Satellites Pinpoint Drivers of Urban Heat Islands in the Northeast
Cities such as New York, Philadelphia, and Boston are prominent centers
of political power. Less known: Their size, background ecology, and
development patterns also combine to make them unusually warm, according
to NASA scientists who presented new research recently at an American
Geophysical Union (AGU) meeting in San Francisco, Calif.
Summer land surface temperature of cities in the Northeast were an
average of 7 °C to 9 °C (13°F to 16 °F) warmer than surrounding rural
areas over a three year period, the new research shows. The complex
phenomenon that drives up temperatures is called the urban heat island
Ping Zhang and Marc Imhoff explain the Urban Heat Island Effect - where
dense urban development raises ambient temperatures dramatically above
the surrounding land - for the AGU conference 2010. Credit: NASA/Goddard Space Flight Center/Scientific Visualization Studio
Heat islands are not a newly-discovered phenomenon. Indeed, using simple
mercury thermometers, weather watchers have noticed for some two
centuries that cities tend to be warmer than surrounding rural areas.
Likewise, researchers have long noticed that the magnitude of heat
islands can vary significantly between cities. However, accurate
comparisons have long eluded scientists because ground-based air
temperature sensors tend to be unevenly distributed and prone to local
bias. The lack of quantifiable definitions for urban versus non-urban
areas has also hindered comparisons.
Satellite technology, which offers a more uniform view of heat islands,
is in the process of changing this. The group of researchers from NASA’s
Goddard Space Flight Center in Greenbelt, Md., presented results based
on a new method for comparing heat islands at the AGU meeting.
Satellite-produced maps of Providence and Buffalo highlight the role
that differences in development patterns and vegetation cover can have
on the magnitude of a city’s urban heat island. Though the two cities
have the same approximate size, Providence has a significantly stronger
heat island. Credit: NASA/Earth Observatory
› Larger images of Providence › Larger images of Buffalo
"This, at least to our knowledge, is the first time that anybody has
systematically compared the heat islands of a large number of cities at
continental and global scales," said Ping Zhang, a scientist at Goddard
and the lead author of the research.
Land surface temperatures in cities, particularly densely-developed
cities, tend to be elevated in comparison to surrounding areas -- a
phenomenon called an urban heat island. Credit: NASA › Larger image
Heat islands can be deadly. This graph shows how the number of deaths spiked in Paris during a sweltering heat wave in 2003. Credit: University of Hawaii at Manoa/Benedicte Dousset › Larger image
Air conditioning systems release waste heat into the atmosphere such
that their widespread use can inadvertently elevate city air
temperatures. This graph shows the result of a model that calculated
the likely magnitude of the effect during the 2003 heat wave in Paris. Credit: Météo France/Cécile de Munck › Larger image
Surface temperatures vary more than air temperatures during the day, but they both are fairly similar at night. Credit: EPA › Larger image
Development produces heat islands by replacing vegetation, particularly
forests, with pavement and other urban infrastructure. This limits plant
transpiration, an evaporative process that helps cool plant leaves and
also cools air temperatures, explained Robert Wolfe of Goddard, one of
the scientists who developed the method.
Dark city infrastructure, such as black roofs, also makes urban areas
more apt to absorb and retain heat. Heat generated by motor vehicles,
factories, and homes also contributes to the development of urban heat
A New View
The new method for comparing cities, which the team of scientists has
honed for about two years, involves the use of maps of impervious
surface area produced by a United States Geological Survey-operated
Landsat satellite, and land surface temperature data from the
Moderate-resolution Imaging Spectroradiometer (MODIS), an instrument
aboard NASA's Aqua and Terra satellites.
Impervious surfaces are surfaces that don't absorb water easily, such as
roads, roofs, parking lots, and sidewalks. Land surface temperatures
tend to be higher and more variable than air temperatures, but the two
generally vary in sync with each other.
