This objective refers to the improvement of the thermal comfort in outdoor spaces by enhancing the quality of outside environments by reducing ambient and mean radiant temperatures, decreasing the amount of solar exposure and thus reduce surface temperatures and the heat stored in the urban fabric, and promoting the use of outdoor spaces for relaxation, leisure, meeting and other activities [Gherraz et al. 2018, CRC 2017] .
This objective refers to the reduction of the number of additional deaths in the population and the negative impacts (illnesses) on people’s health that are attributed to high urban temperature and/or extreme heat events (i.e. UHI and heatwave) [CRC 2017] ; by reducing the public health burden, reducing the heat exposure and increasing the accessibility of vulnerable populations to affordable cooling systems, and preparing and protecting people’s health during hot weather.
This objective refers to the reduction of the maximum electricity demand employed for cooling indoor spaces (buildings) during hot weather conditions, that can be achieved by providing cooler outdoor environments and decreasing the need for air-conditioning in indoor spaces which in turn results in less waste heat production (anthropogenic heat) and reduced carbon emissions [CRC 2017].
This objective refers to the reduction of the maximum water demand employed in buildings and for irrigation during a period of extreme heat by providing an efficient use of water for reducing people’s health problems, sustaining greenery and providing evaporative cooling in urban environments [Santamouris et al. 2017]
The magnitude of UHIs can be quantified by estimating the UHI intensity (ΔTu-r) which is measured by the difference between the highest air or surface temperature of a given location (usually referred as urban area) and the corresponding temperature of a reference point (commonly referred as the rural surrounding) [Tzavali et al. 2015].
The CDD is a measure of how much (in degrees) and for how long (in days) outdoor average ambient temperature is above a base temperature, that is the point above which a place needs to be cooled (this may vary among locations). For the calculation of the CDD, the mean degree hours method can be implemented since it is considered as the most rigorous (and mathematically precise) method [CIBSE 2019]. This estimation consists of subtracting the base temperature from the hourly outdoor air temperatures, and average the sum of positive hourly differences, which are referred as cooling degree hours (CDH) analogously to cooling degree days, over the day. The CDD index can be used to determine the severity of the climate and the calculation of the residential cooling load savings [Santamouris et al. 2017].
There are available several indices that can be utilised to measure the effect of UHIs on perceived human thermal comfort (HTC) and these include:
Universal thermal climate index (UTCI) This is an outdoor thermal comfort (OTC) index which is based on a human energy model that quantifies OTC by integrating the personal parameters (human activity and insulation) and four thermal-physiological factors: ambient temperature, mean radiant temperature, relative humidity and wind speed. The impact of UHI mitigation technologies can be evaluated by estimating the difference in UTCI between the base scenario and the mitigation strategy [UTCI 2019, Blazejczyk 2012].
The Physiological Equivalent Temperature (PET) biometeorological index This index provides is a human biometeorological indicator that describes the thermal perception of an individual. The PET is applicable for both indoor and outdoor environments [Höppe 1999].
The Temperature of Equivalent Perception (TEP) thermal index The TEP index provides a thermal sensation scale based on the air temperature of a reference environment [Monteiro et al. 2009].
Predicted mean vote (PMV) This measure provides an empirical fit to the human sensation of indoor thermal comfort by predicting the average vote of a large group of people according to a seven-point thermal sensation scale ranging from hot (+3) to cold (-3) [Designing Buildings Wiki 2019].
This measure refers to the number of additional deaths in the population that can be attributed to extreme heat events (i.e. UHI and heatwave). This is usually estimated by counting the number of excess deaths in a year per 100,000 inhabitants considering the average number of deaths over a certain period (i.e. five years).
Further, the heat related mortality model developed by Gosling et al. (2007) [Gosling et al. 2007] quantifies the relationship between the increment of ambient temperatures and heat-related daily mortality rate per 100,000 people, as per the following equation:
Mortality Rate (MR) = e 4.898 - 241.2 / Tmax
It should be noted that heat related mortality should be used with caution as this index is also dependent on demographic factors as the proportion of vulnerable populations (i.e. children and elders) may vary among locations [Santamouris et al. 2017].
The calculation of the energy saved because of the application of a UHI mitigation strategy can be quantified by using two empirical indicators as suggested by Santamouris (2014) [Santamouris 2014] [Santamouris et al. 2017]:
The global energy penalty per unit of city surface and per degree of the UHI intensity (GEPSI).
