The way streets and buildings are placed in cities is one of the most important determinants of the urban heat island effect, according to new research from Massachusetts Institute of Technology.
The heat island effect exists due to the fact that building materials like concrete and asphalt absorb heat and radiate it back at night much more than vegetation does, which has led to an increased focus on greening city spaces.
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The MIT research published in Physical Review Letters found that a city’s pattern or “texture” – whether it is ordered in a precise grid or is less structured – was the most important determinant of a city’s heat island effect, a finding the authors said could provide city planners with new ways to tackle the problem.
Mathematical models used to analyse anatomic structures provided a useful tool in which to describe how a city’s design would influence the urban heat island effect, lead author Roland Pellenq said.
“We used tools of classical statistical physics,” Dr Pellenq said.
The researchers adapted formulas typically used to describe how individual atoms in a material are affected by forces of other atoms, and reduced the complex set of relations to simpler descriptions of relative distances of nearby buildings to each other. They then used satellite images of more than 50 global cities to create an index – the local order parameter – ranging from 0 (total disorder) to 1 (perfect crystalline structure).
Using the algorithms the researchers found that cities varied from 0.5 to 0.9 on the scale.
Cities that were laid out on a precise grid – much like the atoms in a crystal – had a far greater buildup of heat compared to their surroundings than cities that were less ordered – much like a liquid or glass.
The research team said the difference in the heat effect seemed to relate to the way buildings reradiated heat that could then be reabsorbed by other buildings that directly faced them.
The findings could have an important influence on how cities are developed, particularly in those countries like China building new cities and others that are rapidly expanding, Dr Pellenq said.
For example, hot locations could be designed to minimise the additional heat loads, while cold areas may like to order buildings to take advantage of the heat.
“If you’re planning a new section of Phoenix [Arizona, US] you don’t want to build on a grid, since it’s already a very hot place,” Dr Pellenq said.
“But somewhere in Canada, a mayor may say, ‘No, we’ll choose to use the grid to keep the city warmer.’
“This gives a strategy for urban planners.”
While planning to a grid can lead to efficiencies in placing electricity lines, sewer and water pipes, and transportation systems, Dr Pellenq said benefits of a less structured layout may outweigh costs for some places. As a possible example, in Florida the urban heat island effect has been estimated to cause $400 million in excess costs for airconditioning.