Long-Term Daily Climate Records from Stations Across the Contiguous United States

USHCN Home Daily Data Monthly Data Monthly Data Documentation Web Interface

Investigators

M.J. Menne, C.N. Williams, Jr., and R.S. Vose
National Climatic Data Center, National Oceanic and Atmospheric Administration

Table of Contents

• Introduction
• Data Sources and Compilation
• Quality Control
• Bias Adjustments
• Station Information
• Data Files
• Data Access
• References
• Contacts

Introduction

The United States Historical Climatology Network (USHCN) is essentially a subset of the U.S. Cooperative Observer Network operated by NOAA's National Weather Service (NWS). The approximately 1200 USHCN stations were originally selected according to factors such as record longevity, percentage of missing values, spatial coverage as well as the number of station moves and/or other station changes that may affect data homogeneity. Most USHCN stations are situated in rural areas or small towns; however, a smaller number of stations are also part of the NOAA NWS synoptic network, whose stations are generally located at airports in more urbanized environments. USHCN datasets have been developed at NOAA's National Climatic Data Center (NCDC) in collaboration with the Department of Energy's Carbon Dioxide Information Analysis Center (CDIAC).

The USHCN project dates to the mid-1980s (Quinlan et al. 1987). At that time, in response to the need for an accurate, unbiased, modern historical climate record for the United States, personnel at the Global Change Research Program of the U.S. Department of Energy and at NCDC defined a network of 1219 stations in the contiguous United States whose observation would comprise a key baseline dataset for monitoring U.S. climate. Since then, the USHCN monthly dataset has been updated several times (e.g., Karl et al., 1990; Easterling et al., 1996). The USHCN Version 2 Serial Monthly data release is the most recent update to the USHCN datasets.

The first database released by NCDC to contain daily data from USHCN stations, the HCN/Daily (HCN/D; Hughes et al. 1992; hereafter H92), contained daily maximum and minimum temperatures and precipitation totals from 138 select U.S. stations. The temperature and precipitation records from these stations were considered to be the most reliable, internally consistent, and unbiased records from the USHCN. These records were compiled from digital and nondigital data sets archived at NCDC that come from a variety of sources, including climatological publications, universities, federal agencies, individuals, and data archives. Records were subjected to extensive manual and automated quality assurance (QA) checks. The selection of stations for inclusion in H92 was performed with the following data quality issues in mind.

• The degree to which each station maintained a constant observation time for maximum and minimum temperatures, excursions from a station's predominant observing time of no more than four years being desired.
• At least 95% of a station's pre-1951 data should be contained in NCDC digital daily archives.
• A station's potential for heat island bias over time should be low.
• Quality assessments based upon the decile ranking assigned by Karl et al. (1990) to the stations' monthly maximum/minimum temperature data for certain quality characteristics.

Since the release of H92, much more work has been conducted at NCDC involving compilation and digitizing of daily data. However, to enable the compilation of a database providing better spatial coverage of the contiguous United States, the four station selection criteria listed above were not strictly adhered to in later versions of the USHCN daily database (e.g., Williams et al. 2004).

The USHCN daily data contain observations of maximum and minimum temperature, precipitation amount (liquid equivalent), snowfall amount, and snow depth. Records from about 700 stations begin before 1900 and most station records are essentially complete for at least 60 years extending through a recent year. It's common for temperature and precipitation records to have started earlier than snowfall and snow depth measurements, especially at stations with longer periods or record.

Data Sources and Compilation

The current version of the USHCN Daily database contains the same variables, and was subjected to the same quality control checks (details below), as NCDC's Global Historical Climatology Network-Daily (GHCN-Daily) database. USHCN-Daily is in fact a subset of GHCN-Daily, which serves as the official archive for daily data from the Global Climate Observing System (GCOS) Surface Network (GSN). Following is a list of the sources of data currently contained in USHCN-Daily along with a brief description of each source.

