• Changnon, S. A., 1990: Temporal changes in the climate of Urbana-Champaign and Illinois. Trans. Ill. Acad. Sci., 83 , 576.

  • Changnon, S. A., 1999: A rare long record of deep soil temperatures define temporal temperature changes and an urban heat island. Climate Change, 42 , 53153.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Changnon, S. A., , and Boyd G. , 1963: History of the urbana weather station, 1888–1963. Circular 88, Illinois State Water Survey, Urbana, IL, 58 pp.

  • Huff, F. A., , and Changnon S. A. , 1987: Temporal changes in design rainfall frequencies in Illinois. Climate Change, 10 , 19520.

  • Karl, T. R., and Coauthors, 1995: Critical issues for long-term climate monitoring. Climate Change, 31 , 18522.

  • Kurtyka, J. C., 1953: Precipitation measurements study Rep. of Investigation 20, Illinois State Water Survey, Urbana, IL, 178 pp.

  • Mitchell, J. M., 1961: The measurement of secular temperature changes in the U.S. Research Paper 43, U.S. Weather Bureau, Washington, DC, 38 pp.

  • Quayle, R. G., , Easterling D. R. , , Karl T. R. , , and Hughes P. Y. , 1991: Effects of recent thermometer changes in the cooperative network. Bull. Amer. Meteor. Soc., 72 , 1718172.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wendland, W. M., , and Armstrong W. , 1993: Comparison of maximum-minimum resistance and liquid-in-glass thermometer records. J. Atmos. Oceanic Technol., 10 , 23323.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • View in gallery

    Moving averages for 5- and 11-yr periods of the annual mean temperatures at Urbana, expressed as departures from the 118-yr mean value (1888–2004).

  • View in gallery

    A curve depicting the changes in the annual mean temperatures at Urbana resulting from shifts in instruments, sites, and the urban environment.

  • View in gallery

    Measured (actual) and adjusted annual mean temperatures for Urbana (1889–2004).

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Changes in Instruments and Sites Affecting Historical Weather Records: A Case Study

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  • 1 Illinois State Water Survey, Urbana, Illinois
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Abstract

All long historical climate records are based on measurements that experienced shifts in instrumentation, site characteristics, or locations. How such changes affect the quality of past data remains an uncertainty for the thousands of historical records, confounding efforts to assess climate change. Fortunately, one station in Illinois with 118 yr of records has also kept detailed records of all such shifts plus overlapping measurements of temperatures and precipitation, allowing exact measurements of how conditions changed over time. This study examined these data and found varying discontinuities of 0.1°–0.9°C in annual temperatures due to various shifts, but no changes in daily precipitation related to site shifts. However, hourly precipitation amounts from recording rain gauges did undergo a considerable shift due to changes in rain gauge types. Similar studies need to be made of other stations with comparable historical records of station and instrument shifts and with overlapping measurements when shifts were made.

Corresponding author address: Stanley A. Changnon, Illinois State Water Survey, 2204 Griffith Dr., Urbana, IL 61820. Email: schangno@uiuc.edu

Abstract

All long historical climate records are based on measurements that experienced shifts in instrumentation, site characteristics, or locations. How such changes affect the quality of past data remains an uncertainty for the thousands of historical records, confounding efforts to assess climate change. Fortunately, one station in Illinois with 118 yr of records has also kept detailed records of all such shifts plus overlapping measurements of temperatures and precipitation, allowing exact measurements of how conditions changed over time. This study examined these data and found varying discontinuities of 0.1°–0.9°C in annual temperatures due to various shifts, but no changes in daily precipitation related to site shifts. However, hourly precipitation amounts from recording rain gauges did undergo a considerable shift due to changes in rain gauge types. Similar studies need to be made of other stations with comparable historical records of station and instrument shifts and with overlapping measurements when shifts were made.

Corresponding author address: Stanley A. Changnon, Illinois State Water Survey, 2204 Griffith Dr., Urbana, IL 61820. Email: schangno@uiuc.edu

1. Introduction

A never ending, unresolved question integral to use and interpretation of atmospheric data concerns the uncertainties in its accuracy and quality. Uncertainties have existed and will continue to exist in all forms of environmental data and these are related to many measurement factors. There is a need to define, measure, and understand these uncertainties, and those associated with historical surface climate data are illustrated by this case study of a unique historical record. Factors that can produce uncertainties in historical observations of surface climate conditions include 1) instrument accuracy, 2) stations moves, 3) changes in observers, 4) changes and/or malfunctions of instruments, 5) changes in observation and reporting times, 6) changes in exposure and site characteristics, and 7) variations in the height of measurements and ground cover.

