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SUSTAINABLE RESIDENTIAL BUILDING ISSUES IN URBAN HEAT ISLANDS
– THE POTENTIAL OF ALBEDO AND VEGETATION
ALAMAH MISNI1, PENNY ALLAN2
1
University of Technology Mara, Malaysia
Victoria University of Wellington, New Zealand
__________________________________________________________________________________
2
TYPE OF PAPER
Scientific Student Paper
ABSTRACT
The ‘urban heat island effect’ influences most of the major cities around the world. This heat island
phenomenon contributes to discomfort and increased air conditioning loads. On hot summer days, this
effect significantly contributes to the urban dweller’s discomfort, resulting in an increase in energy bills
due to increased requirements for cooling. This paper evaluates the potential for light colour surfaces and
landscape design of single family domestic dwellings to significantly minimise the energy demands
associated with air conditioning in the urban environment. Light coloured surfaces reflect solar radiation
and reduce heat absorbed by buildings. Modification of the value of ‘albedo’ (the measure of a surface’s
reflectivity) of a building’s envelope and in particular walls and roof, can potentially reduce the ambient
temperature in an individual building and its surroundings and keep the entire neighbourhood cool. This
research reveals that urban albedo can achieve a decrease of up to 2°C in air temperature, and reduce home
energy use by as much as 50%. In a single family residence, substantial cooling energy and capacity
savings can also be achieved by the direct shading of walls, windows, and roofs by vegetation. The
appropriate amount, type, and placement of vegetation can save as much as 55% of residential cooling
demand on a hot summer day and reduce surrounding air temperatures by as much as 5°C. Results indicate
that the largest reduction of energy consumption occurs with modifications in reflective building envelopes
and strategically positioned vegetation around buildings. This reduces cooling requirements for building
and neighbourhood areas. Both trees and light colour material surfaces can have significant effect on the
temperatures and energy consumption of buildings. A combination of two mitigation strategies,
landscaping and albedo modification can create communities that are cooler, more aesthetically pleasing,
and more energy efficient.
KEYWORDS:
Urban heat island effect; albedo; vegetation; energy consumption; single family domestic dwellings
INTRODUCTION
The ability of a city to generate an urban heat island is now well documented. The urban heat island effect,
together with summertime heat waves, sets in motion conditions that foster biophysical hazards such as
heat stress and increased concentration of secondary air pollutants (Solecki et al., 2005). The
implementation of urban heat island mitigation strategies can reduce the impacts of biophysical hazards in
cities including heat stress related to elevated temperatures, air pollution and associated public health
effects. Two basic strategies to mitigate urban heat island are high-albedo surface material and increased
vegetative cover.
Using light coloured surfaces is potentially the simplest strategy for saving peak energy and improving the
quality of the environment in urban areas. Building envelopes and their environments have a significant
effect on thermal comfort and energy use. One of the most important factors affecting building envelope
design and its local environment is climate. The building envelope can be considered the selective
pathway for a building to work with the climate and respond to heating, cooling, ventilating, and natural
lighting needs (DOE 2004 cited in Okba, 2005). An optimal design of the building envelope may provide
significant reductions in cooling and heating loads. Each type of climate will require different building
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envelope design strategies. Specific designs and materials can potentially provide solutions for any given
climatic conditions. The use of light coloured surfaces on buildings and outdoor urban areas is potentially
practical in areas which have large amounts of sunshine.
Sustainable design of the built environment along with careful landscaping can create cooler communities
and save energy. Careful landscape planning can reduce the amount of sunlight heating building surfaces
and can prevent reflected heat-causing light from entering the home. Architects can create a microclimate
boundary by manipulating the building envelope and exterior environment (Bomberg and Brown, 1993).
The planning and development of exterior spaces can reduce the energy consumption of buildings by
reducing the adverse impact of some climate factors. If the microclimatic condition around the building is
very similar to the desired interior condition, little extra energy is required. Conversely, if the
microclimate is significantly different from the desired interior conditions, large amounts of energy may be
required for heating or cooling (Brown and Gillespie, 1995). However, proper landscaping can also be
used to improve thermal performance. It reduces the amount of radiation falling on the building by
shading, by moderating temperatures, by the evapotranspiration processes, and by controlling wind
direction to assist in keeping the building warm or cool.
This paper describes the heat island phenomenon, sensible heat flow, the importance of light coloured
surfaces and albedo, and the potential of vegetation to reduce the impacts of the urban heat island effect.
THE URBAN HEAT ISLAND EFFECT
The urban heat island effect is well documented as an urban phenomenon where air temperatures in
densely built urban areas are higher than the temperatures of the surrounding rural country. The larger the
city, the more intense the summer heat island effect (Oke, 1973) and the magnitude of discomfort and
associated air conditioning load (Akbari et al., 1986). The heat island effect is caused by complex
interactions between many factors including decreased urban albedo, increased thermal mass per unit area,
increased city roughness (the roughness elements of a city are its buildings whose heights generally
increase toward the city centre), anthropogenic heat released from buildings and vehicles, and decreased
evaporative areas (Taha et al., 1988). All these factors affect the energy and moisture balance in urban
areas. This process decreases the proportion of latent heat in comparison with the sensible heat in
accordance with the Bowen ratio, which in turn leads to higher ambient air temperature. The Bowen ratio
relates to the ratio of sensible to latent heat fluxes of energy balance. On hot summer days, this effect
significantly contributes to the urban dweller’s discomfort resulting in an increase in energy bills due to the
increased requirements for cooling.
As humans alter the character of the natural landscape in the city-building process, the local energy
exchanges that take place within the boundary layer are affected. Therefore, modification of the landscape
influence the microscale, mesoscale, and macroscale climate. The best-known effect of urban areas on heat
islands is the so-called "microclimate" created in city centres. In urban areas, building and paved surfaces
have gradually replaced pre-existing natural landscapes. As a result, solar energy is absorbed and stored
within building structures, roads, and other hard surfaces during the daytime. The absorbed heat is
subsequently re-radiated to the surroundings and increases ambient temperatures at night causing the
surface temperature of urban structure to become 5.5–10°C higher than surrounding areas because dust
and clouds act like a blanket to keep it in (Akbari et al., 1992). As the surface throughout an entire
community becomes hotter, the overall ambient air temperature increases by 2–8°C. Higher average
temperatures also mean that city centres experience less snow and fewer frosts than either their suburbs or
the surrounding rural area. The higher levels of cloud cover and dust pollutants in the atmosphere mean
that city centres also have higher rainfall levels and more fogs than adjacent rural areas. Urban heat
islands are considered as a mild asset in winter and can reduce the demand for heating. However they can
significantly increase the demand for cooling energy during the summer.
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SENSIBLE HEAT FLOW
Local climate affects the rate of heat gain from buildings by contribute to heat flows throughout our
environment. There are three ways the heat can be transmitted through any medium: conduction,
convection, and radiation.
Conduction involves the transformation of kinetic heat in solid mass. This is the process of heat transfer
from warmer to cooler molecules within a solid material. The heat flows through materials or building
fabric from which a home is built. The primary heat transfer paths between a building and its external
environment are: conduction through walls, roofs, doors (opaque conduction) and windows (glazed
conduction); solar radiation directly penetrating through windows; and sensible and latent heat gain from
air exchange.
Convection represents the transfer of heat by circulation or movement of hot particles to cooler areas.
Heat transfer by convection can occur in liquids and gases when molecules move freely and independently
(Moss, 2007). Convective heat exchange occurs in several situations: when heat is transferred from a solid
to the adjacent air, or heat transfer between two surfaces at different temperatures by means of airflow.
Convection is caused by two types of forces: by temperature difference (natural) and forced air motion
generated by wind or fans. Convective heat transfer takes place when the building is infiltrated by wall
surfaces, when outdoor air flows through the interior space. If the ambient temperature is different from
the temperature of any element of the interior space, convective heat transfer takes place.
