The influence of trees and grass on outdoor thermal comfort in a hot‐arid environment

Abstract

ABSTRACT:The effects of vegetation on human thermal stress in a hot-arid region were tested in two semi-enclosed urban spaces with various combinations of mature trees, grass, overhead shading mesh and paving. The index of thermal stress was calculated hourly from measured meteorological data in the studied sites to evaluate thermal comfort in the different spaces based on radiative and convective pedestrian-environment energy exchanges and sweat efficiency, and expressed on a thermal sensation scale ranging from 'comfortable' to 'very hot'. The efficiency of water use in providing improved comfort was gauged for each of the vegetative landscaping treatments by comparing the total evapotranspiration with the reduction in thermal stress, both expressed in terms of their values in equivalent energy. While conditions in a paved, unshaded courtyard were found to be uncomfortable throughout the daytime hours (with half of these hours defined by severe discomfort), each of the landscape treatments made a clear contribution to improved thermal comfort. With shading, either by trees or mesh, discomfort was reduced in duration by over half and limited in maximum severity when the shading was placed above paving. When combined with grass, both shading mechanisms yielded comfortable conditions at all hours. In both cases, the effect of trees was more pronounced than that of the mesh, but by a small margin. With unshaded grass, 'hot' conditions in the courtyard were restricted to a short period in mid-afternoon, a considerable improvement over unshaded paving, attributable mainly to the lower radiant surface temperatures.

Hot playgrounds and children's health: A multiscale analysis of surface temperatures in Arizona, USA

“…However, this study concurs with studies finding T s reduction from many types of shade (e.g., Antoniadis, Katsoulas, Papanastasiou, Christidou, & Kittas, 2015;Ketterer & Matzarakis, 2014b;Lin & Lin, 2010), where all shade, whether tree or artificial (e.g., mesh or shade sails), is found effective in reducing the T s in the hot and arid climate to safe levels. This would in turn reduce thermal discomfort from sensible heat fluxes as found in related shade studies (e.g., Antoniadis et al, 2015;Shashua-Bar et al, 2011). Further microclimate factors influencing heat stress and thermal discomfort-in addition to exchanges of solar radiation and sensible heat (via convection) discussed here-include evaporation and longwave emittance (Kenny, Warland, Brown, & Gillespie, 2009;Vanos, Warland, Kenny, & Gillespie, 2010), with metabolic activity increasing core and skin temperatures due to increased heat generation Yao, Lian, Liu, & Shen, 2007).…”

“…However, this cooling effect as studied costs a substantial amount of water to achieve-an expensive proposition in a desert city. Further bioclimatic design in hot and sunny climates must employ shade as a means of effectively reducing the mean radiant temperature imposed on a human at the hottest time of the day, and thus overall heat stress (Ali-Toudert & Mayer, 2007;Matzarakis & Endler, 2010;Shashua-Bar, Pearlmutter, & Erell, 2011). We show that shade generates a T s that is similar or lower than the reference temperature, T a , reversing the general trend where T s > T a .…”

Land cover, climate, and the summer surface energy balance in Phoenix, AZ, and Portland, OR

“…These findings agree with the interrelated effects of different surface characteristics previously identified by several studies. Shashua-Bar (2011) found that the actual microclimatic effects of vegetation are complex and interrelated with the effects of other built environments. Oke et al (1999) noted that typically, the sensible heat flux is likely to increase with impervious land cover, but at the same time, the relationship is influenced by water availability and the efficiency of storage.…”

Quantification of turbulent heat fluxes for adaptation strategies within urban planning

“…green spaces and water bodies) can serve as key role in mitigating summer thermal loads. The positive effects of non-sealed surfaces and urban parks and gardens, especially in reducing temperatures through shading, evapotranspiration and cool-air production, are as well known as the cooling effects of urban water bodies (Saito et al, 1990(Saito et al, /1991Kuttler, 1991;Spronken-Smith and Oke, 1998;Upmanis, 1999;Saaroni and Ziv, 2003;Bongardt, 2006;Robitu et al, 2006;Pearlmutter et al, 2009;Lin et al, 2010;Hassaan and Mahmound, 2011;Peters et al, 2011;Shashua-Bar et al, 2011). In addition, positive effects may be observed in neighbouring developed areas, provided that the green space is of sufficient size, the wind speed is adequate and the buildings on the fringe of the area are appropriate (Eliasson, 1996;Bongardt, 2006;Ng et al, 2012).…”