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Historical Climatology Network

Project Description
The U.S. Historical Climatology Network (U.S. HCN) was compiled
in response to the need for an accurate, unbiased, modern
historical climate record for climate change research. The
Carbon Dioxide Research Program of the U. S. Department of
Energy and the National Climatic Data Center (NCDC) of the
National Oceanic and Atmospheric Administration (NOAA)
established a network of 1219 stations in the contiguous United
States for the specific purpose of compiling a data set
suitable for detecting and monitoring climate change over the
past two centuries. This network, known as the U.S. Historical
Climatology Network (U.S. HCN), and the resulting data set were
initially documented by Quinlan et al. (1987) and made
available free of charge through the Carbon Dioxide Information
Analysis Center (CDIAC). The data presented in Quinlan et
al. (1987) extended only through 1984. In 1990 NCDC and CDIAC
revised and updated the HCN data records through 1987 (Karl et
al. 1990). In addition, ! using the techniques of Karl et
al. (1988), NCDC generated temperature files in which the
biases introduced by urbanization effects were removed. The
new revision 3 (Easterling et al. 1996) data represent the best
available data from the United States for analyzing long-term
climate trends on a regional scale and may be used for studies
attempting to determine the climatic impacts of increased
concentrations of greenhouse gases. The data for most stations
extend through December, 1994, and a majority of the station
records are complete for at least 80 years. Unlike many data
sets that have been used in past climate studies, these data
have been adjusted to remove biases introduced by station
moves, instrument changes, time-of-observation differences, and
urbanization effects.

Source and Scope of the Data:

One of the objectives in establishing the U.S. HCN was to detect
secular changes of regional rather than local
climate. Therefore, only those stations that were not believed
to be influenced to any substantial degree by artificial changes
of local environments were included in the network. Some of the
stations in the U.S. HCN are first order weather stations, but
the majority were selected from U.S. cooperative weather
stations (approximately 5000 in the United States). To be
included in the U.S. HCN, a station had to be active (in 1987),
have at least 80 years of mean monthly temperature and total
monthly precipitation data, and have experienced few station
changes. An additional criterion that was used in selecting the
1221 U.S. HCN stations, which sometimes compromised the
preceding criteria, was the desire to have a uniform
distribution of stations across the United States. The 1221
station U.S. HCN database contains station histories, monthly
temperature (maximum, minimum, and mean) data, and total monthly
precipitation data that were compiled by NCDC after being
extracted from digital and nondigital data sets archived at
NCDC. These data sets originated from a variety of sources,
including climatological publications, universities, federal
agencies, individuals, and data archives. All stations were
quality controlled by NCDC with the use of outlier and areal
edits, and each station in the network was corrected for
time-of-observation differences, instrument changes, instrument
moves, station relocations, and urbanization effects (Karl et
al. 1986; Karl and Williams 1987; and Karl et al. 1988). A
unique feature of the data set is that, within most temperature
and precipitation data files, both original and adjusted
estimates are given, along with confidence factors for each
adjusted estimate. Another unique feature of the database is
that in r! elation to the long periods of record, a small
portion of the data are represented as missing. In order to make
the U.S. HCN record as serially complete as possible, missing
data have been estimated by using data from neighboring
stations. The majority of the 1221 stations have had data
records that are serially complete since 1900; where serially
complete is defined as having original or adjusted data
available for all months after the reported serially complete
date for a given station.

Applications of the Data:

The U.S. HCN database represents the best monthly temperature
and precipitation data set available for the contiguous United
States. It provides an accurate, serially complete, modern
historical climate record that is suitable for detecting and
monitoring longterm climatic changes on a regional scale and may
be used for studies attempting to determine the climatic impacts
of increased concentrations of greenhouse gases. The U.S. HCN
climate record may also be used by dendrochronologists and
paleoclimatologists for calibrating tree ring growth, pollen,
and marine plankton data or by those studying the climatic
impacts of periodic events such as El Nino/Southern Oscillation
or volcanic eruptions. Those studying longterm climatic changes
on smaller scales, may want to review the information given in
the appendices in order to identify the stations most suitable
for their research needs.

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