Featured Research

from universities, journals, and other organizations

Urban heat: Not a myth, and worst where it's wet

Date:
July 9, 2014
Source:
Yale School of Forestry & Environmental Studies
Summary:
A new quantifies for the first time the primary causes of the 'urban heat island' (UHI) effect, a common phenomenon that makes the world's urban areas significantly warmer than surrounding countryside and may increase health risks for city residents. In an analysis of 65 cities, researchers found that variation in how efficiently urban areas release heat back into the lower atmosphere is the dominant factor in the daytime UHI effect.

This NASA image illustrates the temperature differential between the city of Providence and the surrounding region.
Credit: NASA Earth Observatory

A new Yale-led study quantifies for the first time the primary causes of the "urban heat island" (UHI) effect, a common phenomenon that makes the world's urban areas significantly warmer than the surrounding countryside and may increase health risks for city residents.

Related Articles


In an analysis of 65 cities across North America, researchers found that variation in how efficiently urban areas release heat back into the lower atmosphere -- through the process of convection -- is the dominant factor in the daytime UHI effect. This finding challenges a long-held belief that the phenomenon is driven principally by diminished evaporative cooling through the loss of vegetation.

The effects of impaired "convective efficiency" are particularly acute in wet climates, the researchers say. In cities such as Atlanta, Georgia, and Nashville, Tennessee, this factor alone contributes a 3-degree C rise in average daytime temperatures, according to the study, published July 10 in the journal Nature.

The phenomenon could have profound impacts on human health in cities worldwide as mean global temperatures continue to rise -- and as more and more people move into cities -- said Xuhui Lee, the Sara Shallenberger Brown Professor of Meteorology at the Yale School of Forestry & Environmental Studies (F&ES) and one of the study's authors.

"There is a synergistic relationship between climate conditions and the urban heat island," Lee said. "This relationship suggests that the urban heat island will exacerbate heat wave stress on human health in wet climates where temperature effects are already compounded by high humidity.

"This is a huge concern from a public health perspective."

For years scientists have recognized the primary causes of the UHI effect. In addition to the changes in convection efficiency and evaporative cooling, these include the tendency of buildings, pavement, and other structures to store more heat than vegetation and soil; heat generated by human-built industrial systems; and changes to the albedo of Earth's surface. (Albedo refers to the proportion of sunlight or radiation reflected by the surface of the planet. Light-colored parking lots, for instance, reflect more sunlight back into space than darker surfaces.)

Using satellite data of land surface temperatures and vegetation cover from cities in the United States and Canada, researchers calculated the mean temperature differentials between urban centers and their rural surroundings during both daytime and nighttime hours. They also used climate modeling to produce a more complex range of variables -- from air density to aerodynamic resistance -- which were then used to quantify the roles of each of the primary drivers of UHI (radiation, convection, evaporation, heat storage, and human-generated heat).

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.


Story Source:

The above story is based on materials provided by Yale School of Forestry & Environmental Studies. The original article was written by Kevin Dennehy. Note: Materials may be edited for content and length.


Journal Reference:

  1. 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

Cite This Page:

Yale School of Forestry & Environmental Studies. "Urban heat: Not a myth, and worst where it's wet." ScienceDaily. ScienceDaily, 9 July 2014. <www.sciencedaily.com/releases/2014/07/140709140119.htm>.
Yale School of Forestry & Environmental Studies. (2014, July 9). Urban heat: Not a myth, and worst where it's wet. ScienceDaily. Retrieved November 27, 2014 from www.sciencedaily.com/releases/2014/07/140709140119.htm
Yale School of Forestry & Environmental Studies. "Urban heat: Not a myth, and worst where it's wet." ScienceDaily. www.sciencedaily.com/releases/2014/07/140709140119.htm (accessed November 27, 2014).

Share This


More From ScienceDaily



More Science & Society News

Thursday, November 27, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Ebola Leaves Orphans Alone in Sierra Leone

Ebola Leaves Orphans Alone in Sierra Leone

AFP (Nov. 27, 2014) The Ebola epidemic sweeping Sierra Leone is having a profound effect on the country's children, many of whom have been left without any family members to support them. Duration: 01:02 Video provided by AFP
Powered by NewsLook.com
EU Pushes Google For Worldwide Right To Be Forgotten

EU Pushes Google For Worldwide Right To Be Forgotten

Newsy (Nov. 27, 2014) Privacy regulators recommend Google expand its requested removals to apply to all its web domains. Video provided by Newsy
Powered by NewsLook.com
Who Will Failed Nuclear Talks Hurt Most?

Who Will Failed Nuclear Talks Hurt Most?

Reuters - Business Video Online (Nov. 25, 2014) With no immediate prospect of sanctions relief for Iran, and no solid progress in negotiations with the West over the country's nuclear programme, Ciara Lee asks why talks have still not produced results and what a resolution would mean for both parties. Video provided by Reuters
Powered by NewsLook.com
FCC Forces T-Mobile To Alert Customers Of Data Throttling

FCC Forces T-Mobile To Alert Customers Of Data Throttling

Newsy (Nov. 25, 2014) T-Mobile and the FCC have reached an agreement requiring the company to alert customers when it throttles their data speeds. Video provided by Newsy
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:

Strange & Offbeat Stories


Science & Society

Business & Industry

Education & Learning

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins