AA-MCH Workshop: The climate and the city

Post date: Jul 4, 2011 9:43:36 PM

Last month I participated in a workshop with the Architectural Association and the Master of Collective Housing of the ETSA Madrid. The aim of the workshop was to reflect on specific concepts of Sustainability under the title of “Climate is the issue; city is the answer.” During the first phase we reflected and embarked a debate to put our ideas together in a coherent discourse. Our main concern lied on the dialectical couple of issue and answer. We concluded that there is not such a thing as an issue and an answer, but a series of factors that interact and influence each other.

Thus, we focus in three specific questions about the city and the climate in order to understand the interactions between them. This is a summary of the research’s findings:

1- What are the differences between urban and rural climates?

The rapid urbanization experienced during the last several decades has modified the radiative, thermal, and airflow performance of the urban climate. The progressive conversion of vegetated areas into buildings or paved surfaces has altered substantially the atmospheric characteristics of cities. Specifically, cities experience higher temperatures than natural areas, an effect known as the urban heat island effect. The difference between rural and urban temperatures can be as high as 7°C, reaching its peak at night, as urban materials release heat more slowly.

London’s Heat Island (http://forum.netweather.tv/topic/54544-effects-of-urban-heat-island-on-climate/)

2-Which are the factors influencing urban microclimates?

The main factors are:

-urban morphology

-surface properties

-anthropogenic effects

The climatic differences are caused mainly by two urban characteristics: first is the fact that cities absorb and store more solar radiation in the form of heat due to their own morphology and the common use of dark materials such as asphalt; second is the fact that the predominance of impermeable materials does not allow for heat dissipation through evaporation. In addition, pollution and anthropogenic heat from heating, cooling, transportation, and industrial activities contribute to increase urban temperatures.

3- Which strategies can we apply in outdoor urban spaces to mitigate uncomfortable environments?

-Vegetation and Trees:

Green areas play a vital role in urban areas: they improve air quality, decrease storm water runoff, create more comfortable neighbourhoods, and reduce energy use. They are also very useful in reducing the heat island effect.

-Light coloured surfaces:

Lighter surfaces have greater solar reflectance. Therefore, increasing the albedo of urban surfaces reduces heat gain and heat storage.

-Solar protection:

Canopies and awnings can be used as protection from direct solar radiation. This strategy is more common in middle to low latitudes, where solar radiation is very high in the summer. Canopies, shadow facades, and street pavement reduce heat gain through radiation.

-Water:

Water can also be used to decrease temperature on the surrounding environment. Water has a low reflectivity rate, of around 3%, and absorbs a great portion of solar radiation (up to 80% depending on the thickness of the layer). However, due to its high thermal inertia and the evaporation produced in its surface, it maintains lower temperatures than the air above it.

Use of canopies over Calle Sierpes, Seville (Spain)