes cools the study area because of a more extensive contact between built-up areas and vegetation ^[156][190]. However, the relationship emergence condition is the presence of green spaces in the vicinity. The permeable surface system of green areas may play an essential role during intense solar radiation [54]. A compact green-space layout is generally accompanied by more significant amounts of permeable land ^[179][234]. An evenly distributed complex shape pattern of trees and its high dispersion can provide more shade and enhance energy exchange between green spaces and built-up areas while reducing the LST ^[156][179][190,234]. Moreover, low plants’ scattered distribution and irregular shapes can help mitigate the UHI effect ^[190][122]. In the case of water objects located within the estate, their cooling effect depends on their location in the urbanized area and wind directions ^[103][138].

The layout of buildings may be the most influential factor in changing the thermal environment during the day [54]. Different urban designs affect the insolation and ventilation patterns of the housing estate diversely ^[275][315] and thus determine their thermal efficiency during the day and at night. Clustered blocks of flats raise the AT and LST ^[212][190][121,122] because of the higher roof surface density and a lower greenery percentage. Therefore, the dispersed distribution of buildings facilitates the LST reduction ^[190][122], allowing the introduction of other mitigation features into the estate. It has been proven that the surface temperature and thermal comfort outside free-standing blocks of flats are slightly influenced by street orientation and H/W. The vegetation has a stronger influence ^[174][208]. Thus, in the absence of sufficient greenery, a higher open-space factor increases the UHI effect ^[206][272][255,312] and T_mrt because of increased direct sunlight exposure ^[238][281]. Regular block of flats patterns may facilitate air circulation in the canyon ^[135][170]. Higher irregularity in the layout of buildings, and the complexity of their shapes and diversity, affects wind patterns and also increases LST and UHI levels through higher solar exposure ^[156][259][190,301].

The dense linear apartment block systems that make up the city canyon have the highest UHI level ^[276][316] because they provide a long solar radiation duration for the estate ^[131][169][166,203] in the case of incorrect spatial orientation. By controlling the direction of the linear form, it is possible to influence both passive cooling and heating of the blocks of flats ^[131][166]. This way, the apartment block orientation as a whole has a significant impact on energy consumption, especially for high linear systems ^[269][309]. The windward buildings are blocking the airflow ^[229][274]. Therefore, adjusting the building blocks’ layout and orientation concerning the prevailing wind directions is essential. Proper arrangement of buildings can positively affect urban porosity, create wind corridors, strengthen the narrowing effect in the ventilation corridor, and increase wind speed ^[229][255][274,298] and thus can shorten the duration of high air temperatures in the area ^[141][176]. On the other hand, inappropriate building arrangements and orientation, increased building density, asymmetry, and the number of alleys may block the airflow, resulting in increased temperature at night ^[96][229][131,274]. For example, suppose the binary system is perpendicular to the wind directions. In that case, its ventilation and thermal comfort in summer can be improved by increasing the space between buildings along the road and creating small green areas ^[204][255][253,298]. However, this treatment will lower thermal comfort in winter ^[204][253]. An example of traditional Greek city development shows how to eliminate this problem by using small linear blocks of flats and a complicated street pattern ^[97][132].

Although building arrangements may negatively affect air circulation, they provide more shade ^[229][274]. Therefore, the closed layout of the apartment blocks is characterized by a more comfortable thermal environment during the day ^[229][269][274,309], provided that the H/W ratio is high. However, the low SVF associated with this system may result in poor radiant cooling capacity and thus increase the nighttime temperature ^[264][265][305,306]. The low H/W ratio of the closed apartment blocks layout increases its LST ^[264][305]. Nevertheless, the natural compactness of the apartment blocks with a peripheral configuration makes these systems more suitable in an urban environment than linear systems ^[115][150].

