Almost a Century of “Imaging” Clouds Over the Whole-Sky Dome

Abstract

Meteorological whole-sky photography can be traced back to just after the turn of the century. Capturing an objective and well-determined view of the cloud cover over the whole-sky dome has been one of the principal goals of subsequent developments. Three types of photographic systems have been devised: refracting, reflecting, and moving film systems. A moving film apparatus seems to have been the first to capture a whole-sky view, but the technology has not advanced far since then. Reflecting systems are the cheapest for do-it-yourself enthusiasts, but refracting systems are readily purchased. The problem of selecting the most useful method for projection of the sky onto the film has arisen many times in the last 70 yr. Although an equidistant projection system makes relative distance determination easier, cloud amount can be most readily determined from a photograph produced by an equal-area projection system. If such a system is not used, the grid superimposed on the image must correct for areas distortion. Recent literature describing the use of ?fish-eye? lenses in forest and urban micrometeorology might benefit from cross-referencing with the meteorologists' problems reviewed here. For meteorological and climatalogical application, such as intercomparison with satellite-derived cloud amounts, it must be noted that the precise nature of lens projection for an automated system probably has a much smaller effect than the observer-perceived sky shape on conventional reports.