Radiation occurs without the involvement of a physical substance as the medium. Thermal radiation
involves the interchange of electromagnetic waves between surfaces of differing temperature (Moss,
2007). The sun transfers its heat to the surface of the earth by radiation through space to the earth’s
atmosphere. The ozone layer, atmospheric particles, condensing water vapour and dust, act as a filter to
solar radiation, reducing its intensity at the earth’s surface. Radiant heat transfer in buildings is energy
exchanged between surfaces by electromagnetic waves across space. The radiation wave is transmitted
through space until it strikes an opaque surface, where it is partly absorbed. The absorbed radiation
increases the vibration of the surface molecules and thus raises the temperature of the material where the
absorption took place.
LIGHT COLOURED SURFACES
The use of light coloured surfaces on building and outdoor urban areas is potentially practical in areas
which have a large amount of sunshine. Light coloured surfaces reflect solar radiation and reduce the heat
absorbed by buildings. Through modification, the suitable value of albedo for a building envelope,
particularly walls and roof, can potentially reduce the ambient temperature in an individual building and its
surroundings and keep the entire neighbourhood cool.
Walls. Walls are a major part of the building envelope, usually a solid structure of masonry, wood,
plaster, or other building material. Walls have two main purposes: to support roofs and ceilings, and to
divide space, providing security against intrusion and weather. In addition, the wall may house various
types of electrical wiring or plumbing. Electrical outlets are usually mounted in walls. In conventional
construction, a building wall will usually have the structural elements, insulation and finishing elements, or
surface. The wall thickness, material and finishes can be chosen based on the heating and cooling needs of
the building. Appropriate thermal insulation and air cavities in a wall reduce heat transmission into the
building. During the summer, walls receive large amounts of solar radiation. The thermal comfort
conditions for buildings depend on the heat storage capacity and heat conduction properties of walls.
Walls are the biggest surfaces of buildings. White exterior walls absorb less heat than dark walls.
Therefore, wall surfaces play a large role in temperature and modification of energy consumption.
Modification of the albedo of walls is simplest, it only require routine painting.
Roof. A roof is the top covering on the uppermost part of the building envelope. A roof protects the
building from the effect of weather such as rain, the high summer sun, and also from snow melt coming off
the roof. Roof designs come in 'pitched', 'low slope' or 'flat' forms. Roof shapes and pitches are often the
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product of design, material constraints and climatic considerations. Pitched roofs are the primary design
on residential buildings. About a third of the unwanted heat that builds up in homes comes in through the
roof (DOE, 1994). The need for greater thermal resistance will dictate the type of roof design. The
weather proofing material is the topmost or outermost layer exposed to the weather. Pitched roofs are often
covered with asphalt shingles, thatch, wood shake, steel, corrugated galvanised iron, slate, and tiles.
Drainage on roofs is typically installed at interior locations or on the exterior wall. Light coloured roofing
material has been used to reduce the summer contribution of solar-driven roof gain. Unlike most light
coloured surfaces, white asphalt and fiberglass shingles absorb 70% of the solar radiation making them
less effective (DOE, 1994). Another way to reflect heat is to install a radiant barrier on the underside of the
roof. A radiant barrier is simply a sheet of aluminium foil with a paper backing. When installed correctly, a
radiant barrier can reduce heat gains through a ceiling by about 25% (DOE, 1994). One good solution is to
apply both waterproof with insulation layer and reflective coating properties to the roof.
ALBEDO
The measure of a surface’s reflectivity is called albedo. Albedo is the fraction of sunshine (shortwave
radiation) reflected from any surface on the earth back into space (Brown and Gillespie, 1995). A light
meter is used as a device to measure the amount of light with scale and the unit of light is in lux (measures
intensity of light per unit area). The sensor points upward to measure the incident sunlight, and then
quickly flips upside down to measure the reflected sunlight. The value of albedo is calculated dividing the
second measurement (reflected) by the first measurement (incident). Albedo can assume any value
between 0.0 and 1.0 but these limits do not include natural elements.