The 118 yr of historic climate data collected beginning in 1888 at Urbana, Illinois, forms one of the state’s longest datasets. The Urbana station has been identified in the past as one of the best weather stations in the nation (Mitchell 1961). Fortunately, all the changes made in the station’s location and its instruments over time have been well recorded. Importantly, overlapping temperature and precipitation measurements were made at the times of all the shifts in the station’s location and instrumentation, one of the few weather stations in the nation that has experienced such dedicated dual record keeping over its entire period of operation. The operation and care of the Urbana station reflect 4 of the 10 climate monitoring principles (Karl et al. 1995). These include 1) the need for parallel testing of old and new systems, 2) full documentation of each system and operations, 3) assessment of data quality and homogeneity, and 4) maintenance of a long-term commitment to the observations and data quality. In 1961, the nationally recognized quality and value of the station led the U.S. Weather Bureau’s top climatologists of the mid–twentieth century, Helmut Landsberg and Murray Mitchell, to select the Urbana station as one of 26 benchmark stations in the nation for monitoring the nation’s climate (Mitchell 1961). Of added importance is the fact that from the beginning, atmospheric measurements included a comprehensive set of conditions including humidity, wind, sky cover, pan evaporation, evapotranspiration (ET), and soil temperatures. The goal of this case study was to illustrate how various changes in sites, instruments, and the station’s environment affect a historical climate record, and to encourage similar studies of other stations.

2. Historic events

The high quality of data collection and recording of operational events at the Urbana station were related to those operating the station. The Urbana weather station was started by the University of Illinois on its south campus, operated as part of the new agricultural experiment station started in 1887. The College of Agriculture maintained the station, provided the best instrumentation, and leading scientists supervised its operations from its beginning in June 1888 until 1947. The ET and soil temperatures were measured in special devices designed and constructed by university staff. From 1948 to the present, the Illinois State Water Survey has maintained and staffed the station as part of its research and services program. In 1902 the Urbana station was identified by the U.S. Weather Bureau as one of its cooperative substations.

In early 1897 the station was moved 600 feet from its original site and installed alongside the famed “Morrow Plots,” which are agricultural experimental plots planted in corn since 1876 (to the present) and recognized as a National Historic Landmark. Comparable temperature and precipitation records were made at both sites for a year, revealing no differences. During 1899–1902 several recording-type instruments were added (rain, temperature, humidity, and wind). The weather shelter, which contained the liquid-in-glass thermometers, had been 1.2 m above the ground from 1888 to 1897, but in 1898 the thermometers were installed in a shelter on a platform 3 m above the ground where it remained until 1948 (Changnon and Boyd 1963). In 1948 the weather shelter was again mounted at a standard height, 1.2 m above the ground, and comparisons were made of temperatures at both heights for a year. The station’s recording rain gauge, a tipping-bucket type, was changed in 1948 to a weighing-bucket-style gauge. In 1984 the weather station was established at a rural setting 2.2 km southwest of the 1897–1984 Morrow Plots site. All forms of weather data were collected at the Morrow Plots site and at the new location for 3 yr (1984–86) to measure differences.

A factor that can affect the accuracy of weather data are the weather observers who collect the data. Fortunately, since the inception of the Urbana station in 1888, there have been skilled scientists supervising the station and its operations (Changnon and Boyd 1963; Changnon 1990). The institutional staff employed and trained the station’s observers. There is little reason to suspect that observer errors are a problem in the historical data of Urbana. Observation times have been in the morning hours, a condition not apt to affect the continuity of the values over time. The land cover at all station sites was grass.

3. Effects of changes in sites and instrumentation

a. Precipitation

The official daily precipitation was measured in standard 20-cm (8-in.) diameter nonrecording rain gauges using measuring sticks from 1888 to the present. All gauges were at the same height above ground, and all gauge exposures were relatively open and very similar throughout the station’s 118-yr history. There are no instrumental or siting reasons for temporal shifts in the official precipitation values. The 1897 station relocation was checked by data collected at both sites and the difference was less than 1%. The 1984 relocation was also checked and the 3-yr difference in amounts was 3%, which was within the precipitation variability found across a local dense rain gauge network.

