1. Introduction
Numerous negative manifestations of climate change, such as the changes in humidity, clouds, rain patterns, the strength and frequency of weather events (fog, snow, storms), and the damage done by weather [
1], as well as the increase of air, ground and ocean temperature, being the direct response to global warming, progressively affect local and global environment. To restrain climate change it would be necessary to add ecosystem services to world business [
2]. “Urban areas are especially vulnerable to high temperature, which will intensify in the future due to climate change” [
3] (p. 1). Climate change, its local manifestations, and the features of a city itself together contribute to the occurrence of different negative alterations and phenomena such as the intensification of the urban heat island (UHI). UHI effect is a phenomenon where significant temperature difference between inner micro-climates of a city and their neighboring micro-climates can be perceived [
4]. Increased ambience temperatures deteriorate the physical well-being of a city’s population, as a result of thermoregulatory system damage induced by heat stress in the form of heat syncope, thermal exhaustion, cardiovascular stress, cardiorespiratory diseases, and heat stroke [
4].
The UHI effect in cities and its mitigation measures is the focus of many studies. According to Emmanuel and Loconsole [
5], even when urban growth has subsided, the local warming that results from urban morphology (increased built cover, anthropogenic heat generation, pollution, lack of vegetation) generates local heat islands. O’Malley et al. [
6] demonstrate that building form, orientation, and layout are among the most effective UHI mitigation strategies. Harnessing natural wind patterns in designing the layout of buildings and presence of water in the form of urban water bodies, such as ponds, are also suggested to reduce temperature. Furthermore, the use of high-albedo materials (light-colored paving tiles) is suggested for UHI mitigation [
7]. Materials for pavements with low albedo, like asphalt, brick and stone, intensify the UHI phenomenon [
8]. The reduction of surface temperatures of materials directly mitigates negative UHI effects. Increasing the albedo of the urban surfaces and planting trees could effectively mitigate the UHI phenomenon [
9]. Huang and Ye [
10] demonstrate the significance of vegetation in urban centers on the case study of Beijing. Urban vegetation and urban forest play an important role in decreasing LST (land surface temperature). “The apparent influence of urban vegetation and urban forest on LST varies with the spatial resolution of the imagery, and peaks at the resolutions ranging from 90 m to 120 m (ibidem)”. Vegetation makes the environment cooler through evaporation from the Earth’s surface and transpiration of the vegetation [
8]. The UHI effect is mitigated by grass, shrubs (e.g., hedges of a height of 1.5 m), and trees of 5–10 m in height with dense crowns [
6]. Oke’s model [
11] approaches the heat island phenomenon both vertically and horizontally. According to the vertical scale, he defines different types as: Air UHI (Urban Canopy Layer UCL, and Urban Boundary Layer UBL), Surface UHI, and Subsurface UHI. The UCL encompasses the urban cover layer below the average height of buildings [
12]. Discovering the microscale in urban areas can be clarified through the relationship between urban form, roofing materials, and UHI (including air and surface characteristics), with a particular reference on the impact to the morphology of a housing area.
Today, cities all over the world encounter the increase of UHI intensity, and so is the case with the City of Ljubljana, the largest urban area in Slovenia, where 13.87% of total country’s population reside (in 2016 Slovenia had a population of 2,064,188) [
13]. The City of Ljubljana is located 46°03′20″ N/14°30′30″ E, at 298 m altitude, and has a total area of 274.99 km
2 [
8], with 287,218 inhabitants [
13]; the population density is 1044.4 inhabit./km
2 [
13]. The climate in Ljubljana is characterized by the transition between Mediterranean and Continental climates, with moderately cold winters (with temperatures below 3.9 °C [
13]) and warm summers (with temperature up to 25 °C [
14] (pp. 5–6)). During the winter months, the city area experiences the temperature inversion (often with heavy fog formation) [
15]. The average annual temperature, in 2014, was 12.6 °C and annual precipitation was 1850.5 mm [
16]. In Ljubljana, the average temperature in June 2016 was 19.9 °C, which is 0.8 °C above the long-term average (of the period 1981–2010) and within the limits of normal variability [
14] (p. 6). The average soil temperature at a depth of 2 cm for June 2015 was 22.6 °C (at a depth of 10 cm and 100 cm it was 22.3 °C and 17.8 °C, respectively); for July 2015 it was 26.3 °C (at a depth of 10 cm and 100 cm it was 25.9 °C and 21.3 °C, respectively) [
16]. The wind direction for June (in 2015) prevailed from SSW or W (at 2:00 p.m. with an average speed of 2.5 km/h; and at a 24-h average with an average speed of 1.2 km/h) [
16].