In building surfaces and components, the importance of albedo varies with a location’s latitude. For
example in near tropics, the roof assumes the most critical role in sensible heat exchange but in mild
climates the effect of exterior colour is further complicated (Taha et al., 1988). An increase in the surface
of albedo has a direct impact on the energy balance of the building. Santamouris (2005) states that in all
cases the temperatures of reflective roof surfaces are significantly reduced, but the degree to which the
cooling load decreases depends on the structure of the roof and on the overall thermal balance of the
building. Using high albedo materials reduces the amount of solar radiation absorbed through building
envelopes and urban structures and keeps their surface cooler, for example, white elastomeric coatings
(with an albedo of 0.72) were 45°C cooler than black coating (with an of albedo 0.08) in the early
afternoon of a clear day in summer (Taha, 1997). Taha also added that using a white surface with an
albedo of 0.61 was only 5°C warmer than ambient air whereas conventional gravel with an albedo of 0.09
was 30°C warmer than air. For example, the temperature of dark coloured insulated residential building
surfaces in Washington DC was about 8°C higher than light colour surfaces during peak time at 14.00
hours (Connor 1985, cited in Taha et al., 1988). In general, light-coloured surfaces have high albedo and
dark-colour surfaces have low albedo. The reduction of surface temperature also reduces the intensity of
long wave radiation. Local and downwind ambient air temperatures can be lower because of smaller
convective heat fluxes from cooler surfaces. Computer simulation shows that reasonable increases in urban
albedo can achieve a decrease of up to 2°C in air temperature (Taha, 1997). Reductions in temperatures
and energy use from albedo modifications relate to both individual buildings and entire neighbourhoods
(Akbari et al., 1992). Modifying the albedo material by using light-coloured surfaces on a building will
potentially lower the heat build-up from sunlight on the wall and roofs, and reduce the amount of
electricity needed for air-conditioning.
The effect of albedo on building energy use can be categorised as either local or global. According to
Taha et. al. (1988), local or direct effects refer in particular to the modifications in reflective building
envelopes. Global or indirect effects refer to micro-climatic changes where urban albedo is modified
mainly in dry bulb temperature. Akbari (1992) found that from computer simulation of an individual
typical house in Sacramento, the total air-conditioning bill could be reduced by up to 20% if the albedo of
the walls and roofs were increased from a typical 0.30 to a light-coloured 0.90. Akbari states that if similar
albedo modifications were implemented in large scale urban surfaces, the result would be an indirect
impact of lowered air temperature in the neighbourhood. Akbari showed that the indirect cooling energy
saving for typical houses could be 3–5% for each 0.01 increase in overall city albedo. When the direct and
indirect savings of albedo changes were combined, the simulated total energy saving approached 50%
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during average hours and 31% during peak cooling period (Akbari et al., 1992). Ahbari revealed these
simple changes in albedo levels can reduce home energy use by 10–50% (1992). The studies show that
using albedo can potentially create comfortable environments for buildings and reduce heat in air
temperature. It can save energy by reducing air-conditioning during summer in temperate climates and in
hot climates.
POTENTIAL OF URBAN VEGETATION
Vegetation affects urban climates and building thermal performance and energy by shading, by lowering
and channelling wind speeds, and by evapotranspiration. It modifies the thermal interaction between a
building and its surroundings.
Shade. Tree shading is the cover or shelter provided by the interception of trees and solar radiation.
Using shade effectively depends on the species, and density of the tree and the size, foliage, shape, and
location of the moving shadow that it casts. According to Akhbari et al. (1992), tree shade reduces
cooling energy use inside a building in three ways. First, window shading helps prevent direct solar
radiation from entering the interior of the house. Second, shading the walls, windows, and roofs keeps
them from getting hot, thereby reducing the amount of heat reaching the interior. Third, shade keeps the
soil around the building cool. Efficient shading will be achieved when trees are placed strategically. For
example, at 40°S Latitude evergreen trees should be placed on the northeast and west sides of the building.
The best locations for deciduous trees in the northern hemisphere are on the south and west sides of a
house. During summer, trees will cast long shadows which can be utilised effectively on those sides which
are otherwise difficult to protect from the sun’s heat at this time of the day. When these trees drop their
leaves in the winter, sunlight can reach the house to help in heating the home. Trees with high, spreading
crowns can be planted to the south of a building to provide maximum summertime roof shading. Trees
with crowns closer to the ground are more appropriate to the west, where shade is needed from lower
afternoon sun angles. Evergreen trees on the north and west sides afford the best protection from the
setting summer sun and shelter from cold winter winds. In addition to the obvious use of shade trees to
provide relief from the summer sun, properly placed trees can channel summer breezes to desired
locations. The type, foliage, and species of trees used are very important. Trees should be selected
according to the amount and type of shade they provide.
