Urban environment as a factor in the formation of the area microclimate

. The growth of the leading countries’economies leads to the rapid development of megacities and changes in urban areas. Currently, more and more modern high-rise civil buildings are being built, resulting in an increasing density of urban areas and the activity of transport infrastructure. This causes major changes in the structure of urban development, which leads to a change in aeration and microclimatic conditions in urban areas. Solar radiation plays an active role in shaping the microclimate of the urban space, which aggravates the heat and wind regime of the urban environment. This is especially acute in the southern cities with hot climatic conditions. The article analyzes modern urban areas and factors affecting their microclimate. The role of temperature inversions in the formation of microclimatic conditions is indicated. Using the example of the yard space in Dushanbe, measurements and field studies were carried out; graphs of the climatic indicators of the urban area were plotted. The role of solar radiation in the formation of convective flows and microclimatic conditions of urban space is discussed.


Introduction
A modern city is a complex system of interconnected urban planning and infrastructure facilities. In the city, buildings and structures that form this system, serviced by engineering and transport infrastructure, form the urban environment. The urban environment is an artificially created habitat for citizens within the boundaries of their residence and activities. In connection with urbanization, modern cities are developing at an active pace. At the same time, urban space is developing, increasing the number and density of civil facilities with a developed engineering infrastructure. An increase in the building density of urban areas and the development of transport infrastructure causes a change in the microclimatic parameters of urban space. A modern city dweller, in the course of his active urban life, often or periodically staying in the urban environment, feels these changes..
Despite the fact that the main goal of the urban development is to transform the public urban space in such a way that it meets the requirements of comfort, modern urban development negatively affects the biosphere. As a result of the high density of modern urban spaces, the aerodynamic, temperature and environmental conditions of the environment are changing [1].
Thus, urbanization changes the modern urban environment by increasing the density of urban buildings, developing transport and engineering infrastructure. As a result of this, the population density and their increased activity affect the existing climate of the area and the environment. In this regard, the issue of forming the climate of a modern city and the microclimate of urban spaces is becoming increasingly important.

Materials and Methods
It is known that solar radiation, being a climate-forming factor, affects the heat and wind conditions of the area. The intensity of exposure to solar radiation forms a climate that is characteristic of a certain geographical area. Depending on the geographical location, large metropolitan areas are in certain climatic conditions. Large regions, megacities and urban spaces are characterized by macroclimatic, mesoclimatic and microclimatic natural phenomena.
Macroclimatic natural phenomena are the climatic phenomena of a large geographical landscape. An example is the climatic conditions characteristic of the territory of Siberia, the republics of Transcaucasia, and the republics of Central Asia. People are unable to regulate andcontrol macroclimatic phenomena.
Mesoclimatic natural phenomena are climatic changes intermediate in scale between macroclimatic and microclimatic phenomena. An example is the climatic conditions characteristic of cities. By creating cities with high-rise buildings, artificial paving, and areas with artificial surfaces, a person can influence mesoclimatic phenomena.
Microclimatic natural phenomena are climatic changes occurring on a small scale of specific areas, i.e. on the streets of cities, sites, courtyards between houses, premises, on the slopes of mountains, etc.
These phenomena play a decisive role not only in the formation of natural and climatic factors of the area, but also in changing the environment of the areas of human activity [2,3,4].
Thus, urban spaces are characterized by mesoclimatic and microclimatic phenomena of nature, which can change during the development of urban areas.
Another factor in changing the microclimate of the urban environment is temperature inversions and the formation of a "heat island" (Fig.. 1,2) [5].This is caused by dense buildings and the impact of solar radiation on the horizontal and vertical surfaces of urban facilities and the accumulation of solar heat by the urban area. At the same time, the violation of natural aeration in densely built-up individual territories violates the natural aeration of urban space and contributes to the formation of a "heat island" that affects the temperature and aerodynamic conditions of the air basin.
A layer of heated air that forms over the city prevents the dissipation of heat accumulated by the urban space, which occurs due to exposure to solar radiation [6].
The combination of a surface inversion with weak winds is called a stagnant state of air and is one of the most unfavorable weather conditions, since the microclimate parameters deteriorate sharply. Source: [5] Obviously, the microclimatic conditions of the urban environment are subject to changes in the process of changing urban development and the active activity of citizens. This is especially pronounced in megacities with a hot climate, where temperature inversions are most often observed. High temperatures characteristic of hot cities, the intensity of solar radiation affecting the active surface of urban spaces can create extreme, local meteorological conditions in the urban environment [7,8].
Local meteorological conditions influence the climate and microclimate of urban areas. Solar radiation, being a climate-forming factor, in urban conditions affects the surfaces of buildings and adjacent territories, as a result of which local microclimatic conditions are formed within the boundaries of yard spaces. Aeration, being one of the indicators of the temperature and wind regime, also forms the microclimate of urban areas [9,10]. The intensity of insolation of yard spaces contributes to the accumulation of heat, while increasing the standard temperature of the area during the day. Such temperature conditions in closed areas of urban space, in the absence of aeration, create a feeling of stuffiness and uncomfortable conditions for residents. Ensuring adequate air exchange in urban areas can reduce discomfort. The formation of a small movement of air currents can contribute to the body's heat transfer and create more comfortable conditions. In this regard, the issue of aeration of modern urban development in the hot season is becoming increasingly important.
Modern urban development with a complex structure has certain aerodynamic properties that depend on planning factors and landscaping. Wind flows are subject to deceleration and deformation due to the flow around urban areas and dense green spaces [11].At the same time, there is a decrease in wind speed by 50-70% in the areas of habitable residential development. In low-wind cities, wind flows up to 2 m/s are observed and calm conditions are formed. In such conditions, the development of air flows of thermal origin caused by the temperature difference of the yard space surfaces during their insolation plays an important climate-forming role [12,13].The speed of such convective flows can reach up to 3.3 m/s, in some cases up to 10 m/s. Thus, by optimizing the building structure depending on the insolation regime, it is possible to ensure the air exchange of urban areas, which will affect the microclimate of the territory.