Differences over time occurred in the recording rain gauge data. The tipping-bucket gauge used from 1902 to 1948 produced lesser amounts at high rainfall rates than did the weighing-bucket gauge used since 1948. The temporal response of the tipping mechanism in such gauges is sufficiently slow to produce lesser rain values when heavy rainfall rates occur than does the weighing mechanism in bucket-type gauges. Differences in heavy rainfall rates, defined as 2.5 cm or more per hour, between these two types of rain gauges have been measured as being 15%– 35% less by the tipping bucket (Kurtyka 1953), with the difference dependent on the rate of rainfall (i.e., bigger percentage with heavier fall). This is a factor in the noted increase in heavy rainfall amounts that have occurred since 1940–50 in Illinois (Huff and Changnon 1987). The shift from the wide use of tipping-bucket gauges to weighing-bucket types by the Weather Bureau occurred during the 1940–50 period, and the 1-day, 2-yr rainfall frequencies in Illinois showed an increase of 17% from 1901–40 to 1941–80.

Records of snowfall observations since 1888 do not reveal shifts that produce possible effects on snowfall amounts. Assessment of the annual snowfall amounts during 1888–1988 at Urbana provided no indication of changes due to measurement and observations (Changnon 1990).

b. Temperature

The annual mean temperatures, expressed as departures from the 118-yr mean, for 5- and 11-yr moving averages are presented in Fig. 1. Effects of the noted changes in station sites, instrumentation, and urban environment are not identifiable in these curves. The curves show 1) a major temperature increase that began in the 1920s; 2) a peaking of values from 1933 to 1956; 3) a rapid descent in values from 1957 into the 1960s; and 4) fluctuations around a rather constant value for 1961–2004.

Liquid-in-glass (LIG) thermometers were used for maximum and minimum daily values from 1888 until 1988 when maximum–minimum temperature sensors (MMTS) became the official Weather Bureau method for obtaining the daily maximum and minimum temperatures at Urbana and most other cooperative stations (Quayle et al. 1991). This shift resulted in a change in the Urbana values with maximum temperatures from MMTS being 0.6°C lower than LIG values, whereas the minimum MMTS values averaged 0.06°C higher than LIG values (Wendland and Armstrong 1993). The shift in average daily mean temperature was 0.3°C lower.

The shift in shelter heights from 1.2 to 3 m from 1904 to 1948 resulted in a lowering of annual mean temperatures by 0.17°C. This was determined from measurements of temperatures taken during 1947–48 in the shelters at each height.

Another factor affecting air temperatures at Urbana was the local growth of Champaign and Urbana communities. In 1888 the local total population was only 7700 and located more than 1 km from the weather station. By 1960 the population had reached 100 000 and the urban settlement had grown very close to the Morrow Plots area. The potential for effects from the urban heat island was real. Fortunately, deep soil temperature data were collected at a nearby rural site during the 1889–1952 period, allowing assessment of the heat island effect on local air temperatures. A study of the air and soil temperatures and their differences over time revealed that the urban heat island had increased the annual mean air temperature by 0.7°C from 1900 to 1952 (Changnon 1999). Ensuing growth from 1952 to 1980 had less effect and raised the annual mean temperature by another 0.2°C by 1980.

The relocation of the station to a rural area in 1984 was assessed over a 3-yr period by taking measurements at the Morrow Plots site and the new site. This produced a reduction in values with annual mean temperatures being 0.8°C lower. This new rural site effectively eliminated the urban heat island effect.

The overlapping measurements made during the instrument and site changes allow assessment of the changes these shifts in sites and equipment produced, and the changes in annual temperature (in degrees Celsius) for each are

  • higher shelter from 1898 to 1948 lowered by 0.17°,
  • urban heat island from 1900 to 1983 increased by 0.9°,
  • rural setting from 1984 to the present lowered values by 0.8°, and
  • shift to MMTS from 1988 to the present lowered values by 0.3°.

These are illustrated for the 118-yr record (Fig. 2) using a curve constructed to reveal the decrease in 1898 (higher shelter), the continuing increases thereafter (urban heat island), the increase in 1948 (lower shelter), the sharp decrease in 1984 (station moved), and the decrease in 1988 (MMTS installed). These adjusted annual values were applied to the actual measured annual temperatures to create the “adjusted” curve shown in Fig. 3, which also has the unadjusted curve based on the annual mean temperatures recorded for the 4-yr periods from 1889 to 2004.

