Assessment Of Urban Versus Rural In Situ Surface Temperatures In ...
VOL. 16, NO. 18
J O U R N A L O F C L I M A T E
15 SEPTEMBER 2003
Assessment of Urban Versus Rural In Situ Surface Temperatures in the Contiguous
United States: No Difference Found
THOMAS C. PETERSON
National Climatic Data Center, Asheville, North Carolina
(Manuscript received 26 May 2002, in final form 23 February 2003)
ABSTRACT
All analyses of the impact of urban heat islands (UHIs) on in situ temperature observations suffer from
inhomogeneities or biases in the data. These inhomogeneities make urban heat island analyses difficult and can
lead to erroneous conclusions. To remove the biases caused by differences in elevation, latitude, time of ob-
servation, instrumentation, and nonstandard siting, a variety of adjustments were applied to the data. The resultant
data were the most thoroughly homogenized and the homogeneity adjustments were the most rigorously evaluated
and thoroughly documented of any large-scale UHI analysis to date. Using satellite night-lights–derived urban/
rural metadata, urban and rural temperatures from 289 stations in 40 clusters were compared using data from
1989 to 1991. Contrary to generally accepted wisdom, no statistically significant impact of urbanization could
be found in annual temperatures. It is postulated that this is due to micro- and local-scale impacts dominating
over the mesoscale urban heat island. Industrial sections of towns may well be significantly warmer than rural
sites, but urban meteorological observations are more likely to be made within park cool islands than industrial
regions.
1. Introduction
urban was sought. A good current source of information
is night-lights data from the Defense Meteorological
a. Impetus for this analysis
Satellite Program (DMSP). Owen et al. (1998) devel-
As just about every introductory course on weather
oped an approach to identify locations as urban, rural,
and climate explains, urban areas are generally warmer
or suburban using night-lights data as have other re-
than nearby rural areas. Often referred to as the urban
searchers (e.g., Hansen et al. 2001). These night-lights
heat island (UHI) effect, urbanization has long been
rural/urban metadata avoid some of the shortcomings of
regarded as a serious contamination of the climate signal
the map-based metadata.
(e.g., Landsberg 1956). Those of us working with cen-
To find out how contaminated global temperature
tury-scale instrumental climate data strive to remove all
trends were from the UHI, Peterson et al. (1999) iden-
sources of artificial biases from the data. So the UHI
tified each station in GHCN using both the map-based
contamination is one aspect dataset creators seek to ad-
and the satellite-based metadata. Two time series were
dress. For example, the Global Historical Climatology
then created. One was the time series from the full da-
Network (GHCN; Peterson and Vose 1997) consists of
taset, the one used routinely to determine global tem-
over 7500 temperature stations around the world that
perature trends over land areas at the National Climatic
were identified as rural, urban, or an in-between class
Data Center (e.g., Lawrimore et al. 2001), and another
of small town using information on operational navi-
one produced using only data from stations that were
gation charts and a variety of different atlases. A rural
station was any station not associated with a town of
identified as rural by both techniques. The two time
over 10 000 population.
series were very similar. The linear trend from 1880 to
However, there were problems with the operational
1998 was 0.65 C century 1 for the full dataset and the
navigation charts. Much of the information going into
slightly higher 0.70 C century 1 for the rural-only sub-
the charts was over a decade old. Some towns that were
set. The resulting conclusion was that the well-known
rural a decade or two ago have since been engulfed by
global temperature time series from in situ stations was
sprawling urban centers. Therefore, another approach to
not significantly impacted by urban warming.
identifying which stations were rural and which were
The research presented here attempts to unravel the
mystery of how a global temperature time series created
partly from urban in situ stations could show no con-
Corresponding author address: Dr. Thomas C. Peterson, National
Climatic Data Center, 151 Patton Avenue, Asheville, NC 28801.
tamination from urban warming. This is important to
E-mail: Thomas.C.Peterson@noaa.gov
improving our understanding of the UHI contamination
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