Why Smooth Surfaces Make Your City Sizzle in Summer

It's not just you, city-dwellers. Urban areas actually do get noticeably hotter than the rural areas around them, and that's especially problematic in summertime. Why does that happen? Well, a new study says it all has to do with the aerodynamic shape of your city. In other words, the smoother your skyline, the hotter you'll get.

The study, published this week in Nature, takes a new look at the familiar phenomenon of hotter temps in cities. Previous theories held that increased city temps were due to a lack of vegetation (which can cool the local environment through water evaporation) or building materials that absorb heat from baking in the sun all day and release it at night, or even tall buildings that trap heat with reflective facades.

The new study, written by a team at Nanjing University of Information Science and Technology in China, adds another theory: since cities are made of smooth, rather aerodynamic surfaces, it's harder for the heat they generate to escape via air movement. As Smithsonian Magazine puts it, "the dense vegetation of the rural area is aerodynamically rougher than the city, which increases the efficiency of convection, letting more heat move from the land into the atmosphere."

The research team used mathematical modeling to study this theory, and came away with an interesting supporting finding: In desert areas like Las Vegas, where the rural outskirts of town are actually more aerodynamically smooth than the built-up city, the urban areas show about 20 percent better heat dissipation.

Clearly, we're still learning about the thermodynamics of big cities. It's an increasingly important topic: EPA studies say that heat island effect can raise the ambient temperature of large urban areas (cities over 1 million people) by as much as five degrees during daylight and a whopping 22 degrees at night. With summer heat waves becoming increasingly common, this phenomenon could lead to drastic problems of electricity demand, strained water supplies, and heat-related health problems.

Maybe louder, more turbulent, less-aerodynamic buildings are the key to cooler cities after all. [Nature via Smithsonian Magazine]

Image: Shutterstock / Joshua Haviv