• U.S. Cooperative Summary of the Day (NCDC DSI-3200) - Includes daily observations at 19,000 stations in the United States and its territories. While most measurements are taken by volunteer observers as part of the U.S. NOAA National Weather Service's Cooperative Observer (COOP) Program, manual and automated measurements from some "First Order" sites are also included. Some records extend back to the late 1800s, but most do not begin until 1948 or later.
• CDMP U.S. Cooperative Summary of the Day (NCDC DSI-3206) - Contains mostly pre-1948 observations from more than 11,000 COOP stations that were keyed as part of NCDC's Climate Data Modernization Program (CDMP).
• U.S. First Order Summary of the Day (NCDC DSI-3210) - Contains historical and present-day manual and automated observations from approximately 1600 sites, including U.S. First Order stations, a selection of Canadian sites, and U.S.-operated stations in other countries.

Quality Control

As part of the GHCN-Daily quality control procedures, USHCN-Daily data have been subjected to a variety of internal consistency, frequent-value, outlier, and spatial consistency checks. The quality control efforts are always improving and expanding, so users are urged to keep up with them by visiting the Quality Control section of the NCDC/GHCN website.

Bias Adjustments

At present, the USHCN daily data contain no adjustments for biases resulting from historical changes in instrumentation and observing practices. This is true of the GHCN-Daily database as a whole, which includes the USHCN daily data. However, there is ongoing work at NCDC to develop adjustments that can be applied to daily maximum and minimum temperatures, and a GHCN daily derived product containing adjusted daily temperatures may become available in the future.

Station Information

The format of each record in the USHCN Daily station inventory file (ushcn-stations.txt) is as follows.

These variables have the following definitions:

TABLE 1. State numbers and abbreviations for the contiguous United States.

Data Files

USHCN data files may be downloaded from CDIAC's anonymous FTP area (see the USHCN Data Access page).

There are 48 state files (ASCII) using the following naming convention: stateno_stateabbrev.txt.gz.

Example: state01_AL.txt.gz.

station_file_format.txt contains ASCII file format information and variable definitions.

There are also five NetCDF files containing data organized by variable type (e.g., ushcn_prcp.nc.gz).

ushcn-stations.txt contains a listing of all of the stations along with their latitude and longitude.

us.txt.gz contains data for the 48 states represented.

The format of each record in an ASCII data file, be it a state-level file (e.g., state01_AL.txt) or the file for the entire U.S. (us.txt) is as follows. (Each record in a file contains one month of daily data.)

These variables have the following definitions:

Data Access

The USHCN Daily data are available via FTP or a Web interface that allows users to query, plot, and download data for individual states, stations, and variables. Please see the USHCN Data Access page.

References

• Easterling, D. R., T. R. Karl, E. H. Mason, P. Y. Hughes, and D. P. Bowman. 1996. United States Historical Climatology Network (U.S. HCN) Monthly Temperature and Precipitation Data. ORNL/CDIAC-87, NDP-019/R3. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee. 280 pp.
• Hughes, P. Y., E. H. Mason, T. R. Karl, and W. A. Brower. 1992. United States Historical Climatology Network Daily Temperature and Precipitation Data. ORNL/CDIAC-50, NDP-042. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee. 140 pp.
• Karl, T. R., C. N. Williams, Jr., and F. T. Quinlan. 1990. United States Historical Climatology Network (HCN) Serial Temperature and Precipitation Data. ORNL/CDIAC- 30, NDP-019/R1. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee.
• Quinlan, F. T., T. R. Karl, and C. N. Williams, Jr. 1987. United States Historical Climatology Network (HCN) serial temperature and precipitation data. NDP-019. Carbon Dioxide Information Analysis Center. Oak Ridge National Laboratory, Oak Ridge, Tennessee.
• Williams, C. N., R. S. Vose, D. R. Easterling, and M. J. Menne, 2004. United States Historical Climatology Network Daily Temperature, Precipitation, and Snow Data. ORNL/CDIAC-118, NDP-070. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee.

Contacts

Questions regarding the USHCN Daily web site or data may be directed to Dale Kaiser at CDIAC.