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A method for measuring shade

Survey looks into five species of tree planted in urban areas and points out which are the most appropriate to provide to the inhabitants

A study with five species of tree in urban areas of Campinas (SP) – the sibipiruna, the ipê-roxo( it might be translated into English as Pink Trumpet Tree, Purple Trumpet Tree and Tabebuia Ipe) , the magnolia, the chuva-de-ouro ( Golden Shower ) and the jatobá (courbaril tree) – show that all significantly reduce the effects solar radiation and offer comfort in the heat. And, as Campinas has a similar climate to that of other regions in the Brazilian Southeast, these trees should serve for many other towns. Where the climate is different, however, a new study will be needed with native or adapted species. This is now quite easy: the Campinas study established a method that is available to all interested parties.

Financed by FAPESP, the Thermal Comfort in Towns project: Effect of Trees in Controlling Solar Radiation was developed between 1997 and 1999 by the physicist Lucila Chebel Labaki, head of the Department of Architecture and Construction at the School of Civil Engineering of the Campinas State University  (Unicamp), jointly with the biologist Rozely Ferreira dos Santos, a professor at the Department of Sanitation and the Environment at the same college and a specialist in environmental planning.

The work also focused on the thermal comfort provided by certain green areas in the central area of Campinas and showed that the mitigation of the solar radiation reaches 99.06% in old and dense woods and 88.24% in a recently built square.

It was confirmed that the vegetation dramatically lowered the discomfort of the so-called islands of heat in urban centers, created by the combination of factors such as large concrete and mortar masses – which reduce wind and concentrates pollutants – the waterproofing of the ground by the pavement and the presence of heat absorbing construction materials

As a suggestion, the study indicates some species of tree as being more appropriate for making things more comfortable for people in areas of transit, such as streets and avenues, or leisure areas, such as squares and parks.

What influences
Lucila explains that thermal comfort depends on four environmental factors and two personal ones. The environmental factors are: temperature, average radiant temperature, relative air humidity, and air speed.

Average radiant temperature is the average between the thermal radiation falling on the local surfaces – objects and living creatures – heating them up and the radiation that they emit back into the environment. This value gives an idea of the effects of thermal radiation on a person, as there are significant temperature differences between the surrounding surfaces – in the case of outdoors  areas, these are the trees themselves, the grass, the pavement and buildings around them . An example of the importance of this parameter is that of a crowd packed together in a public square on a sunny day: although the temperature is the same at the edge as in the middle of the square, those in the center feel hotter – because, besides solar radiation and reflected radiation by the area’s asphalt, they receive the heat coming out from the bodies around them.

The personal factors influencing thermal comfort are the clothes the person is wearing and the physical activity in which he or she is engaging. If on a very sunny day, we compare an individual wearing black clothes with another wearing white one, the one wearing white will feel more comfortable: white reflects light and, therefore, absorbs practically no radiation – in contrast to black. As for activity, we just have to think of doing manual labor or practicing sport under a hot sun to remember that the feeling of heat will be lower at rest. The six factors must be taken into account in a tree-planting project. For places in a hot climate and those intended for sports, for example, trees that efficiently absorb solar radiation are recommended. On the other hand a park bench intended for resting and reading, we can think of vegetation that filters the sun’s rays a little less, since the body will resting.

Among the project’s objectives was the analysis and knowledge of the capacity of various species to attenuate solar radiation, with a view to tree-planting projects. “But the main objective”, emphasizes Lucila, “was to establish a method through which, and at any time, and in any climate, similar studies can be carried out”.

In short, the method consists of placing measuring equipment under the canopy of each tree studied – which must be planted in an isolated place, without buildings or large-scale vegetation nearby. Similar equipment is placed nearby, under the sun. Afterwards the data are compared and the attenuation of the solar radiation is obtained as a percentage.

“An important detail of the research is that the instruments were placed 1.30 meters above the ground, corresponding to the average height of a person’s chest”, points out Lucila. This is so because it would be useless measuring the radiation at ground level or a spot taller that average person’s height.

The civil engineer Carolina Lotufo Bueno Bartholomei – who, in mid -1998, completed her master’s degree in the field of the project, at Unicamp’s Civil Engineering Faculty, followed these procedures. For each species studied, she collected the data from the equipment recorded for five days from 7 a.m. to 5 p.m.

