This study examines the climatic behavior of different designs of urban parks during hot and humid summer conditions, and their influence on human comfort in Tel Aviv, Israel. The research was conducted in three different types of urban parks: a park with grass and a few low trees, a park with medium sized trees and a park with high and wide-canopied trees. The results showed that an urban park that contains high trees with a wide canopy has the maximum cooling effect during daytime, reduces temperatures by up to 3.5°C and lowers heat stress values despite increasing relative humidity values. An urban park that contains dense, medium sized trees can also reduce temperatures during daytime by up to 2.5°C as well as slightly lower heat stress. However, during nighttime it can create uncomfortable climatic conditions owing to the reduction of wind velocity and increase in relative humidity. An urban park covered with grass can be warmer and sometimes even more humid than the built-up area during the day, which increases heat stress values.

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ABSTRACT:Microclimate formation and its significance in urban planning was examined through two components that predominate in affecting the city's climate: built-up morphology and urban shade trees.The methodological approach focus is on a generalization procedure for quantifying the thermal effect of any studied situation through parameterization of the vegetated variables and the built-up forms. The analysis is integrative, using empirical climatic data followed by an analytical study for generalization and sensitivity analysis using an integrative model, the Green CTTC model.Three urban tree species predominant in the Tel Aviv gardens and streets, with different canopy characteristics, and three levels of building densities were analysed to determine their thermal effect on an urban street microclimate. The variables were parameterized according to six basic cooling attributes for the studied tree species in urban gardens in Tel Aviv, and according to three geometric built-up parameters for the studied urban street.The integrative modelling approach of considering all changes simultaneously was illustrated on an urban boulevard in Tel Aviv. The analysis demonstrates the shortcomings of piecemeal modelling and the merits of the integrative approach.The study indicates the importance of urban trees in alleviating the heat island effect in a hot and humid summer. The tree cooling effect was found to be strongly related to the built form geometry. In all the studied cases, the thermal effect of the tree was found to depend mainly on its canopy coverage level and planting density in the urban street and little on other species characteristics.The methodology of analysis presented in this paper can be applied to develop an operational tool in assessing for an urban open space the integrative thermal effects of different tree species, and of the varied urban morphology and the interaction between them.

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