From 1989 to 2004, the annual values were raised by 0.3°C to adjust for the MMTS effect. The effect of the urban heat island during 1900–80 was used to lower values but it was partially reduced by the increases due to MMTS and the lower temperature resulting from the higher shelter during 1898–1947. The adjustments to the time series produced some important changes in the interpretation of the multidecadal variability at this site. Although 1953–56 remains the warmest 4-yr period, several recent periods were nearly as warm and comparable to other warm periods of the 1930s. Also, the most recent 20 yr are decidedly warmer than the cool period of 1961–80, a feature not evident in the unadjusted actual time series.

4. Conclusions

Analysis of the extensive Urbana station climate records revealed several significant changes in temperature values occurred during 1888–2005 as a result of instrument shifts, a station relocation, and urban growth. The official precipitation amounts were measured over the 118 yr by the same type of nonrecording rain gauges and at similar sites, leading to no human-made shifts. A change in the type of recording rain gauges in 1948 produced a shift in the sampling of heavy rainfall, leading to increased heavier rainfall amounts.

Few other long-term U.S. stations are apt to have records that compare with those at Urbana. Particularly important to the Urbana assessment is the thoughtful keeping of time-overlapping measurements when changes were made. These have allowed quantification of the changes in values resulting from shifts in instruments and station locations. Most importantly, they allow one to understand the uncertainties inherent in the historical climate record everywhere. These simply cannot be measured because of the lack of detailed record keeping and the important collection of dual data for shifts in instruments or sites. Comparable studies should be made of other weather stations that possess quality records documenting station location and instrument shifts plus overlapping measurements of temperature and precipitation when shifts were made.

REFERENCES

  • Changnon, S. A., 1990: Temporal changes in the climate of Urbana-Champaign and Illinois. Trans. Ill. Acad. Sci., 83 , 576.

  • Changnon, S. A., 1999: A rare long record of deep soil temperatures define temporal temperature changes and an urban heat island. Climate Change, 42 , 53153.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Changnon, S. A., , and Boyd G. , 1963: History of the urbana weather station, 1888–1963. Circular 88, Illinois State Water Survey, Urbana, IL, 58 pp.

  • Huff, F. A., , and Changnon S. A. , 1987: Temporal changes in design rainfall frequencies in Illinois. Climate Change, 10 , 19520.

  • Karl, T. R., and Coauthors, 1995: Critical issues for long-term climate monitoring. Climate Change, 31 , 18522.

  • Kurtyka, J. C., 1953: Precipitation measurements study Rep. of Investigation 20, Illinois State Water Survey, Urbana, IL, 178 pp.

  • Mitchell, J. M., 1961: The measurement of secular temperature changes in the U.S. Research Paper 43, U.S. Weather Bureau, Washington, DC, 38 pp.

  • Quayle, R. G., , Easterling D. R. , , Karl T. R. , , and Hughes P. Y. , 1991: Effects of recent thermometer changes in the cooperative network. Bull. Amer. Meteor. Soc., 72 , 1718172.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Wendland, W. M., , and Armstrong W. , 1993: Comparison of maximum-minimum resistance and liquid-in-glass thermometer records. J. Atmos. Oceanic Technol., 10 , 23323.

    • Crossref
    • Search Google Scholar
    • Export Citation

Fig. 1.
Fig. 1.

Moving averages for 5- and 11-yr periods of the annual mean temperatures at Urbana, expressed as departures from the 118-yr mean value (1888–2004).

Citation: Journal of Atmospheric and Oceanic Technology 23, 6; 10.1175/JTECH1888.1

Fig. 2.
Fig. 2.

A curve depicting the changes in the annual mean temperatures at Urbana resulting from shifts in instruments, sites, and the urban environment.

Citation: Journal of Atmospheric and Oceanic Technology 23, 6; 10.1175/JTECH1888.1

Fig. 3.
Fig. 3.

Measured (actual) and adjusted annual mean temperatures for Urbana (1889–2004).

Citation: Journal of Atmospheric and Oceanic Technology 23, 6; 10.1175/JTECH1888.1

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