120 Years of Daily Sunshine Observations from the Blue Hill Meteorological Observatory

A nearly continuous record of daily bright sunshine from 1889 through 2011 has been collected and digitized at the Blue Hill Observatory in Milton, MA. The Blue Hill Observatory is singular among North American weather stations for maintaining the longest continuous record of meteorological observations in the United States. Moreover, many of the measurements continue to be collected using traditional observing methods and instruments, and in a few cases the same instruments have been in use for much of the time since the observatory's founding in 1885. This provides an unprecedented degree of continuity within the records.

Existing digital datasets of temperature, precipitation and wind have now been expanded to include a new record of daily bright sunshine collected with a Campbell-Stokes sunshine recorder. The original sunshine recorder remained in use for over a century from 1886 until its replacement with a modern equivalent in the 1990s. The Campbell-Stokes instrument measures the duration of bright sunshine in hours per day. Daily bright sunshine is determined from the length of a burn on a calibrated paper card caused by unobscured direct sunlight being focused by a glass sphere into a small point on the card throughout the day. The total burn length is manually translated into minutes, and subsequently hours, of bright sunshine at a precision of 0.1 hours. We use the term "sunshine" because the Campbell-Stokes instrument does not directly measure solar radiative flux at the surface. During a short period of time after sunrise and before sunset, insolation is too weak to burn the card. Hence, the total possible bright sunshine duration measured by the instrument on a clear day will be slightly less than the time between sunrise and sunset. While a simple correction can account for the non-burning time, the data presented are uncorrected.

There are wide-ranging research applications of the sunshine dataset. We have already begun to explore a variety of discernable features such as global dimming, the 11-year solar cycle, and possible periods of increased atmospheric aerosol loading. It is likely that many other signals of both natural and anthropogenic phenomena exist within a sunshine record of this length. Ongoing digitization of other historical weather records at Blue Hill will provide additional research opportunities to utilize the sunshine data.