Urban microclimate in temperate climates: a summary for practitioners

1. Introduction

The dramatic growth in the scientific understanding of the microclimatic consequences of urban growth is increasingly yielding policies, programmes, protocols and planning codes to mitigate its negative consequences. The aim of this briefing note is to succinctly capture key considerations and current developments that have a bearing on planning practice. It highlights the current science findings as well as knowledge gaps, closing with key issues for the integration of urban heat resilience policy with climate change mitigation.

The urban microclimate anomaly is at its most distinctive state on clear (cloudless) and calm (little or no wind) nights (ranging from soon after sunset to just before dawn) and manifests itself in air temperature, relative humidity, wind (speed and direction) as well as precipitation in the manner described in Table 1. It further impacts building energy needs, human comfort and wellbeing, as well as regional climate change.

2. Causes

Cities tend to absorb more of the total available heat (i.e. solar radiation plus heat generated from human activities), thereby raising the urban air temperature. The way land is used and covered, the configuration (massing) of buildings relative to each other and in relation to streets, the thermal properties of building materials and pollution from human activities all add to the unique urban signature on local climate.

At street level, the height of buildings with respect to the width of streets (also known as the aspect ratio) and the direction of the wind with respect to the street layout determine the nature of air flow within resultant canyons. These influence both their temperature as well as their air quality. Differential heating of canyon walls (especially the windward walls) and the addition of roughness elements (such as awnings, protrusions and setbacks) in combination with an appropriate canyon aspect ratio (especially a square canyon, where the average height of buildings is equal to the width of street) could enhance street ventilation (Fellini et al. 2020).

The influence of urban as well as street-level parameters on local climate is dependent on the location of the city (the ‘latitude effect’). At lower latitudes with high levels of solar insolation (i.e. the amount of energy from the sun), street-level conditions are less likely to be affected by building shading. At higher latitudes, the effect of solar insolation is highly dependent on the geometry of building forms. Street layout (e.g. diagonal streets versus a grid pattern oriented along the cardinal directions) add further complications. Urban density—whether induced by ‘vertical’ parameters such as the floor area ratio (FAR) or facade area as indicated by the facade-to-site ratio, or ‘horizontal’ density as indicated by plan area density (PAD) that indicates the extent of plot coverage—also influences the local climate (Chatzipoulka & Nikolopoulou 2018). High horizontal density would lead to stronger urban climate anomaly in winter (Table 1), while an increase in vertical density would result in an intense summertime urban heat island (UHI) (Salvati et al. 2019).

Planners and designers may be able to influence the horizontal density (the compactness of an urban arrangement) and the vertical density (the height of buildings as well as the efficiency with which the built volume is enclosed by the building envelope). These could be used to enhance local thermal comfort at street level.

Figure 1 summarises these interactions at both the street and citywide scales to highlight the planning controls that could be used to ameliorate the negative consequences of urban climate. Street geometry, built form, vegetation and the thermal properties of materials are the key levers to control urban climate at local scales. The use of these need to be context specific and wider questions relating to urban activities, land use and functions should also be considered (Jamei et al. 2016). Human thermal comfort depends not only on air temperature but also on other factors such as wind, solar reflectance and humidity (Heris et al. 2020). Thus, urban climate mitigation needs an integrated approach that addresses all local climate conditions simultaneously.

Figure 1

3. Approaches to mitigation

Attempts to mitigate the negative consequences of urban climate need to consider the questions of WHAT we want to change as well as WHERE we want to achieve this. In terms of the ‘what’ question, the goals of mitigation options can be grouped into three:

• Climate improvement: temperature control to manage heatwaves and mitigate overheating, reduce interference with humidity and precipitation.

• Health and wellbeing: thermal comfort and air quality enhancement; health, productivity and comfort; and improvements to biodiversity.

• Carbon management: efficiency enhancement in energy and water use.

Tools and approaches to achieve these at scale are three-dimensional urban form, nature-based solutions (NbS) and the manipulation of the surface properties of buildings.

In terms of urban form, volumetric compactness (horizontal density), aspect ratio and form factor (vertical density), plot surface, openness to sky (as given by the sky view factor and distance to the nearest wall), and land cover types (especially hard surfaces that act as heat sources or soft surfaces that act as heat sinks) are all important for mitigating the negative consequences of urban climate (Giridharan & Emmanuel 2018). The presence of open spaces, the shape of building envelopes and the symmetry of the street further add to local-level (at the street scale) thermal comfort variations during the day (Guo et al. 2019).

Solar geometry dictates the prioritisation of streets and facades. East–west canyons receive more direct sun than north–south canyons and therefore solar radiation falling on eastern and western facades has the most impact on the microclimate. Its impact on north and south facades makes negligible contributions to pedestrian thermal comfort (Taleghani et al. 2021).

