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

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Please cite data as:
M. J. Menne, C. N. Williams, Jr., and R. S. Vose, 2011.
United States Historical Climatology Network Daily Temperature, Precipitation, and Snow Data. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee.
Last updated August 2011.

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

The United States Historical Climatology Network (U.S. HCN) is essentially a subset of the U.S. Cooperative Observer Network operated by NOAA's National Weather Surface (NWS). The approximately 1200 HCN 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 HCN 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. U.S. HCN 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 U.S. HCN 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 U.S. HCN monthly dataset has been updated several times (e.g., Karl et al., 1990; Easterling et al., 1996). The U.S. HCN version 2 serial monthly data release is the most recent update to the HCN datasets.

The first database released by NCDC to contain daily data from HCN 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 HCN. 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 HCN daily database (e.g., Williams et al. 2004).

The HCN daily data contain observations of maximum and minimum temperature, precipitation amount (liquid equivalent), snowfall amount, and snow depth. Records from 700 stations begin 1900, with Tarboro, NC recording the earliest data with some precipiation measurements taken in 1870. Most station records are essentially complete for at least 60 years. 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. Records from 1191 stations extend through at least 2000, while 1092 stations extend through 2009.

The current version of the HCN 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.
  
  
  

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 15 temperature, 12 precipitation, 14 snowfall, and 12 snow depth checks are performed in a deliberate sequence, with each procedure operating on only those values that did not fail any of the preceding tests. The performance of each check has been rigorously evaluated using the methodology of Durre et al. (2008) in order to minimize the numbers of false positives and misses.

A list of all checks performed is provided below. Checks are divided into four groups: temperature checks, precipitation checks, snowfall checks, and snow depth checks. Within each group, procedures are listed in the order in which they are applied. Further details about the procedures will be contained in a journal article which is currently in preparation.

• Naught check - Checks for days on which maximum and minimum temperature are both equal to 0°C at stations not operated by the United States or are both equal to -17.8ºC (0ºF) at United States stations.
• Duplicate check - Checks for duplication of the data between entire years, different years in the same calendar month, different months within the same year, and maximum and minimum temperature within the same month.
• Bounds check - Identifies maximum and minimum temperature that fall outside the world extremes for the highest and lowest temperatures ever observed.
• Streak check - Checks for sequences of 15 or more identical values in time series of nonmissing daily maximum or minimum temperature.
• Gap check - Identifies maximum/minimum temperatures that are at least 10ºC warmer or colder than all other maximum/minimum temperatures for a given station and calendar month.
• Climatological outlier check - Checks for daily maximum and minimum temperatures that exceed the respective 15-day climatological means by at least six standard deviations.
• Internal consistency check - Checks for days on which the minimum temperature is greater than the maximum temperature.
• Interday consistency check - Checks for daily maximum temperatures that are less than the minimum temperatures on the preceding, current, and following days as well as for minimum temperatures that are greater than the maximum temperatures during the relevant three-day window.
• Lagged range check - Identifies maximum temperatures that are at least 40ºC warmer than the minimum temperatures on the preceding, current, and following days as well as minimum temperatures that are at least 40ºC colder than the maximum temperatures within the three-day window.
• Temporal consistency check - Checks whether a daily maximum (minimum) temperature exceeds the maximum (minimum) temperatures on the preceding and following days by more than 25ºC.
• Spatial consistency check - checks for temperatures whose anomalies differ by more than 10ºC from the anomalies at neighboring stations on the preceding, current, and following days.
• Megaconsistency check - Looks for daily maximum temperatures that are less than the lowest minimum temperature and for daily minimum temperatures that are greater than the highest maximum temperature for a given station and calendar month.
  
  
  

• Trace flag consistency check - Checks for days on which the data measurement flag indicates a trace of precipitation yet the precipitation amount is nonzero.
• Duplicate checks - Check for duplication of the data between entire years, different years within the same calendar months, and different months within the same year, using only years and months with at least three nonzero totals.
• Bounds check - Identifies precipitation totals that exceed the world extreme for the highest 24-hour total ever observed.
• Streak check - Checks for sequences of 10 or more consecutive identical values in Time series of nonzero precipitation observations.
• Frequent-value check - Checks for clusters of 5-9 identical moderate to heavy totals in time series of nonzero precipitation observations.
• Gap check - Identifies values that are at least 300 mm larger than all other precipitation totals for a given station and calendar month.
• Climatological outlier check - Checks for daily precipitation totals that exceed the respective 29-day climatological 95th percentiles by at least a certain factor (9 when the day's mean temperature is above freezing, 5 when it is below freezing).
• Snowfall to precipitation ratio check - Checks for cases in which snowfall is excessively large compared to precipitation. I.e., if the current day's snowfall is more than 100 times larger than both the current+previous and current+following days' precipitation sums, then the current day's precipitation and snowfall totals fail the check.
• Snow depth increase to precipitation ratio check - Checks for cases in which a day-to-day increase in snowed depth is excessively large compared to precipitation. I.e., if the snow depth increase between the previous and current days is more than 100 times larger than both the current+previous and current+following days' precipitation sums, the current day's precipitation total and the current and previous day's snow depths fail the check.
• Spatial consistency check - Checks for precipitation totals that differ significantly from totals reported at neighboring stations on the preceding, current, and following days.
  
