Fabio Bertrand Elsa

The urban heat isles and the micro-climatic variations brought about by vegetation

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.

A tree lined square of 100x100 m. can reach a level of transpiration of 50,000 liters a day. Thus for the change of the state of water approximately 31,650,000 cal are used, and taken from the outside environment (this energy would other wise be absorbed by buildings and reflected as heat).
It has been verified that the cooling off due to transpiration of a plant of large dimension equals the capacity of five small air conditioning devices working for 20 hours a day.
The microclimatic effects due to the evapotranspiration are particularly visible in areas with little wind but exposed to strong solar radiation.
One s also to consider that the reduction in temperature, caused by the transpiration processes of plants, is minimized by the presence of a single tree, but is sensibly increased when the green area is vast. In fact, experiments conducted in Germany highlight the difference in temperature between parks and the surrounding urban areas to be as high as 7°C.
The temperature reduction effect due to green areas, can be examined only on a local scale and at relative distance, for the air exchanges reduce the influence on a larger scale. At the same time it was reveled how the increase of green areas in a city, through the process of evapotranspiration, significantly contributes to improve the summer global temperature and subsequently to reduce the electric and air-conditioning consumption in the world.
These premises lead to the conclusion that green areas are to be opportunity integrated with edifices in the city (both for more edifices as for the single units) to improve the summer microclimate and the quality of the air.
The possible intervention strategy, integrating vegetation with construction, thus permits us to reduce the entering thermal flux through shading, reflect solar radiation, reduce the convective exchanges, and the absorption of solar energy used for the processes of transpiration and photosynthesis.

Vegetation and atmospheric pollution

The vegetation of cities can be an organ of enviromental control being activly a filter for gas and dust and beign passivly a precious noticer of their presence. Many studies on phytotoxic effects of pollution pointed out how some vegetal species react differently in relation to a certanin kind of polluting substance. Their response to pollution could range from very sensitive (they report damages even wth small quantities) to very strong.
Sensitive plants can be used as monitors to calculate atmospheric pollutin levels; actually, they react weakining and with various symptoms which need a complex interpretation, such as:

Water and thermic stresses combined with lack of nutrition can lead to such symptoms which are very similar to the ones associated with pollution. Anyway, there are many plants which present a sensitivity to one ore more specific polluting substances, and therefore can be used as ecological sentinel (e.g. lichen). The use of plants as monitors should be run alongside with electrical observation and surveying points. The strongest species of plants can represent those elements able to absorb polluting substances in urban spaces, as thery're strong enough to survive and metabolise them. This reduction happens on the surface of the leaves and in their vegetal tissues through the disarming of gas by absorbing toxic composits, by the disarming of the vegetal tissues of the cells, by precipitation and storage, and because plants use the composits during the oxidizing metabolisation.
Other studies let us know some relevant data about the quantities of gas substances reduced by plants.
In USA, they discovered that vegetation is able to subtract from air some polluting substances, as explained in the following table.

Reduction of pollution in the atmosphere, near plants:



m g/mq per hour



m g/mq per hour



m g/mq per hour



m g/mq per hour



m g/mq per hour



m g/mq per hour



m g/mq per hour



m g/mq per hour


Even lead contained in air could be reduced by the presence of the plants.
We must not forget that environmental conditions intervene on plants ability to reduce polluting substances or augment their damaging action. For example the stagnation of air (fog) or excessive dryness can raise intolerance phenomenon of sensitive species. In particular, in big cities as Milan, the scarce ventilation and the high level of humidity in air worsen the damages provoked by pollution and especially by sulfate. In these situation , we found out that conifers are more appropriate, even if deciduous leaves plants reduce pollution by storing it in their leaves and eliminating it when the leaves die and fall during winter.
The evergreen plants work even in wintertime ( when pollution reaches its high levels) and helps the elimination of pollution stored in their leaves, instead of letting it fall to the ground.
The strongest species can be as useful, because they help to discover and filter the dust in the atmosphere. The filter action is related with the diameter of particles and results more efficent in motionless leaves and with a wrinkled epidermis. A sensible reduction of dust presence has been registered of about 38 - 42% thanks to the evergreen plants and 27-30% thanks to the deciduous species. We believe, then, that the action filter can reach values that range between 200 to 1000 kg/ha.

The productive function of public green

An easily ignored and not considered function of vegetation in urban green planning is the productive one. This function develops itself by the production of food genres, and also through the utilisation of wood refuses as a raw material for several uses especially for the production of energy. A study embarked in Tourin shows some quantitative data about the energetic potential deriving from wood: all the matrials coming from a complete prune cycle (120-260 thousands quintals of in 5-6 years) can hypothetically represent an energetic potential equivalent of 4 thousands of oil.
A rapid glance goes on that kind of interstitial green whose diffusion is limited to the milanese urban area ( in a disorganised and abusive way) and widley developped in the Nordic countries: the urban orchard. This represents a new and interesting way to concieve the management of green areas given to private users. In Germany, this has been experimented for a long time ( since 1830) , giving great results. In Nordic urban ambient this kind of green areas don't concern only the suburbs but even the central zones of the city.
In the Milanese areas, we must mention an experimental project conducted by AEM (Municipal Energetic Comapany) which gave to some private citizens ( a hundred ) some orchards to cultivate them.
This experience stimulated new ideas and interesting proposals; among them we can find the estabilishment of a ecological station (Urban Ecologic Centre) for the study, the analysis, and the disclosure of problems strickly connected with public health and liveability of the urban ambient.

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