The GEPSI index (kWh/m2/K) is highly influenced by building density and considers the average UHI characteristics of a given place. Thus, higher GEPSI values correspond with a higher building density and UHI intensity.
The global energy penalty per person and per degree of the UHI intensity (GEPPI).
The GEPPI index (kWh/p/K) is highly influenced by population density and average UHI intensity characteristics of a given place. Thus, higher GEPPI values correspond with a lower population density and higher UHI intensity.
This calculation refers to the maximum electricity demand employed for cooling registered during a specific period. The impact of UHI mitigation strategies can be measured by comparing the electricity demand of a base case scenario (when no mitigation strategy has been implemented) versus the electricity reduction achieved by implementing different mitigation strategies [Santamouris 2016, Santamouris et al. 2017].
This calculation refers to the maximum water demand registered during a specific period (i.e. a heatwave) compared to a reference case [Brazel 2009]. The impact of UHI mitigation strategies can be measured by comparing the water consumption demand of a base case scenario (when no mitigation strategy has been implemented) versus the expected water consumption estimated when different mitigation strategies are implemented. It should be noted that the peak water demand may be affected by water restrictions such as droughts.
The following climate region is defined according to NatHERS climate zones and weather files [NatHERS 2019, Climate Zone Map 2019]
The following climate region is defined according to NatHERS climate zones and weather files [NatHERS 2019, Climate Zone Map 2019]
The following climate region is defined according to NatHERS climate zones and weather files [NatHERS 2019, Climate Zone Map 2019]
The following climate region is defined according to NatHERS climate zones and weather files [NatHERS 2019, Climate Zone Map 2019]
The following climate region is defined according to NatHERS climate zones and weather files [NatHERS 2019, Climate Zone Map 2019]
The following climate region is defined according to NatHERS climate zones and weather files [NatHERS 2019, Climate Zone Map 2019]
The following climate region is defined according to NatHERS climate zones and weather files [NatHERS 2019, Climate Zone Map 2019]
The following climate region is defined according to NatHERS climate zones and weather files [NatHERS 2019, Climate Zone Map 2019]
The following climate region is defined according to NatHERS climate zones and weather files [NatHERS 2019, Climate Zone Map 2019]
The following climate region is defined according to NatHERS climate zones and weather files [NatHERS 2019, Climate Zone Map 2019]
The following climate region is defined according to NatHERS climate zones and weather files [NatHERS 2019, Climate Zone Map 2019]
This zone corresponds to a range of low-density residential uses such as dwelling houses and dual occupancies as well as non-residential uses to meet the day to day needs of residents. This zone also includes residential housing in rural settings and/or large lots.
This zone corresponds to a broader range of residential dwellings such as residential flat buildings, attached dwellings (terraces), and multi-dwelling housing (townhouses).
This zone provides a variety of housing types within a high-density residential environment. Also, it permits other land uses that provide facilities or services to meet the day to day needs of residents.
This zone refers to small and medium scale centres that provide a range of retail, business, entertainment and community functions that serve the day-to-day needs of residents.
This zone provides a wide range of retail, business, office, entertainment, community and other suitable land uses that serve the needs of the local and wider community.
This zone corresponds to centres containing an integrated range of businesses, residential and retail uses that are in proximity to public transport. These centres are generally characterised as corridor or strip centres
This zone provides a wide range of mix of business (i.e. business parks and corridors), light industrial, warehouse and related land uses that support the viability of centres. It also provides facilities or services to meet the day to day needs of workers in the area.
This zone provides suitable areas for those industries that need to be separated from other land uses. This zone also includes to areas dedicated to industrial and maritime activities that require direct waterfront access (i.e. ports).
This zone corresponds to a range of government and community facilities and institutions including schools, signal stations, police stations, council buildings, etc.
This zone applies to publicly- and privately-owned land and touristic areas such as open spaces (i.e. plazas, squares), recreation land, local, regional and national parks, and reserves.
This zone corresponds to a range of infrastructure including, major roads (i.e. highways), railway stations and transport corridors, energy, water and sewage supply systems and facilities.
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It should be prioritised the use of cool or reflective materials in roofs with high emissivity, high albedo and high reflectivity properties. These can significantly reduce the absorption of solar radiation and thus, decrease the amount of heat emitted to the atmosphere, improve perceived outdoor and indoor thermal comfort, and decrease cooling energy demand.