  
  

• Trace flag consistency check - Checks for days on which the data measurement flag indicates a trace of snowfall yet the snowfall amount is nonzero.
• Duplicate checks - Check for duplication of the data between entire years, different years within the same calendar months, and different months within the same year, using only years and months with at least three nonzero totals.
• Bounds check - Identifies snowfall totals that exceed the world extreme for the highest 24-hour total ever observed.
• Streak check - Checks for runs of 10 or more consecutive days on which the same nonzero snowfall total is reported.
• Warm check - Checks for nonzero snowfall totals that occur when daily minimum temperatures at the same station are equal to or warmer than 7°C on the preceding, currentt, and following days.
• Snowfall to snow depth increase consistency check - Checks for days on which the increase in snow depth from the previous day to the current day exceeds the current+previous and current+following days' snowfall sums by more than 25 mm. In such cases, the current day's snowfall and snow depth as well as the previous day's snow depth fail the check.
• Snowfall to precipitation ratio check - Checks for cases in which snowfall is excessively large compared to precipitation. See the same entry under the Precipitation Checks above.
• Spatial warm check - Checks for nonzero snowfall totals that occur when daily minimum temperatures at neighboring stations are equal to or warmer than 7°C on the preceding, current, and following days.
• Megaconsistency checks - Look for (1) nonzero snowfall totals for stations and calendar months whose lowest reported minimum temperature is 7°C or warmer and (2) warm season n onzero snowfall totals at stations where no valid cold season snowfall was ever reported.
• Climatological outlier check - Identifies daily nonzero snowfall totals on dates outside the likely snow season as calculated from the snowfall records at the station and its suitable neighbors.
  
  
  

• Trace flag consistency check - Checks for days on which the data measurement flag indicates a trace of snow depth yet the snow depth amount is nonzero.
• Bounds check - Identifies snow depths that exceed the world extreme for the highest snow depth ever observed.
• Temporal consistency check - Checks for day-to-day increases in snow depths that exceed the world record 24-hour snowfall total.
• Streak check - Checks for runs of 90 or more consecutive days on which the same nonzero snow depth is reported.
• Gap check - Identifies values that are at least 35 cm larger than all other reported snow deaths for a given station and calendar month.
• Warm check - Checks for day-to-day increases in snow depth that occur when daily minimum temperatures at the same station are equal to or warmer than 7ºC on the preceding, current, and following days.
• Snowfall to snow depth increase consistency check - Checks for days on which the increase in snow depth from the previous day to the current day exceeds the current+previous and current+following days' snowfall total by more than 25 mm. See the same entry under Snowfall consistency check.
• Snow depth increase to precipitation ratio check - Checks for cases in which a day-to-day increase in snow depth is excessively large compared to precipitation. See the same entry under Precipitation Checks above.
• Spatial warm check - Checks for day-to-day increases in snow depth that occur when daily minimum temperatures at neighboring stations are equal to or warmer than 7ºC on the preceding, current, and following days.
• Megaconsistency checks - Look for (1) nonzero snow depths for stations and calendar months whose lowest reported minimum temperature is 7ºC or warmer and (2) warm season nonzero snow depths at stations where no valid cold season snow depth was ever reported.
• Climatological outlier check - Checks for day-to-day increases in snow depth on dates that fall outside the likely snow season as calculated from the snow depth records at the station and its suitable neighbors.
  
  
  

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.

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

Variable		Columns		Type
COOP ID		1-6		Character
LATITUDE		8-15		Real
LONGITUDE		17-25		Real
ELEVATION		27-32		Real
STATE		34-35		Character
NAME		37-66		Character
COMPONENT 1		68-73		Character
COMPONENT 2		75-80		Character
COMPONENT 3		82-87		Character
UTC OFFSET		89-90		Integer

These variables have the following definitions:

COOP ID		is the U.S. Cooperative Observer Network station identification code. Note that the first two digits in the Coop ID correspond to the assigned state number (see Table 1 below).
LATITUDE		is latitude of the station (in decimal degrees).
LONGITUDE		is the longitude of the station (in decimal degrees).
ELEVATION		is the elevation of the station (in meters, missing = -999.9).
STATE		is the U.S. postal code for the state.
NAME		is the name of the station location.
COMPONENT 1		is the Coop Id for the first station (in chronologic order) whose records were joined with those of the HCN site to form a longer time series. "------" indicates "not applicable".
COMPONENT 2		is the Coop Id for the second station (if applicable) whose records were joined with those of the HCN site to form a longer time series.
COMPONENT 3		is the Coop Id for the third station (if applicable) whose records were joined with those of the HCN site to form a longer time series.
UTC OFFSET		is the time difference between Coordinated Universal Time (UTC) and local standard time at the station (i.e., the number of hours that must be added to local standard time to match UTC).

