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Sunday, July 13, 2014

Urban heating and what drives it

Sou | 3:23 AM Go to the first of 7 comments. Add a comment

I was surprised that Anthony Watts hasn't copied and pasted a press release about a new paper on urban heat island effect. It's one of his very favourite subjects. I guess he missed it.

The paper has been published in Nature and I spotted the press release about it a couple of days ago when scanning The article describes research from scientists Lei Zhao, Xuhui Lee, Ronald B. Smith and Keith Oleson. It's an international collaboration. The scientists hail from Nanjing University of Information Science and Technology, Yale University and the National Center for Atmospheric Research in Boulder, Colorado.

What they did was look in some detail at the various contributions to warming associated with the growth of towns and cities. In particular, they analysed data for 65 cities across North America.

Urban heating has implications not just from a comfort perspective but from a health perspective. Research on the subject can help cities mitigate against this human-caused warming.

After dark, the main contributor to urban heating is the buildings and roads etc releasing heat stored during the day. That confirmed what was already thought.

During the daytime, however, it's related to the efficiency of convection and humidity. As described in the abstract:  "If urban areas are aerodynamically smoother than surrounding rural areas, urban heat dissipation is relatively less efficient and urban warming occurs (and vice versa)."

There's more. Here is an extract from

Their results reaffirmed the consensus view that, regardless of the local climate, the release of heat stored in human-built structures is the dominant contributor to UHI during the nighttime.
But during the daytime, researchers found, convection is the dominant factor -- particularly in "wetter" cities of the southeastern United States. In those places, the smooth surfaces of buildings and other human-made features are far less conducive to heat diffusion than the densely vegetated areas that surround them. Overall, in wetter climates urbanization reduces convection efficiency by 58 percent.
"The 'rougher' surfaces of the vegetation triggers turbulence, and turbulence removes heat from the surface to the atmosphere," said Lei Zhao, a doctoral student at F&ES and lead author of the study. "But where there is a smoother surface, there is less convection and the heat will be trapped in the surface."
Convection plays a key role in drier cities, too -- albeit with far different consequences. In those settings -- including in urban areas of the U.S. Southwest where surrounding vegetation is typically shorter and scrubbier -- the rural areas are less effective at dissipating heat. As a result, the urban landscapes are actually 20 percent more efficient in removing heat than their rural surroundings, triggering a 1.5-degree C cooling within the cities.

That's interesting. I wonder if Anthony Watts has compared temperature trends in drier cities and surrounding rural areas with those in wetter cities and surrounding rural areas?

Lei Zhao, Xuhui Lee, Ronald B. Smith, Keith Oleson. "Strong contributions of local background climate to urban heat islands". Nature, 2014; 511 (7508): 216 DOI: 10.1038/nature13462


  1. That last paragraph may explain why WAW hasn't taken notice of the paper. Ooops....

  2. Kudos to you Sou for reporting on both ends. This article was interesting and I appreciate you presenting it to read. I would love to see a study of wet vs. dry cities as you suggested. Regardless, it doesn't explain temp increase for all those cities above the Arctic Circle :)

  3. Something I wonder about, especially for the suburban cities in the USA, is how much cooling we have in cities in dry regions due to irrigation. People poor a lot of water on their gardens and also the irrigated growing of vegetables is more often around cities because the ground is expensive.

    1. I was kind of hoping you might shed some further light on this, Victor. The study does raise some interesting points, doesn't it. I wonder if it's on target?

    2. :o) I mainly work on the algorithms to remove non-climatic effects. For that I have to know a little about the causes, but the literature about the urban heat island is huge. That means a lot to read to become expect and a small probability to find something interesting, because most things have been studied already.

      If I would study it, the main reason would be that I am also blogging and that urbanization is so important in the blogosphere. There are so many other non-climatic effects that have not been studied well yet. That is where I would concentrate my attention as a scientist.

    3. I can understand that, Victor. Would you agree that the blogosphere often tends to distort the overall science? Perhaps sometimes making mountains out of molehills while ignoring the real mountains?

    4. I would say so and interestingly the distortions go in both ways.

      The main example would be the hysteria about the apparent pause in the surface temperature. It is such a minor deviation on climatic time scales and only in the temperature, not in the warming of the full climate system. It is interesting to scientists specialising in natural variability, but not for the main story.

      On the other hand, there are large uncertainties in feedbacks (clouds and land surface) and impacts. They do get some mention, but hardly as much as they deserve.


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