The Urban Climate

The city with its built environment has a significant effect on the local climate, be it on a street level or on the atmosphere surrounding it. The urban climate is best understood as multi-layered, each layer affecting how a city’s outdoor spaces are being used. The extent of which the local climate is modified by the urban environment depends on the physical aspects of a city. The thermal comfort of citizens and utility of public spaces is therefore a combination of weather and location as well as the physical urban characteristics. It is these physical aspects which can be modified to improve climatic conditions in urban spaces. Let's look at elements of each urban climate layer:

The macroscale represents the meteorological context which forms the baseline of the local urban climate. Climate change is altering this baseline and should therefore be part of the planning horizon. The macroscale affects all aspects of urban planning and design, therefore being aware of the components that make up the macroclimate is very useful to adapt cities for the future. A first step therefore is an analysis of the current and future climate, which includes average monthly maximum and minimum temperatures, relative humidity, prevailing wind patterns, the amount of solar radiation and sun hours, as well as seasonal variations in these variables. Finally, climate projections can inform about changing temperatures, weather patterns and extreme weather events. The macroscale can be addressed through sustainable planning and practices.

One of the important manifestations of the urban climate on the mesoscale is the Urban Heat Island (UHI), where the air and surface temperatures of a city are higher than that of the surrounding landscape. How pronounced the UHI is varies in time and place as a result of the local macroclimate and the built environment. There are several known factors which influence the UHI effect: 

• The main source of heat is solar radiation and as it hits the city is either reflected (light, high albedo) or absorbed (dark, low albedo) by its surfaces. Absorbed heat is released back over time. Urban environments have more surface area and materials which store more heat (asphalt, concrete, stone) than its surrounding rural areas.

• The building density and height of buildings creates urban canyons, which inhibit heat loss over time, radiating it back into the urban fabric. 

• Less vegetation and more sealed surfaces mean less evaporation, resulting in more sensible heat and less cooling through evaporation. Tree coverage not only increases cooling, but also reflects incoming solar radiation, making it a key factor of the UHI. 

• Another source of heat is human activity (anthropogenic heat) from combustion by cars, industries, building heating and air conditioning. The pollution from combustion also absorbs and re-emits heat back to the urban environment.

• Wind is the main cooling component, however the built environment can decrease wind cooling on the street level and obstruct natural air flows resulting from the local topography (such as cool air from surrounding hills)

Urban Heat Islands are a phenomenon observed on the scale of the whole city and therefore can only be addressed by citywide urban planning.

On the microscale temperatures are not rising linear from the edge of the city to the center, but are varied with hot and cool spots. The design of streets, parks, squares and other public spaces can amplify or mitigate the basic climate conditions (macroscale) and UHI effect (mesoscale). Wind can be regulated to reduce or allow for wind chill, where it's needed. Shade can be implemented in locations with a lot of sun exposure. More vegetation can also add shade and bring evaporation. Surfaces can be chosen to reflect more and store less heat, as well as allow water to percolate when it's wet and evaporate when it's warm. In other words microclimate design can create spaces for people that are thermally comfortable,  reduce heat stress and related health risks despite climate change and the effect of urban heat islands.