The average building height significantly affects the ambient temperature ^[122][163][157,197] and wind patterns ^[169][187][203,241]. It also determines the shading value in densely populated urban areas ^{[255][259][267]}[298,301,308]. Therefore, greater building heights indicate a lower AT and a better outdoor comfort level ^[268][261][217,302]. Building height can influence cooling efficiency through its relationship to the UHI formation and propagation. Up to a certain height, the building’s height strongly influences the evapotranspiration cooling and the shade provision by the vegetation in the estate ^[143][178]. When buildings are lower than trees, the buildings do not obstruct the cool horizontal airflow and do not reduce the cooling capacity ^[117][230][152,275]. As the buildings’ heights increase, the average wind speed above buildings increases ^[217][265]. However, it harms the ventilation at the pedestrian level ^[230][275] and increases the internal heat gain by thermal trapping ^[104][139]—especially in public and residential areas ^[190][122]. The buildings’ heights increase the urban unit’s FAI, thus blocking the natural wind corridors ^[131][166]. Due to all these unfavorable relationships in the case of high buildings, the influence of ventilation significance on city air temperatures rises [44]. It is worth introducing a gradual lowering of buildings in the direction of prevailing winds to reduce the FAI ^[131][166]. It is also possible to modulate the urban roughness, determined by the differences in the buildings’ heights. This affects both the shadow distribution and the wind speed and direction in urban areas ^[141][176]. Hence, it may also impact the UHI effect ^[191][163][126,197]. Identical building heights lead to a stronger vertical turbulent movement favorable for pedestrian-level ventilation ^[230][275] and increase the albedo ^[268][217]. However, an increase in the difference in building height increases the shadow area and the aerodynamic roughness. That affects convective district cooling positively, especially at night ^[191][126]. It should be remembered that high surface roughness values make it challenging for adequate wind ventilation ^[141][176] through the vortices generated. It can be concluded that the benefits of this parameter modulation in an urban environment can be obtained by carefully mixing the building heights ^[115][131][150,166].

For the elongated apartment blocks’ shapes and increased H/W ratios, block orientation becomes a vital parameter influencing temperature rise and energy consumption ^{[184][213][269]}. Buildings facing south have the highest daytime temperatures for the northern hemisphere because of insolation ^[229]. Although these are suitable conditions for a low-energy urban form[239, this solution’s balance of losses and profits loses to other savings forms ^[269].

3. Conclusions and Recommendations for Future Research

3.1. Guidelines for Microclimatic City Design

The complete elimination of the UHI effect seems impossible in the current urbanization form. However261, it is possible to improve the thermal city environment309]. Following some universal guidelines to improve the existing urban tissue and create the new one makes it possible to influence the entire city PET and UHI effect significantly. Only holistically planned preventive measures and interventions can make the city climate-neutral and make it resistant to the rapidly approaching climate change.

The guidelines based on the discussion chapter show a strong shifting vector of changing the current urban environment toward de-urbanization. This rather old concept returns in the climate change context of the 21st century and sheds a new “green light” on the city’s appearance in the future. As in the de-urbanization concept, the microclimatic design reduces and distributes rationally the built-up areas’ coverage and the density and height of the buildings, concerning the natural landscape and well-known weather patterns. The main principle of this design trend is to adjust the solar intensity as the leading energy carrier to the needs of the specific temperate climate type. In warmer varieties, less radiation will be needed to provide the same conditions as in colder regions. When planning the radiation balance in the urban areas, one cannot forget the vegetation duration appropriate for each space. It is necessary to consider both in situ and ex situ factors in microclimatic design because the city’s energy flow remains open ^[116]. The designers should not rely on wind cooling a city with a poor wind environment and avoid creating large and compact urbanized areas ^[273]. They should also intertwine building areas with forests and lakes, develop urbanized areas toward greenery, and create decentralized systems with highly amorphous, organic shapes. Inside the designed cities, they should diversify the environment, improve the distribution of buildings, roads, and BGI elements ^[133][137], reduce the amount of industrial and commercial areas, limit road transport (by planning ring roads and smart transport links), and promote a zero-emission life ^[90].

