The estimated relationship between the cooling effect of a site and the background temperature, as proposed in this work, provides the means for proper comparison of sites with different background temperatures. To our knowledge, the proposed technique is novel.
The tree shading cooling. as expected from previous studies in non-wooded urban spaces, the statistical analysis of our data indicates also that the shade factor plays a major role in determining the cooling effect of the site. Its effect is more or less the same for all 11 sites. In the studied sites, shading in summer is provided by the trees: on the average, about 80% of the cooling effect was contributed by tree shading.
The shading coverage factor, besides its uncontested role in the cooling process, is also a control variable. It can be regulated by the cultivation regime and by pruning, and in new sites by proper choice and placement of the shade trees.
Following the empirical model, a more general analytical model is being developed based on the cluster thermal time constant (cttc) model. This model will consider the theory leading to the above two effects.
The site specific effect. The site specific effect stands for the effects of many unaccounted variables such as tree characteristics, the site’s geometric configuration, the water regime, etc. a priori, we would have expected a major role for this factor. However, apart from the tree’ shading effect, what remains to be explained is minor. On the average, the specific effect contributes about 0.5 K of cooling in addition to the shading effect. The variation of the specific effects among the sites is small.
The effect of trees in the street. The shading effect of trees in the streets was found to have the same magnitude of cooling effect as in the other sites. However, heavy traffic has an opposite effect of about 2 K .this can explain the fact that the specific effect of herzl street is 0.75 K compared to-0.5 K, the average of the sites. The two sites rothschchild and hen avenues are also streets with heavy traffic, but with on noticeable heating effect. This may be due to the fact that these two avenues are wide (about40 m) and their tree canopies relatively high (10 to15 m high). The absence of heating effects in these two sites suggests that ventilation can be an important factor and should be taken into consideration in the design of trees in a street. In any case, it is important to stress the fact, as found in this study, that even a moderate tree shading coverage (say 60% as in the herzl street) more than offsets the heating effect of heavy traffic.
The cooling effect on the site surroundings. The range of the cooling effect was found to be rather narrow and is perceivable up to 100m from the site boundary. The fact corroborates earlier studies, although in our case we are dealing, by design, with much smaller green areas not wider than 60 m. the cooling values were found to follow an exponential decay function.
The cooling effects of small green areas as found in this study are significant. The effects are, however, local, and as such can be used to suggest some policy measures for alleviating the so-called urban heat island effect the urban environment.
(a) The range of the cooling effect being perceivable up to 100m suggests small gardens, 200m apart. These gardens can be designed to accommodate the recreational needs of young children and senior citizens. The proposed size of such gardens is 0.1 ha, equal to the area of the apartment building commonly found in the urban tel- aviv complex.