The sibipiruna (Caesalpinia peltophoroides), a tree native to the Mata Atlântica ( Atlantic Rain Forest)  and of the same genus as the Brazil-wood stood out in the survey: with beautiful yellow flowers, around six meters in diameter at the crown and average density,  it was the champion as a regulator of solar radiation, getting 88.5% of attenuation. There was initial surprise at the result, since the leaves of the sibipiruna are as tiny as those of a fern, around 0.9 centimeters long by 0.5 centimeters wide. It was then realized that several small leaves squeezed together offer a greater area of exposure to the sun – and, consequently, absorb more radiation – that a single large leaf.

A close second , almost tied to the sibipiruna, were the chuva-de-ouro, with 87.3% solar radiation attenuation, and the jatobá, with 87.2%. Asiatic in origin, the chuva-de-ouro or golden shower (Cassia fistula) has a crown of around 7 meters in diameter, average density, handsome, perfumed yellow flowers and leaves around 11.5 centimeters long by 4.5 wide.

The majestic crown of the jatobá (Hymenaea courbaril), a native of Brazil, is also of average density, but can reach 23 meters in diameter. The flowers are white and the leaves are 5 centimeters long by 2 wide.

In third place came the magnolia (Michelia champaca), with 82.4% solar radiation attenuation: also of Asian origin, it has a dense, 8-meter-wide crown and leaves 23 centimeters long by 7 wide.

Finally, with 75.6%, came the very Brazilian ipê-roxo (Tabebuia impetiginosa), with a thin crown around 10 meters wide, splendid lilac flowers and leaves 17 centimeters long by 9 wide.

Other results refer to the environmental comfort of leisure areas. In the three green areas studied in the center of Campinas, measurements were also taken under the crown of trees grouped together and directly under the sun.

In the center of Campinas
As expected, the traditional Bosque dos Jequitibás (Cariniana grove), denser and older, giving out 99.03% solar radiation attenuation, stood out. Second, the Parque dos Guarantãs (Gasparillo Park), with medium-sized  vegetation, neither very dense nor thin, relieved  88.9% of the radiation. Finally, a surprise: the Bosque dos Artistas (Artists’ Grove), a new, and not yet very dense, square, achieved 88.24% of reduction.

The work on squares and groves resulted in the master’s thesis of the architect Larissa Fonseca de Castro’s, also of the Civil Engineering School at Unicamp,. When the project was completed, she and Carolina continued their studies and produced doctoral theses on the same topic. Now, the architect Érika Lois, a graduate student in Civil Engineering at Unicamp, has joined the group and uses the equipment already purchased to survey the degree of thermal comfort in relation to planting trees along watercourses.

The tools of the study

Two thermometers, one with a dry bulb and the other with a  wet one  – a set called a natural ventilation psychrometer – enabling the relative air humidity to be calculated, were fixed to one of the equipment’s supports. The same support had a globe thermometer attached to it, consisting of a hollow sphere, painted matte black, close to being the ideal black body – which absorbs radiation emitted by the surrounding environment and provides the so-called globe temperature.

Generally at every hour, the engineer Carolina Bartholomei, in charge of measuring, wrote down the readings given by the equipment and measured the air speed with a portable anemometer. With the globe temperature, air speed, and ambient temperature readings, she figured out the radiant temperature through an equation.

Carolina installed the linear solarimeter, equipment used by agronomists and botanists since the 70s to monitor plant growth, on another support, to measure the degree of solar radiation. The solarimeter was coupled to an automatic data recorder, thelogger, programmed to record the data every 10 minutes and download them directly to the computer.

Some practical adaptations were made, required by the whims of nature and by the urban conditions. The wind, for example, insisted on blowing down or shifting the position of the equipment supports. So, the environmental comfort laboratory technician, Obadias Pereira da Silva Júnior, also engaged in the project, built sturdier tripods and made suggestions to ensure the reliable reading of the data. Another problem, the risk of theft, was avoided by not leaving the solarimeters with the loggers attached permanently installed: the supports were assembled in the morning and collected at the end of the afternoon.

The project
Thermal Comfort in Towns: Effect of Planting Trees on the Controlled of Solar Radiation (nº 96/01262-1); Type Support for research project; Coordinator Lucila Chebel Labaki – Civil Engineering Faculty at Unicamp; Investment US$ 11,145.80