the Campbell–Stokes recorder uses solar power. Disadvantages When the sun is low in the sky it may not have enough strength to properly burn the card and thus can only measure the amount of bright sunshine as opposed to visible sunshine. This often occurs at dawn and dusk. Rain may cause the card to be torn when removing it and thus making it difficult to read. In areas of high frost and during periods of freezing rain the sphere may be difficult to clean and may not be removed before the sun is shining again. It is also susceptible to external, non-weather factors such as dust, dirt or dried bird droppings accumulating on the glass sphere which requires frequent inspection and cleaning. The single biggest problem is in the reading of the cards. On days when the sun is alternately covered and exposed by clouds, the amount of burn on the card may be the same for 30 seconds as for 5 minutes. Thus, the reading of the card may differ from one observer to another. Comparisons with automatic instruments at German stations revealed that during summer the differences of the two measurement systems can reach up to 4 h per day. The mean difference was −0.23 h, i.e. the measurements of the Campbell–Stokes recorder are larger than the automatic. Instruments cost $US500–2,000. References Further reading Department of Transport – Meteorological Branch (Canada) – Sunshine Recorders – Manual 81, Second Edition, 1966. External links Campbell's manuscript notes 3D view of a Campbell–Stokes recorder (requires QuickTime) – Alistair Fraser, Penn State University AMS Glossary of Meteorology: Campbell–Stokes Recorder – American Meteorological Society Los Angeles Pierce College Weather Station – provides examples of exposed record cards and photos of the recorder unit Photograph of an 1876 version of the Campbell recorder – National Maritime Museum, UK Photograph of the Campbell–Stokes recorder – Environment Canada Blake-Larsen Sun Recorder Web Site – Blake Larsen Sun Recorder Sunshine recorders