The NbS can also be applied to urban climate mitigation, including green (vegetation) and blue (water) infrastructures. The evapotranspiration-based cooling influence of both green and blue space is critical to the lower atmosphere in cities (i.e. the space from ground up to the average height of roofs—the ‘urban canopy layer’). The magnitude and spread of cooling by NbS depends on the size, spread, distribution and geometry of such solutions. Solitary large parks offer minimal cooling across large areas (Gunawardena et al. 2017), whereas a more distributed provision of green spaces spreads the cooling benefit more widely. However, city-scale cooling is mainly derived from the increased surface roughness of green space (i.e. improved convection efficiency) rather than evaporation (Gunawardena et al. 2017). Blue space cooling during the day can be substantial, but there is a nocturnal warming penalty. When both green and blue spaces are employed together, they can offer many synergistic ecosystem benefits, including cooling (Gunawardena et al. 2017).

For thermal comfort in the streets and neighbourhoods (as opposed to mere air temperature at the street level or across the city as a whole), trees in combination with buildings, grass verges against paved surfaces, and aligning the streets along predominant wind directions can all help to mitigate thermal discomfort in summer (Kleerekoper et al. 2017) without a significant wintertime penalty. In terms of ‘efficient’ positioning of trees, the north-east side of a north-west to south-east street or the north side of an east–west canyon is more useful while their positioning in north–south or north-east to south-west street canyons is more flexible (Chatzidimitriou & Yannas 2017).

Building envelopes provide the third set of urban climate mitigation possibilities. The choice of materials depends on street orientation (Taleghani et al. 2021). A high albedo (i.e. high solar reflectance) enables heat to be quickly radiated back into the atmosphere (Hoverter 2012). High-albedo roofs are widely used in the US (so-called ‘cool roofs’), where a combination of building codes, grant programmes and utility rebates are deployed to reduce both the building energy needs as well as urban air temperature. However, there are uncertainties related to the value for money of the high-albedo approach at city scales as opposed to energy savings to individual buildings (Pomerantz 2018).

The final question to be answered in urban climate mitigation is WHERE interventions should be attempted. The scale of interventions could either be macro (city wide) or micro (neighbourhoods to single street blocks). The former is more beneficial in terms of climate improvement and carbon management, whereas the latter has more immediate health and wellbeing benefits. All three approaches (urban form, NbS and building surfaces) are amenable to both scales.

4. Urban climate mitigation in practice

Practical approaches to mitigate the negative consequences of urban climate are emerging around the world in terms of mapping and analytical tools as well as planning codes (Table 2).

Applying current knowledge about urban climate to urban planning requires the accurate characterisation of land use/land cover and the functions of urban neighbourhoods. Typically, planning considers urban space in terms of discrete sets of urban form or land-use-based zoning, or a combination thereof. However, amelioration of urban climate calls for an integrated, goal-oriented approach (climate improvement, health and wellbeing, or carbon management) to better describe the urban form and to deal with the intrinsic complexity of urban spaces (Vanderhaegen & Canters 2017). The recently introduced Local Climate Zone (LCZ) approach (Stewart & Oke 2012) provides a useful method to combine both the form and function of urban spaces from a local climate point of view.

5. Action points

Planning approaches to urban climate mitigation should be seen as part of the wider, climate change vulnerability actions. The mapping of heat vulnerability provides a way to couple surface UHI measures with socioeconomic indicators of vulnerability. Figure 2 provides a framework to tackle heat vulnerability and bring adaptation responses to global climate change together with urban climate-mitigation measures to reduce risks at both scales simultaneously.

Figure 2

Having mapped the heat vulnerability, planning action could focus on enhancing heat resilience. Heat resilience in this context refers to urban planning, building design, public health and infrastructure provision to increase the quantity and quality of coping strategies. Table 3 provides a policy framework indicating four areas where action is needed for such heat resilience.

Global climate change will impose additional pressures on urban climates. This is likely to result in an increasing frequency and length of warm events, significantly influencing how the local climate is experienced by urban dwellers. The ‘southward shift’ of climate in cities provides an easy way to understand the urban climate of the future. Cities in the Northern Hemisphere, especially in Europe, follow a north-to-south transect at about 3–13 km/year. This average southward velocity is expected to double throughout the 21st century (Rohat et al. 2018). It is therefore possible for ‘northern’ cities to learn planning and urban design lessons from cities further south, including the benefits of shading, more compact development and more explicit use of NbS.

Such lessons should focus on climate-sensitive urban design, integrated land use and urban densification. Using the action areas highlighted in Table 3, planning authorities could establish a set of context-specific principles in terms of the three ‘goals’ of urban climate mitigation (see ‘Approaches to mitigation’) at different levels of granularity (site, street, neighbourhood, city).

Not all lessons from warm places are positive. Special mention must be made of the urban densification pressures (both horizontal and vertical density) that can exacerbate the urban microclimate anomaly. The heat vulnerability framework in Figure 2 shows a way to balance the drivers and responses to enhance resilience to heat and therefore urban liveability in the face of climate change.

Competing interests

The author has no competing interests to declare.