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 file format information and variable definitions.

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.

USHCN daily data are available as ASCII and netCDF (coming soon) files. 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. (ushcnd_all.txt) is as follows. (Each record in a file contains one month of daily data.)

Variable		Columns		Type
COOP ID		1-6		Character
YEAR		7-10		Integer
MONTH		11-12		Integer
ELEMENT		13-16		Character
VALUE1		17-21		Integer
MFLAG1		22		Character
QFLAG1		23		Character
SFLAG1		24		Character
VALUE2		25-29		Integer
MFLAG2		30		Character
QFLAG2		31		Character
SFLAG2		32		Character
.		.		.
.		.		.
.		.		.
.		.		.
VALUE31		257-261		Integer
MFLAG31		262		Character
QFLAG31		263		Character
SFLAG31		264		Character

These variables have the following definitions:

COOP ID		is the U.S. Cooperative Observer Network station identification code. Note that the first two digits in the Coop Id correspond to the state.
YEAR		is the year of the record.
MONTH		is the month of the record.
ELEMENT		is the element type. There are five possible values:
		PRCP = precipitation (hundredths of inches)
		SNOW = snowfall (tenths of inches)
		SNWD = snow depth (inches)
		TMAX = maximum temperature (degrees F)
		TMIN = minimum temperature (degrees F)
VALUE1		is the value on the first day of the month (missing = -9999).
MFLAG1		is the measurement flag for the first day of the month. There are five possible values:
		Blank = no measurement information applicable
		B = precipitation total formed from two 12-hour totals
		D = precipitation total formed from four six-hour totals
		L = temperature appears to be lagged with respect to reported hour of observation
		T = trace of precipitation, snowfall, or snow depth
QFLAG1		is the quality flag for the first day of the month. There are fourteen possible values:
		Blank = did not fail any quality assurance check
		A = failed accumulation total check
		D = failed duplicate check
		G = failed gap check
		I = failed internal consistency check
		K = failed streak/frequent-value check
		M = failed megaconsistency check
		N = failed naught check
		O = failed climatological outlier check
		R = failed lagged range check
		S = failed spatial consistency check
		T = failed temporal consistency check
		W = temperature too warm for snow
		X = failed bounds check
SFLAG1		is the source flag for the first day of the month. There are fifteen possible values:
		Blank = No source (e.g., data value missing)
		0 = U.S. Cooperative Summary of the Day (NCDC DSI-3200)
		1 = U.S. Preliminary Cooperative Summary of the Day -- Transmitted
		2 = U.S. Preliminary Cooperative Summary of the Day -- Keyed from paper forms
		6 = CDMP Cooperative Summary of the Day (NCDC DSI-3206)
		A = U.S. Automated Surface Observing System (ASOS) real-time data (since January 1, 2006)
		B = U.S. ASOS data for October 2000-December 2005 (NCDC DSI-3211)
		F = U.S. Fort Data
		G = Official Global Climate Observing System (GCOS) or other government-supplied data
		H = High Plains Regional Climate Center real-time data
		I = International collection (non-U.S. data received through personal contacts)
		M = Monthly METAR Extract (additional ASOS data)
		R = NCDC Reference Network Database (Climate Reference Network and Historical Climatology Network-Modernized)
		S = Global Summary of the Day (NCDC DSI-9618) NOTE: "S" values are derived from hourly synoptic reports exchanged on the Global Telecommunications System (GTS). Daily values derived in this fashion may differ significantly from "true" daily data, particularly for precipitation (i.e., use with caution).
		X = U.S. First-Order Summary of the Day (NCDC DSI-3210)
VALUE2		is the value on the second day of the month.
MFLAG2		is the measurement flag for the second day of the month.
QFLAG2		is the quality flag for the second day of the month.
SFLAG2		is the source flag for the second day of the month.
...and so on through the 31st day of the month. Note: if the month has less than 31 days, then the remaining variables are set to missing (e.g., for April, VALUE31 = -9999, MFLAG31 = blank, QFLAG31 = blank, SFLAG31 = blank).

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.

Durre, I., M. J. Menne, and R. S. Vose. 2008. Strategies for evaluating quality assurance procedures. Journal of Climate and Applied Meteorology 47:1785-1791, DOI: 10.1175/2007JAMC1706.1.

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.

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