The Emerging Science of Urban Skylines
MIT Technology Review, December 14, 2015. Image credit: ahundt
Schlapfer and co simply downloaded this information from around five million buildings from 12 North American cities. These ranged from the largest, New York and Los Angeles (with populations of 20 million and 13 million respectively), to midsized cities such as San Francisco and Austin (with populations of four million and two million respectively) to small cities such as Ann Arbor and Santa Fe (with populations of 300,000 and 100,000 respectively).
The size distribution shows a clear pattern. Schlapfer and co say that at first glance the data matches the general expectation that the average height of buildings increases with city size and that within a city, buildings get taller nearer the centre.
But a closer look at the data reveal some more detailed patterns. For a start, at the center of cities, average building size increases with population over two orders of magnitude. And this reflects a change in the shape of buildings from larger, flatter structures in smaller cities to taller, narrower ones in bigger cities.
In theory, this trend should be good for energy efficiency. Taller buildings become more like cubes and so generally have a smaller ratio of surface area to volume. This helps make buildings more energy efficient. “We see that building sizes do increase with city size, creating the conditions for greater energy efficiency in terms of climate control,” say Schlapfer and co.
Up to a point. In cities such as New York and Boston this trend has led to the construction of much taller skyscrapers in which are less energy efficient. “The surface-to-volume ratio increases again in the downtown cores of large cities, due to the proliferation of tall, needle-like buildings,” they say.
They conclude that on average, the shapes of buildings in North American cities converge on a cube-like shape as cities get bigger—that’s the most energy efficient shape.