By analyzing data from thousands of settlements around the world, the
Goddard team has pinpointed key characteristics of cities that drive the
development of heat islands. The largest cities, their analysis shows,
usually have the strongest heat islands. Cities located in forested
regions, such as the northeastern United States, also have stronger heat
islands than cities situated in grassy or desert environments.
Most recently, the Goddard group has shown that a city's development
patterns -- whether a city is sprawling or compact -- can also affect
the strength of its heat island.
By comparing 42 cities in the Northeast, they found that
densely-developed cities with compact urban cores are more apt to
produce strong urban heat islands than more sprawling, less
The compact city of Providence, R.I., for example, has surface
temperatures that are about 12.2 °C (21.9 °F) warmer than the
surrounding countryside, while similarly-sized but spread-out Buffalo,
N.Y., produces a heat island of only about 7.2 °C (12.9 °F), according
to satellite data. Since the background ecosystems and sizes of both
cities are about the same, Zhang's analysis suggests development
patterns are the critical difference.
She found that land cover maps show that about 83 percent of Providence
is very or moderately densely-developed. Buffalo, in contrast, has dense
development patterns across just 46 percent of the city. Providence
also has dense forested areas ringing the city, while Buffalo has a
higher percentage of farmland. "This exacerbates the effect around
Providence because forests tend to cool areas more than crops do,"
Cities in desert regions, such as Las Vegas, in contrast, often have
weak heat islands or are actually cooler than the surrounding rural
area. Providence, R.I.; Washington, D.C.; Philadelphia, Pa.; Baltimore,
Md.; Boston, Ma.; and Pittsburgh, Pa.; had some of the strongest heat
islands of the 42 northeastern cities analyzed.
"The urban heat island is a relative measure comparing the temperature
of the urban core to the surrounding area," said Marc Imhoff, the leader
of the Goddard research group. "As a result, the condition of the
rural land around the city matters a great deal."
Heat Island Impacts
Ratcheting up temperatures can have significant -- and deadly --
consequences for cities. Heat islands not only cause air conditioner and
electricity usage to surge, but they also increase the mortality of
elderly people and those with pre-existing respiratory and
The U.S. Environmental Protection Agency estimates that, between 1979
and 2003, heat exposure has caused more than the number of mortalities
resulting from hurricanes, lightning, tornadoes, floods, and earthquakes
"It is the lack of cooling at nighttime, rather than high daytime
poses a health risk," said Benedicte Dousset, a scientist from the
University of Hawaii who also presented data about heat islands at the
Dousset recently analyzed surface temperature images of Paris and showed
the spatial distribution of heat-related deaths during a sweltering
heat wave in 2003. Some 4,800 premature deaths occurred in Paris during
the event, and excess mortality across Europe is thought to be about
The risk of death was highest at night in areas where land surface
temperatures were highest, she found. Buildings and other infrastructure
absorb sensible heat during the day and reradiate it throughout the
night, but the cooling effect of evaporation is absent in cities. The
lack of relief, particularly among the elderly population, can be
deadly, she explained.
Ramped up air conditioning usage may have even exacerbated the problem,
other data presented at the meeting suggests. Cecile de Munck, of the
French Centre for Meteorological Research of Meteo-France, conducted a
series of modeling experiments that show excess heat expelled onto the
streets because of increased air conditioner usage during heat waves can
elevate outside street temperatures significantly.
"The finding raises the question: what can we do to design our cities in
ways that will blunt the worst effects of heat islands?" said de Munck,
who notes also that her research shows that some types of air
conditioning exacerbate heat islands more than others.
Making sure cities have trees and parks interspersed throughout the
compact urban cores can also help defend against heat islands. And
studies shows that painting the surfaces of roads and buildings white
instead of black and creating “green” roofs that include vegetation can
soften urban heat islands.
"There's no one solution, and it's going to be different for every city," said Dousset. "Heat islands are complex phenomena."