Solar heat passing through windows and being absorbed through the roof is the major reason for airconditioner use. Shading is the most cost-effective way to reduce solar heat gain and air-conditioning
costs. Field measurements have suggested that the shade of trees and shrubs planted immediately adjacent
to buildings can directly reduce cooling loads (Simpson and McPherson, 1996). Their study in Sacramento
showed that trees shading a west exposure from afternoon sun had the greatest impact on cooling savings
for all climate zones and insulation levels. In a single family residence, substantial cooling energy and
capacity savings can be achieved due to the direct shading of walls and roofs. Akbari et al.’s (1986) study
in Los Angeles stated that planting trees can save as much as 34% of residential cooling energy on a hot
summer day. They added that the net effect of shade in warm climates was beneficial. Buffington’s
(1980) study showed that the effect of uniform shading on the walls and roofs of a concrete block
residence in central Florida reduced annual cooling by 14%. Heisler (1984) stated that the average
reductions of solar radiation on walls on conventional houses in Davis, California, in the shade of midsize
dense trees, may be 30-35% on sunny days, while the reduction in the shade of large trees may be 45%.
Parker (1983) described that in his study of a child care centre at Florida International University, Miami,
landscaping reduced air conditioning costs during warm summer days by approximately 58%. The
magnitude of these savings suggests that trees and shrubs reduce cooling requirements not only by shading
but also by reducing warm air infiltration and creating cool microclimates by evapotranspiration near the
residence. Proper landscaping around the home can potentially provide shade to save energy and reduce
the urban heat island effect.
Wind modification. During summer, wind is an asset which can provide natural ventilation and
convective cooling of hot exterior building surfaces. Olgyay (1963) suggests that wind is particularly
important for comfort when temperatures are above 29°C and relative humidity above 50%. In these
cases, cooling needs are high and landscaping around the buildings should be directed towards channelling
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cooling breezes, and to minimize humidity close to the home. Heat removal from a building during the
warmer months can be influenced by the placement of plants, and by their shape. Airflow can be increased
by strategically locating a combination of hedgerows, and tree canopies. McPherson, Herrington and
Heisler (1988) suggest that high-branching and wide canopy trees and low ground covers should be used to
promote shade and encourage wind. Trees with tall trunks and spreading canopies in combination with
grasses and low flower beds are the most effective planting for providing usable shade and influencing the
movement of wind around and through a building. Directing wind through a building and against the walls
and roof permits rapid removal of heat. The effect of vegetation on wind conditions depends to a great
extent on the type of the vegetation, and on the details of the planting pattern. Properly selected and placed
landscaping can provide excellent wind channelling during summer. This will reduce cooling costs
considerably. During the summer, strategically planting trees in urban housing areas can act as a wind
channel and potentially have a substantial impact on a residential building’s energy performance.
Evapotranspiration. Evapotranspiration is the combined loss of water from vegetation to the atmosphere
by evaporation and transpiration. This is a major mechanism through which trees contribute to decrease
urban temperatures (Santamouris, 2001). Evaporation accounts for the movement of water to the air from
sources such as soil, canopy interceptions, and water bodies. Transpiration accounts for the movement of
water within a plant and the subsequent loss of water as vapour through the stomata of leaves.
Transpiration puts moisture into the air and moderates air temperature changes. Evapotranspiration can
create oases that are 2–8ºC cooler than their surroundings (Oke, 1988). The oasis effect is a change in the
microclimatological conditions in a vegetated area as distinguished from an area without vegetation
(Saaroni et al., 2004). The change is manifested in lower temperatures and higher relative humidity (Oke,
1988). Oke describes green areas as more humid. These processes increase the proportion of latent heat in
comparison with the sensible heat in accordance with the Bowen ratio. However, the amount of water that
plants transpire varies greatly geographically and over time. It is determined by a number of factors such
as temperature, relative humidity, wind, air movement, soil-moisture availability, and type of plant.
Temperature is an important aspect of transpiration. Temperature can control the opening of the stomata.
Higher temperatures cause the plant cells to open allowing water to be released to the atmosphere, whereas
colder temperatures cause the openings to close. As the relative humidity of the air surrounding the plant
rises, the transpiration rate falls. When water transpires from a leaf, the water saturates the air surrounding
the leaf. Wind movement will increase the movement of air around a plant and this will result in a higher
transpiration rate. Shade from vegetation also prevents direct radiation from reaching the ground surface
and warming it, thereby resulting in lower air temperatures above these surfaces. As a result, when shade is
combined with evapotranspiration process, the air temperatures near the ground in green areas will be
lower.
The evapotranspiration process from vegetation contributes significantly to cooling cities and saving
energy. The lower evapotranspiration rate in urban areas is a major factor in increasing daytime
temperatures. The evapotranspiration process is predominantly effected and monitored surrounding air
temperatures. Vegetation produces evapotranspiration around individual houses and their neighbourhood
areas, thus reducing urban temperatures at the microclimatic scales. If ample soil moisture is present and
environmental conditions are suitable, water in the leaves evaporates and the air is cooled. Heisler has
stated that trees are effective for cooling because they absorb 70–85% of the heat from solar radiation by
transpiration (cited in Akbari et al., 1992). According to Taiz and Zeiger (2006), leaves absorb about 50%
of the total solar energy. Since leaves are generally dark and coarse and thus reflect very little light, they
make ideal solar radiation absorbers and controllers. Temperature measurements in suburban Davis and
Sacramento, indicate that the air temperature in neighbourhoods with mature tree canopies are 3–6°F lower
in daytime than newer areas with no trees (Akbari et al., 1992). Similarly, groundcovers such as grasses or
turf also have a cooling effect from evapotranspiration. The temperature above a groundcover can be 10–
15°F cooler than above a heat absorbent material (Morgan et al., 2000). Computer simulations have shown
that residential neighbourhoods with extensive vegetation can reduce air temperature by as much as 10ºF
compared with nearby areas with little vegetation (McPherson and Simpson, 1995). The amount of tree
cover and its placement influences the reduction in cooling energy. Huang et. al. (1987) studies in the
USA found that a 10% increase in tree cover per house would result in a cooling energy reduction of 24%
in Sacramento, 12% in both Phoenix and Lake Charles. A 25% increase in tree cover, corresponding to
three trees per house on the southeast and west sides, was estimated to have an even more dramatic impact
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with reductions in cooling energy use of 57% in Sacramento, 17% in Phoenix, and 23% in Lake Charles.
Huang et al. (1987) found that the direct effect of shading accounted for only 10–35% of the total cooling
energy saving; the remaining savings were due to evapotranspiration. This confirms that a minimum of a
mature tree strategically placed around residential buildings, combined with groundcovers can reduce
urban heat islands effects and cooling energy use, and can create a comfortable place for house interior and
their surrounding.
CONCLUSION
The major factors increasing daytime temperatures and creating heat island in a residential area are low
albedo in the building envelope and lower vegetative planting around buildings. Proper house
configuration and surrounding landscaping also can influence thermal performance and energy use. Simple
changes in albedo levels can reduce home energy use by 10–50%. Using high-albedo surface materials can
potentially create a comfortable environment for buildings, and reduce air temperature, and save energy by
reducing air-conditioning during summer. Vegetation reduces the cooling requirements not only by
shading but also by reducing warm air infiltration and creating cool microclimates around buildings by
evapotranspiration. In addition, during the summer season, vegetation in urban housing areas can act as a
wind channel and potentially have a substantial impact on a residential building’s energy performance.
Vegetation can reduce temperature and energy use by doubling the amount of albedo modification on
building. By combining of every type of vegetation and strategically placed with right amount, size,
species and form around residential buildings will save energy and produce a more comfortable house, and
can reduce urban heat island effects to the surrounding city. Therefore, mitigating strategies such as using
high-albedo surface material for the building envelope, and increased urban vegetative cover aimed at
reducing temperatures and energy use will become increasingly important under dynamic climate
conditions.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge Professor Dr George Baird, at School of Architecture, Victoria
University of Wellington, New Zealand, for very helpful and valuable review comments.
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