It should be prioritised the use of cool or reflective materials in walls (external facades) with high emissivity, high albedo and high reflectivity properties. These can significantly reduce the absorption of solar radiation and thus, decrease the amount of heat emitted to the atmosphere, improve perceived outdoor and indoor thermal comfort, and decrease cooling energy demand.
Green roofs are roof systems that partially or completely covered with plants and a growing medium. Green roofs can provide solar and heat protection to buildings and contribute to decrease surface temperatures, improve indoor thermal comfort and decrease cooling energy demand of spaces directly below or separated by one or two floors from the roof. Green roofs can also contribute to increase local biodiversity, reduce the reflection from surrounding glazed surfaces, harvest rainwater for irrigation, and reduce stormwater runoff and peak flow rates. The cooling performance and effectiveness of green roofs depend on external factors such as climatic conditions (solar radiation, ambient humidity, wind speed, precipitation) and construction parameters (types and amounts of plants, depth of growing medium, irrigation levels).
Vertical greenery systems (VGS) –also referred as green/living walls– integrate plants onto buildings facades and other vertical structures. These can increase evaporative cooling and provide solar and heat protection (shade) by reducing surface temperatures, improving indoor thermal comfort of buildings and contributing to energy conservation. In addition, VGS can contribute to human wellbeing, regulate stormwater impacts, reduced water usage, increase local biodiversity, improve the efficiency of building integrated PV (BIPV), and increase property values and amenity for people.
Balconies sometimes are highly exposed or unprotected and may contribute to decreased HTC conditions and higher peak electricity consumption by increasing the need of air-conditioning on adjacent indoor spaces. Thus, these building elements should be well designed from a climate perspective [City of Adelaide 2019].
Buildings provide shade over different urban surfaces, that may vary during the day and seasons, which can help block solar radiation and decrease mean radiant temperatures (MRT) leading to enhanced outdoor thermal comfort in indoor and public areas. Some building elements such as awnings, verandas and arbours can be used as shading devices to provide additional solar protection to footpaths and open spaces adjacent to buildings. However, they should be carefully designed to cut off incident radiation from the sun while enabling light penetration.
A cool pavement can be defined as a street pavement that absorbs less solar radiation than a traditional dark-coloured concrete or asphalt pavement. Significant advances in the development of ‘cool pavements’ have been achieved in recent years and two main technologies are already available to be implemented in urban development. On one hand, developments can apply cool pavements with a high solar reflectivity and high emissivity characteristics that cause a minimal glare effect on pedestrians. On the other hand, there are ‘water retention pavements’ that use the infiltrated water to decrease surface and near-surface air temperatures through evaporation. A detailed list of different cool pavements and technologies and their application on the built environment is presented in the ‘Guide to urban cooling strategies’ developed by Osmond, P., & Sharifi, E. (2017) (page 18) [CRC 2017].
Street trees and plants can positively moderate the urban microclimate of urban canyons and significantly reduce surface and ambient temperatures and improve outdoor thermal comfort. Nevertheless, a unique design, pattern and palette of tree plantings should respond to particular characteristics (i.e. street orientation, level of pedestrian activity, sense of place, character, and type of surrounding activities/uses) and street sections (i.e. widths and heights of frontages). Accordingly, five ‘planting schemes’ have been generally defined according to typical urban areas found in cities [City of Adelaide 2019]:
In addition, three main planting types with anticipated canopy covers are proposed as general guidelines to accommodate trees according to street sections, available space for plantings (considering ground, underground and overhead constraints) and amount of shading required [City of Melbourne 2019]:
A combination of planting schemes (1-5) and canopy covers (A-C) enables a more versatile and flexible design and distribution of trees in different urban contexts (i.e. 1A corresponds to a civic and commercial street with a canopy cover of 20%). Specific planting configurations would depend on certain constraints (i.e. footpath widths, traffic lanes, light-rail/tram routes, etc.); despite this, general provisions, suggestions and guidelines are recommended below.
These streets are generally characterised by high pedestrian activity, such as outdoor dining, retail and shops, business activities and public transport (bus lanes, metro, train, light-rail/tramways, etc.)
These streets are usually located in residential and mixed-use areas of medium and high densities, with local services, restaurants and shops located in the ground floor.