UHI of Ljubljana is distinct, affected by different factors such as green areas, location, land use, types of building and roofing materials, layout of buildings, their energy efficiency, and other; some of Ljubljana’s districts are constantly warmer that others, and specific areas are definitive hot spots [
17] (pp. 328–330). Referring to Komac [
17], there are significant differences between the UHI index for urban (inner-city Ljubljana Bežigrad station, 46°07′ N, 14°52′ E, 299 m a.s.l.) and rural (rural station Brnik, 46°22′ N, 14°22′ E, 364 m a.s.l.) areas of Ljubljana [
17]. The UHI intensity-cycle (calculated by Komac [
17] for the period from 20 to 26 July 2011) of daily heating/nocturnal cooling reached the third highest value in the selected period (4.95 K) [
17]. The wind varied from calm in the night/early morning time to having a top speed of around 2–3 m/s in the afternoon (both stations); air humidity was the lowest at that time (both stations). The weak SW wind and low humidity caused low night-time temperatures, especially at the rural station where the UHI intensity rose by 0.5–2 K [
17] (pp. 363–365).
Nonetheless, a major part of the City of Ljubljana (
Figure 1) is currently not intensively affected by the UHI phenomenon (
Figure 2). In general, temperature variations in urban areas with a low UHI effect have not been studied sufficiently in the literature. The greatest attention is placed on the studies proving its existence and proposing methodologies for mitigating high UHI intensity. Thus the paper aims to address the following research questions: Is there a relationship between urban climate concentration and urban morphology in residential areas with low UHI, and if so, what are the differences in locations with similar parameters? Additionally, how to identify relevant research parameters when there is a multitude of indicators available?
Following the adoption of the master plan of urban development of Ljubljana in January 1966, there was an expansion of housing construction, which provided for the development in a shape of a five-pointed star whose vertices expand along the main access roads to the urban center. Housing development focused on two categories: neighborhoods with apartment blocks for concentration of large-scale population and residential areas on the outskirts with low buildings, corresponding, in structure, more to rural rather than urban areas. When studying the quality of the living environment in terms of impacts of temperature on UHI creation, we were mostly interested in the neighborhoods considered as high-quality products of Slovenian urban design and architecture. “In many cases the planning of residential buildings is no longer based on the fundamental elements which helped form the types of residential buildings (users’ basic needs, functional processes which dictate organization and gabarits of buildings and external spaces, location characteristics, etc.), but on other criteria (economic feasibility study, analysis of residents’ purchase power, allowed utilization of land for construction, etc.) which often fail to consider the complexity of programmes and spatial design of residential buildings” [
18] (p. 138). On the other hand, citizen science and crowdsourced information offer new insights into the urban parameters and allow for collection of “large datasets [...] that would otherwise not be possible” [
19] (p. 2). This also creates new, unexploited opportunities for the City of Ljubljana.
We limited our study to the period of modernist neighborhoods (1960–1985) and some contemporary examples built after 2000.
Figure 3 shows that most of these areas had lower UHI (up to a maximum of 29.5 °C—a parameter defined by Pogačar [
20] as a threshold value for detecting heat waves), which means that urban design of these neighborhoods, both modernist and contemporary, included environmental parameters which favorably affect the comfort of living. We investigated the relationship between urban morphology and overheating in terms of Oke’s [
11] vertical scale: Air UHI, Surface UHI, and Subsurface UHI.