135 Years of Daily Observations at the Blue Hill Meteorological Observatory

The Blue Hill Meteorological Observatory (BHO), located on the 635-foot summit of Great Blue Hill ten miles south of Boston, Massachusetts, has been the site of continuous monitoring of the local weather and climate since its founding on February 1, 1885. For its duration and consistency, this station was recently recognized by the World Meteorological Organization as a Centennial Observing Station, one of only several sites so designated in the United States. The meticulous, extensive and high-quality climate record maintained at this location has included the measurement of temperature, precipitation, snowfall, wind, surface pressure, water vapor pressure, sunshine, cloud amount and cloud type, visibility and other parameters. For many of these measurements, traditional observing methods and instruments remain in use to provide a high-degree of continuity. Such instruments include a hygro-thermograph used within a large Hazen shelter, an ombroscope, which records the time of rainfall, a late 19^th century barometer, a weighing recording rain gauge, and a Campbell-Stokes sunshine recorder. A recent effort to assemble a carefully quality-controlled database of the BHO daily measurements of maximum and minimum temperature, precipitation, snowfall and sunshine has been completed, and these data are now available for analysis at unprecedented detail for a period covering the last 135 years. Digitization of other daily parameters remains in progress. Annual mean temperature data show an upward trend that is statistically significant to better than 99.9% confidence of +0.173 °C/decade since the late 19^th century, and this increase is larger than the trends over the continental United States and the globe over that time. An upward trend in annual precipitation of +1.53 cm/decade since the 1880’s is also statistically significant. Annual mean wind speeds have dropped by 17 percent since 1980 consistent with the stilling being documented at other locations in North America and Europe, though the specific cause remains under investigation. Sunshine duration data show decadal variations in response to changes in atmospheric aerosols since the 1960’s. The extensive BHO daily data also provide an ideal context in which to examine correlations across multiple parameters and to analyze changes in diurnal variability and daily extremes at this location, and an assessment of these diagnostics will be presented. Maintaining the extensive BHO climate record into the future and using it to enhance public awareness of atmospheric science are critically important objectives of the Blue Hill Observatory Science Center that will help ensure that this irreplaceable scientific and educational resource will continue to provide an invaluable historical perspective on ongoing, long-term, local climate changes.