Heat Islands

Linking Urban Form, Function, and Climate

Gerald Mills, a Professor of Geography at University College Dublin, joined the call for a presentation on the relationship between urban form, function, and climate. He began by noting that urban areas create distinctive climates due to factors including: (1) Physical form and composition, (2) Urban activities that alter the atmospheric composition by emitting waste heat and materials. Dr. Mills then said that these aspects of cities can be modified with adequate consideration of technology innovation, design change, and behavior modification. As a result, there is great demand for stronger linkages between urban climatologists, planners, and designers.

Dr. Mills also pointed out the relationship between the magnitude of an urban heat island and the physical structure of the settlement, which is largely determined by city-scale transportation planning. A policy prescription following from this observation is that "sustainable" cities can be achieved through mixed, high-density land-use that reduces travel demand.

Dr. Mills presented results suggesting a clear relationship between population density and automobile fuel consumption. He also showed data indicating that smaller settlements consume less energy and generate less waste.

Dr. Mills noted that a potential problem with implementing the "sustainable" strategy of increasing urban density to decrease energy use and accrue additional environmental benefits is that as buildings of similar height move closer to each other, air flow between and around buildings is reduced. Thus, air mixes much less between buildings - such as in street canyons between tall buildings - and instead air passes over the tops of the buildings. This phenomenon, ironically, means that although there may be fewer emissions due to less vehicle traffic, the emission concentrations between buildings may be higher, due to the lack of air flow and mixing. Thus, compact, high density urban developments must consciously incorporate surface roughness - diverse building heights and potentially orientations - into their design. This will increase air flow between buildings resulting in better street level air quality compared to developments that have buildings of the same height and orientation that are built closely together.

To conclude his presentation, Dr. Mills described the key take-aways from his research. One is that establishing relationships between measures of urban form and activity is necessary to link urban planning decisions to urban climate effects. Another is that this scenario illustrates the potential pitfalls of implementing measures based on a single perspective. In particular, it illustrates the importance of good design at the building group scale to ensure that decisions at the settlement scale do not have unintended micro-scale consequences.

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