3.1.1. Material

When creldings fating, introducing, or modifying city materials, one should pay particular attention to some of their properties. It is recommended to use materials with increased diffusivity, transmission, and a reduced speed of radiation absorption ^[165]. It is necessary to increase pavement permeability and t south have the materials’ porosity ^[172]. Permeability can be increased by using various types of vegetation surfaces. Cities’ poor ventilation requires the extension of road material porosity ^[147]. When night ventilation is provided at a high level, materials with a high heat capacity, such as grass, stabilized sand, and granite, can be used ^[159]. In anst day other case, it is recommended to reduce the heat capacity and thermal mass using, for example, reclaimed asphalt pavement (RAP) materials ^[172] or limiting the thickness of the foime temperatundation layers ^[165]. The use of asphalt and concrete as a surface is strongly discouraged ^[159]. T for thermal insulation of building partitions should be increased, especially in colder climate zones.

Tnorthern he urban areas’ albedo should be varied ^[154]. Retroreflective materials can be safely used on all hard surfaces and contexts ^{[277][278][279]}. The guidelines for high albedo materials are despendent on the area density. In the densely built-up areas, the albedo should not be increased. In this situation, attention should also be paid to the leaf’s albedo. It is safe to use high albedo materials only on the highest roofs. Increasing the albedo on each surface for low H/W ratio values is desirable. When increasing the albedo, the PCM and RR coatings, white paints and membranes, bituminous surfaces modified with oxides, colored roof tiles ^[280], and resin surfaces with exposed light-colored aggregate can be used ^[164]re because of insolation [274]. It is necessary to ensure high color durability. Resin surfaces have significant limitations in use. It is recommended to use them as pavement surfaces for pedestrian use ^[178].

3.1.2. Building

In the case of buildings’ geometries, it is necessary to reduce both their floor and wall areas. This is essential for commercial and residential buildings. Facilities must take on a compact, regular, and simple form ^[185]. The best solution would be to use spherical, domed shapes ^[281]. In addition, the buildings’ heights should not be significant. It is suggested their heights should be in the range of 20–50 m ^[217] and much lower for buildings located near main roads ^[251]. The best solution is to place buildings on high pillars ^[255].

Increasing the buildings’ energy efficiency and reducing their thermal emissions is necessary ^[163][195]. For this purpose, one can use location-related, orientation, greenery, building covering materials, and cooling or heating solutions. One should also avoid using the A/C. If this is not possible, it is recommended to use A/C and heating in energy-saving mode and timers ^[100]. Instead, it is possible to thouse ground heat exchangers and water cooling ^[142]. It is also worth using oth ther low-energy solutions. For example, RR materials are desirable to limit A/C usage ^[282]. A se are suimilar task is fulfilled by increasing the building albedo ^[163][199] andble applying better insulation ^[163]. Green roofs effectively reduce cooling and heating loads ^[106][143]. In addindition, buildings should be reasonably located in relation to chiaroscuro and wind corridors, depending on the desired effect.

3.1.3. Street Canyon

Propfortions, orientation, and greenery patterns should be selected appropriately ^[174][221]. The designer’s primary goal should be to obtain a loa low solar exposure of the canyon ^[283], but the optimal value for a given climatic zone should be considered ^[207][234].

The most crucial guideline for canyon geometry is to reduce its height while increasing the tree crown cover ^[234][284]. In hotter types of climate where tree shade cannot be introduced, canyon height can be increased ^[209]. Limiting the canyons’ heights is especially important in the case of canyons that are transverse to the main winds. In this case, it is worth increasing a building’s height slightly on the leeward side and increasing the spacing between the buildings along the road. The depth of the street canyon oriented parallel to the airflow may be higher, but it is worth increasing their horizontal asymmetry and smoothness. Nevertheless, it is worth reducing the heights of buildings in the vicinity of intersections. Reducing the road surface area or increasing shading solutions is another critical issue. The answer is to use traffic circles, reducing traffic congestion and the anthropogenic heat increase.

