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Climatology - Research - GES

Thermal Benefits of Street Trees

Urbanisation can significantly alter the climate of a landscape. Urban temperatures, although significantly variable, are generally greater than neighbouring rural temperatures. The height of buildings, width of roads, presence of water and vegetation and anthropogenic heat sources contribute to modified urban temperatures. Increasing building densities with materials of high thermal admittance are likely to increase the temperatures of the surrounding area, especially during the night. This is enhanced by the high run-off rate within urban areas due to presence of impervious surfaces. The cumulative effect of these warming influences results in the generation of the Urban Heat Island (UHI).

Street trees are highly effective at ameliorating urban warmth at the micro-scale, referring to ground to building height. Tree shading reduces the amount of heat stored within urban surfaces. Shading has been observed to decrease late afternoon street temperatures by 3° Celsius (Ali-Toudert & Mayer 2007b), and it has been suggested that 80% of vegetative cooling effectiveness in hot, dry climates is attributed to shading (Shashua-Bar et al. 2010a). Evapotranspiration processes also lessen the levels of atmospheric heating of the surrounding area, acting to mitigate "the heat island not by cooling the air, but by warming the air less" (Kurn et al. 1994). The cooling potential of street trees is variable and dependent upon the level of canopy coverage, planting density and geometry of the street (Shashua-Bar et al. 2010a). Cooling from vegetation is generally greater with high canopy coverage and shallow, wide streets.

Street tree cooling can be utilised as a mechanism for improving the climate as experienced by people, or the Human Thermal Comfort (HTC) of an environment. HTC is primarily influenced by humidity, air movement and mean radiant temperature. Consequently, the shading and evapotranspirative effects of trees largely impact HTC levels. Ali-Toudert & Mayer 2007b cite solar exposure to be the primary determinant of thermal stress, highlighting the importance of shading for heat stress reduction. It has been estimated that Physiological Equivalent Temperature (PET - an indices of Human Thermal Comfort) can be reduced by 22° when standing directly beneath a tree canopy (Ali-Toudert & Mayer 2007b). Thus, street trees generally improve thermal conditions for pedestrians.

The objective of the joint research project between Monash University and the City of Melbourne is to:

- Quantify actual temperature and thermal comfort benefits of street trees;
- Assess the benefits of different tree canopy covers and placement within different street types

This will be achieved by monitoring temperature and human thermal comfort of selected streets within the City of Melbourne boundary, namely Bourke St East (Central Business District), Gipps St (East Melbourne) and George St (East Melbourne). The selected streets represent three arrangements of varying canopy coverage, as well as two canyon geometry types (high/narrow, shallow/wide), see Figure 1.

Figure 1. Location and relative tree coverage

The installation network will include 24 weather stations, encompassing the following components:
- HMP-155A temperature and relative humidity sensor
- MET-1A wind speed sensor
- SP-212 solar radiation sensor
- Black globe thermometer

                              
Figure 2. Example of monitoring system   Figure 3. Monitoring system installed in Bourke St East.

The resultant data will typically include average air temperatures and calculated thermal comfort indices (PET). Further data analysis will be conducted in order to assess the thermal benefits of street trees within Melbourne.

References

Ali-Toudert F & Mayer H. 2007. ‘Thermal comfort in an east-west oriented street canyon in Freiburg (Germany) under hot summer conditions'. Theoretical and Applied Climatology 87: 223 - 237.

Kurn D K, Bretz S E & Akbari H. 1994. ‘The potential for reducing urban air temperature and energy consumption through vegetative cooling'. Proceedings of the 1994 Summer Study on Energy Effects in Buildings, Pacific Grove, CA.

Shashua-Bar L, Oded P, Arieh B, Dalia B & Yaron Y. 2010. ‘Microclimate modelling of street tree species effects within the varied urban morphology in the Mediterranean city of Tel Aviv, Israel'. International Journal of Climatology 30: 44 - 57.