These streets are typically found in medium and low-density residential areas characterised by wide urban canyons and varying vegetation cover.
These streets are typically located in industrial sites and warehouse land uses and characterised by wide sections and lack of vegetation.
These streets are mostly located in historic precincts/locations with a distinct character and important heritage value.
Green open spaces (including trees in open spaces) have numerous of benefits including climate moderation (reduction of air and surface temperatures), improved outdoor thermal comfort and amenity, increased biodiversity value, improved human health and social cohesion, energy savings, enhanced air quality, among many others. In this sense, green open spaces should be a primary aspect to consider in the design of any urban development, as it is highlighted as a mainstream strategy to mitigate UHI more effectively.
Provisions:
The excess heat of the urban environment can be effectively dissipated by using natural heat sinks that usually present much lower temperatures than the surrounding ambient air. Water bodies act as major heat sinks as they are excellent heat absorbers, and along with natural ground cover (i.e. pervious surfaces), can be implemented for passive cooling dissipation to decrease cooling loads of buildings, reduce surface temperatures and improved outdoor thermal comfort [Santamouris et al. 2017, Koc 2018, Natural resources 2019].
Since water properties clearly contrast with those of land (terrestrial) surfaces, water features (i.e. fountains, lakes, rivers, ponds, ocean, marshes, wetlands, etc.) are the most efficient in reducing surface temperatures during the day (especially large water bodies). However, they provide a relative heating effect during the night which is a condition that is more desirable in winter seasons [Santamouris et al. 2017].
On the other hand, as water needs energy to change phase from liquid to vapour, evaporative cooling refers to the process of removing heat from the atmosphere through evaporation .
Provisions:
Shading structures are cost-effective solutions that offer protection to public space users from direct solar radiation and enhance outdoor thermal comfort by decreasing mean radiant temperatures [CRC 2017]. Also, they provide shade over urban surfaces that help reduce surface and near-surface air temperatures.
Water filtration, harvesting and treatment systems are essential components of WSUD that can contribute to mitigate UHI by ensuring evapotranspirative cooling and managing storm- and rain-water for treatment and irrigation purposes. Thus, WSUD strategies and technologies should be included and prescribed in legislation and planning controls [DEPI 2014].
A raingarden or bioretention basin is a planted basin designed to collect and clean incoming stormwater runoff by using an underlying infiltration medium or a set of transitional layers.
Compared to raingardens, bioswales do not only harvest and filtrate incoming stormwater, but also move water along the drainage system. Bioswales are mostly grassed or vegetated, these promote water infiltration into the underlaying medium, this water is then collected into perforated pipes and redirected to the drainage system.
Constructed wetlands are temporarily or permanently inundated areas that can help detain and control stormwater for a certain period of time and are designed for water management and treatment. They vary in scales and style and may include zones with shallow and deep water [WSUD 2019, CRC 2015].
Anthropogenic heat produced by human activities (i.e. industrial activities, transport, air conditioning) can significantly influence the formation and magnitude of the UHI; hence several strategies and provisions can be implemented to reduce the amount of heat originating from transport systems, assisted cooling/heating and urban energy in general [Santamouris 2015].
Heat refuges or public cooling centres are recommended for a more efficient heat management aimed at reducing heat-related morbidity and mortality on normal hot days and extreme heat events (i.e. heatwaves). In practice, air-conditioned facilities such as council libraries, halls and private facilities like shopping malls are commonly utilised as cool refuges during heatwaves. As these may be insufficient in some cases, additional special-purpose refuges could be strategically provided in public spaces including hydration stations, air-conditioned refuges, and emergency refuges [Fraser et al. 2018, UNSW 2019].
Several systems and technologies might be combined to mitigate the UHI. However, it is important to mention that any combination of strategies should be quite lower compared to the sum of the contributions of each mitigation technology separately [Santamouris et al. 2017]. The effectiveness of the strategies listed above were estimated from 55 urban projects from all over the world including reflective materials (cool roofs and pavements), greenery, water systems, shading devices, which represent the most popular combinations of mitigation technologies applied by real projects. [Santamouris et al. 2017]
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The heat vulnerability index measures the propensity of a population within the Sydney Metropolitan area to be negatively affected by extreme heat events. The index results from three key components measuring the vulnerability of population: exposure, sensitivity, and adaptive capacity.