There is no single preferred city canyon orientation. The desired direction varies with the climate zone and the need for sunlight or shade ^[120]. The best general solution seems to be the use of irregular road runs, in which the orientation changes from time to time, creating a balanced thermal environment. In addition, it is worth using diagonal streets ^[216][285]. In warmer temperate climates, the orientation of the roads along the N–S axis is suggested as the most appropriate ^[204][238]. In this situation, it is worth ensuring a considerable height of the canyon ^[219][285]. One can also reasonably increase the streets’ width to provide better comfort of their use ^[283]. It is also good to extend the road length unless it is an arcaded street. Streets oriented on the E–W axis are suggested as optimal for colder regions ^[242]. They may also be optimal in warmer climate zones, but only if the canyon is deep ^[240][285]. In that case, the canyon’s length should be shortened ^[201], and the spaces between buildings along the street should be increased.

Shaders (artificial or natural) in canyon space are generally considered a good solution ^[214][222]. A tree crown cover is preferable to artificial shaders for many ecological and microclimatical reasons ^[101]. The trees should be as tall as possible to create a lot of shade and ultimately cover most of the street. However, their use legitimacy should be considered, and the optimal tree species should be selected for the given canyon openness and orientation ^[238][244]. It is necessary to introduce trees into vast canyons ^[202][240]. In wide canyons, dense canopy cover and compact artificial shaders should be used. It is not recommended to introduce many trees into high canyons, and their canopy cover should be sparse ^[100][243]. Shading with greenery, arcades, or artificial shaders should be used to avoid the E–W-oriented streets overheating ^[210][240].

3.1.4. Neighborhood

During the construction of new neighborhood units, it is recommended to generallergy reduce the intensity of urban developmentrban ^[256] and manipulate the parameters of the spatial configuration to create the most diverse landscape. In this way, certain areas will lower daytime temperatures, and others will reduce nighttime temperatures, and the whole will ensure correct cross-ventilation ^[116]. It might seem the best development strategy for an rm, this solurbanized area for its microclimate is the foundation of skyscrapers surrounded by greenery ^[286]. However, the efficient creation of an optimal thermal environment is also possible in other already existing conditions.

The main goal of microclimatic design on the city scale is to reduce the urban surfaces cover (impermeable) in favor of natural ones (pervious surfaces). The buildings and roads should be reduced, thus increasing the fraction of the green spaces and water object areas ^[252][258]. That is especially important in the context of very intensively built-up areas ^[201][286]. It is sometimes enough to decanalize rivers hidden underground during intense urbanization to increase the number of water objects.

Reducing the road and built-up area densities and volumes to increase the BGI coverage density is generally required to improve the urban climate ^[258][287]. The legitimacy of modulating the built-up area density depends on the climate type and the possibility of introducing tall greenery. For this reason, in warmer temperate climate zones, it will be justified to increase the built-up area density to ensure better thermal comfort in the urbanized area ^[207]. The transit road density may be increased, but only when accompanied by shading trees. It is not recommended to excessively increase the tree-planting density in green areas; instead, tree species with the desired parameters should be selected appropriately for the available space ^[179].

