Urban Vegetation di Arch. Antonella Bellomo

The climatic unease in urban environments is brought by the overheating of the air, due to the heat, dust, pollutants from the city's activity, and to the network of the city. The center of the city absorbs 10% more solar energy than a corresponding green area, this is due to the concentration of constructions, the asphalt pavement and the high heat conductivity of most materials used, such as reinforced cement. Furthermore, "cemented" spaces tend to heat up rapidly and to cool slowly, the opposite of what happens in the near-by countryside. In fact the difference between the city and the country's temperature reaches its highest a few hours after sunset, and its lowest in the first afternoon hours. The accumulation of thermal energy, and the difficulty to disperse it in space is due to the shape of the urban spaces themselves, often densely settled.

The sections of narrow streets determine a multitude of reflection/radiation effects between the near by walls of the buildings, consequently overheating the air they come in contact with.
During the night the situation does not improve. The heat's infrared radiation which has accumulated during the day is intercepted by the buildings, instead of dispersing in space. The air-conditioning devices and traffic only worsen the situation, generating other artificial heat.
It was measured that during summer, at medium latitudes, the temperature increase due to artificial heat is of 5-10% of the solar energy, increasing the average temperature almost by one degree, and of more degrees if one considers the single situation of a micro-climate.

With equal humidity and temperatures, the summer thermal comfort in densely settled areas is worst than that in rural or peripheral areas, due to the diminished intensity of the wind (20-30%). For example, the difference in temperatures between the center of Milan and its periphery reaches 2/3° C.
Different studies highlight how the presence of vegetation in a city drastically improves the micro-climate, sensible reducing the temperature.

Temperature variations, and relative humidity in the air induced by the presence of vegetation are principally due to:

a) reduction of the solar radiation on edifices shaded by vegetation

The solar energy which hits a mass of vegetation is in part reflected, absorbed and transmitted, in part dispersed in the atmosphere as latent heat and sensible heat, and in part utilized in the plant's metabolic processes.
Through photosynthesis, plants transform solar energy in biochemical energy, particularly absorbing the visible radiation (hottest ones), thus their presence becomes relevant in the determination of the micro-climate of a specific area.

It has been calculated that plants absorb a percentage equal to 60-90% of the solar radiation, in relation to a series of variables which determine the shading/absorption of the solar radiation, such as the density of the foliage (dense or sparse), the growth cycle (evergreen or deciduous plant), and the dimension and shape of the plant (maximum height and its structure).Along with this one must have knowledge of the phenology of the single species, in order to select the best ones during the project for a green space.

There exist instruments (radiometers) and analytical methods which enable us to determine the reduction of the sun's intensity, according to the plant's foliage.
The choice of plants among the deciduous (diverse density of foliage) is as important as the choice of evergreen or deciduous.
To grants us cooling off in summer and warmth in winter, one must choose a plant with a dense foliage in summer and a low shading capacity in winter. For some plants the shading coefficients in summer and winter have been calculated, this information should be highly regarded when choosing plants that are to be put in proximity of edifices.

The selection of plants with more or less dense foliage can contribute to changing the energy flow of near-by buildings, thus changing the internal temperature.
The density of the foliage and thus the capacity to filter solar radiation can depend both on the environmental conditions (from this the importance of ecological amplitude of the species which are to be inserted in a specific environment, meaning, are they adaptable or not to the climatic conditions and can they resist water stress situations) and cultivation practices (the importance of trimming as a mean to control density and new branches).
The shading of vegetation con contribute in a relevant way to the cooling off of buildings, it can determine a reduction of the internal temperature and a rounding of the maximum temperature (the temperature of the surrounding air, from which depends the thermal behavior of the shaded edifice, reaches its zenith 2-3 hours after the maximum solar radiation). Through the use of vegetation near edifices one can contribute to moderate the use of air conditioning, which in Italy has increases of 20%, with a consequent consumption of electricity in summer and great emission of CO2 in the atmosphere.

b) Modify the exchange of solar radiation and long waves between surfaces and outside environments.

A green coat emits less infrared radiation than the ground or artificial materials, and thus reduces the average radiant temperature of the environment. The buildings that face green surfaces (with radiant temperatures lower than those of sun hit surfaces) resist less to high radiant temperatures than streets and adjacent buildings.

c) Processes of evapotranspiration

Evapotranspiration of plants is a phenomenon tied to photosynthesis, plants, in order to assume carbon dioxide from the atmosphere, must keep its STOMI open and in this way they loose water. A great quantity of water is pumped from the ground into the atmosphere under shape of vapor.
The change from liquid to vapor occurs in the leaves and requires an absorption of thermal energy, for each gram of vapor there occur 633cal.
Considering that the quantity of heat dissipated for the transpiration of green surfaces, not subject to water stress, is high, one can conclude that the presence of green areas in urban settings con drastically contribute to correct the summer overheating, and locally reduce the temperature.

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