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.