One should reasonably manipulate the area density by distributing the buildings horizontally and vertically ^[234]. A desirable solution is to lower the buildingsion’ heights and carefully differentiate the buildings’ sizes in apartment block areas ^[131][234], especially those with high buildibalang density ^[94][141]. In that case, the climate variation must be considered. In warmer regions, it will be preferable to increase the buildings’ heights to increase density ^[190] and d of lossecrease the heights of south-facing apartment blocks. Moreover, to maintain the same built-up areas density, it is possible to reduce the coverage area with low buildings in favor of tall buildings ^[137]. An essen and profitial aspect of creating a housing estate microclimate is to ensure even wind access throughout its area ^[141]. F loses to or this purpose, tall buildings must not be located on the neighborhood units’ outskirts from the prevailing wind’s side ^[131]. The same principle applies to tall buildings near water objects and green areas. The buildings’ heights may increase with the distance from the boundary and cooling objects. Neer savertheless, it is worth adjusting the heights of the tallest buildings so that the roofs are under the tree crowns. The designers should modulate the buildings’ sizes to allow the wind and humid airflow to penetrate the housing estate’s interior ^[251]ngs forms [309]. They should plan uniform and compact green areas that are spatially interconnected ^[179]. In high-density built-up areas, one should create irregular green areas and fill any free space with them ^[147]. Green roofs and facades are recommended ^[107]. Even distribution of the tree crowns in the housing estate area should cover as much area as possible ^[54][251]. Water objects should be located from the windward side. It is crucial to not canalize streams and rivers on a given site ^[251].

The relationships of the shape, length of apartment blocks, and spaces between buildings should be rationally planned, considering the creation of insolation areas and wind corridors ^[229]. The best buildings’ covering pattern is dispersed distribution in the neighborhood greenery ^[54][147]. This principle also applies to the housing estate layout design ^[251], especially if the surroundings are excessively dense built-up areas ^[212]. It may also be helpful to use a variable apartment building configuration ^[212]. One should avoid building an estate as a closed structure of excessive depth ^[54]. Instead, it is better to use average building heights, which let the sunlight reach the bottom of the courtyard. Furthermore, designers should orient regular patterns relatively parallel to the prevailing wind directions while controlling the rotation relative to the optimal solar exhibition. Finally, free spaces between buildings appropriately oriented to the wind corridors are recommended.

3.2. Vision of Climate-Resilient and Climate-Neutral Cities

The urbanization principles need to be changed to eliminate the UHI effect and thus ensure optimal PET comfort in the city. In addition, the microclimatic approach to housing estate design may affect the scale of the changes. Still, the vision of a future temperature increase in a temperate climate zone requires researchers and designers to brainstorm and create ideas for preventing a simulated crisis.

According to the ecosystem pulse theory, each civilization’s violent disintegration is associated with an excessive complexity increase ^[288]. Research clearly shows that urban development and uncontrolled spatial configuration exacerbate the temperature difference between urban and non-urbanized areas. Vegetation is essential to achieving ecosystem climax, and progressive surface reduction leads to climate anomalies and crisis deepening.

In the contemporary understanding of urban planning, solar exposure is the most critical factor influencing the overheating of urbanized areas. Of course, there are more reasons. Still, using already known smaller-scale solutions, solar exposure can be transformed into something desirable together with the underground placement of the building. With this change of approach, the problem becomes the solution. Earthship-type buildings show that construction can be energy-saving and reduce emissions to virtually zero. In that case, the large thermal mass of buildings also becomes an advantage, and its internal microclimate is easier to control because of the constant lower temperatures prevailing underground ^[289]. Despite the modern name, life underground does not bear the hallmarks of modern times. Since prehistory, people worldwide have lived in earthworks ^[290][291] and archaeological research shows that even entire cities can be created this way ^[292].

The vision of the underground city seems abstract, but only in this way is it possible to ensure a parallel, undisturbed existence of nature and architecture. Building cities underground does not seem to be technically impossible, and that concept appears from time to time in urban planning and architecture ^[293]. The earthscraper is a skyscraper, only upside-down—instead of rising, the earthscraper is built deep into the ground. Building heating requirements will force designers working in the field of temperate climate area to change the apartment blocks’ spatial patterns, taking inspiration from warmer climates ^[169][294]. Creating neighborhoods covered with the earthscrapers and Earthship-type buildings surrounded by greenery, water features, and underground public transport can effectively solve the upcoming climate challenge.