Results and Discussion
To assess the role of convective flows in the formation of the microclimate of urban areas, studies were carried out of the courtyard space with typical buildings of a characteristic orientation in Dushanbe in the hottest month of July.
The main objectives of the study were: 1. To determine the role of insolation affecting the vertical and horizontal surfaces of the building in the formation of air flows of thermal origin; 2. To obtain a diagram of the movement of the air flow at the outer fence with active insolation; 3. To obtain a qualitative and quantitative picture of the temperature and velocity field of a near-wall convective flow. To achieve these objectives, field studies were carried out: air temperature measurements and actinometric measurements were carried out for parallel 9-storey buildings of latitudinal and meridional orientation. Measurements of the air temperature of the wall air and the surface of the facades were carried out at a distance of 14, 30, 50 cm, at 1.5; 4, 5; 7.5; 10.5; 13.5; 16.5; 19.5; 22.5; 25.5; 27.0 m (Fig. 3). Actinometric measurements were carried out at the level of the 5th floor of a 9-storey residential building. The measurements were carried out in July to identify the relationship between the total radiation coming to the wall of the building and surface heating at different albedo of the facade wall. Based on the results obtained, graphs were plotted (Fig.4).  It can be seen from the graph that the maximum value of solar radiation up to 800 W / m 2 fell at 9:90 on the eastern facade, at 17:30 -on the western facade. At the same time, at 13:30, solar radiation worth 900 W/m 2 fell on the horizontal surface, which caused the wall surface to heat up to 58°C, and the horizontal surface to 60°C. The measurements and field observations showed that the difference in air temperatures in the near-wall layer of buildings irradiated by the sun and located in the shade was 16-19°C, which contributed to the formation of air flows of thermal origin [14,15]. The development of convective flows with a longer duration was observed near the western and southern walls. Onthewesternwall, convectiveflowstookplaceintheeveningat 17:00-21:00.
According to the results of observations of air flows near the wall of the building on the outer side of the wall, schemes for the movement of convective flows were built As a result of the analysis of field observations near the building and the adjacent territory, characteristic air cushions, differing from each other in heat and wind indicatorswere identified. The microclimatic indicators of these areas depended on the conditions of insolation of the vertical and horizontal surfaces of the building area. The directions of convective flows near vertical surfaces wereorganized and clear. The speed of ascending convective flows increasedwith the height of the building, and its maximum value reached 3.5 m/s near the western orientation when the wall and the adjacent territory were irradiated. At the same time, ascending and descending convective flows were observed near the insolated surfaces of the facades The observations showed that the direction of convective flows near horizontal surfaces were energetically diverse with chaotic motion.
As a result, it can be noted that the thermodynamic and aerodynamic changes that occur between the air of the yard space, the wall layer of the facades and the vertical surface of the territory were found. Thus, it is possible to draw up a diagram of the heat balance of the inter-building space, which depends on the building structure.

Conclusions
1. Modern urban environment, being an artificially created environment, constantly changes the aerodynamic and microclimatic conditions of the area in the process of development.
2. The main factors of the city that affect the air exchange and temperature indicators of urban areas are dense typical buildings and the resulting temperature inversions.
3. In the hot season, in large cities, air exchange is disturbed, and uncomfortablemicroclimatic conditions are formed. With active insolation of building surfaces, air flows of thermal origin, which can compensate for the lack of aeration in urban areas,are formed.
4. Regulation and correct consideration of convective flows in the planning of urban areas can improve the temperature conditions and air exchange of the territories of modern cities. Optimization of the building structure, landscaping in the design of buildings can influence the formation of natural aeration and improve the microclimate of the urban environment.