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.
Progress
of the temperature in London: the temperature values increase
proportionally to the density of the construction's volume.
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.
The
solar energy which hits a certain green area is greatly used by the
vegetation for its transpiration and photosynthesis processes,
causing a sensible reduction in temperature
In
highly settled urban areas, solar energy is reflected and absorbed
by the building's vertical surfaces, increasing its thermal charge.
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.
Screen
effect of vegetation.
1. Shading - 2. Reflect solar radiation - 3.convective exchanges -
4. 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:
variation of their growth (asymmetric reduction);
chlorosis ( coloration at the borders of the leaves, for injuries to the
chlorophiyl);
necrosis ( death of the cells).
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:
CARBON
MONOXIDE (CO)
2500
m
g/mq per hour
CHLORINE
(Cl)
2000
m
g/mq per hour
FLUORINE
100
m
g/mq per hour
AZOTO
OXIDE (NO)
2000
m
g/mq per hour
OZONE
(O3)
80000
m
g/mq per hour
PAN
2000
m
g/mq per hour
ANHYDRIDE
SULFATE (SO2)
500
m
g/mq per hour
HAMMONIUM
(H2 NO4)
400
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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