Campbell-Stokes recorder
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The Campbell-Stokes recorder (sometimes called a Stokes sphere) is a kind of sunshine recorder. It was invented by John Francis Campbell in 1853 and later modified in 1879 by Sir George Gabriel Stokes. The original design by Campbell consisted of a glass sphere set into a wooden bowl with the sun burning a trace on the bowl. Stokes's refinement was to make the housing out of metal and to have a card holder set behind the sphere.
This basic unit is still in use today with very little change. It is probably the most common sunshine recorder in use today, outside of the United States where the Marvin sunshine recorder is the instrument generally used by the National Weather Service.
The unit is designed to record the hours of bright sunshine which will burn a hole through the card. However, at sunrise and sunset the sun is lower in the sky and will tend to leave a scorch mark on the card which may at the extreme end be difficult to see.
The glass sphere - typically 10 cm (4 inches) in diameter - is designed to focus the rays from the sun onto a card mounted at the back and is set on a stand. The card is held in place by grooves of which there are three overlapping sets, to allow for the height of the sun during different seasons of the year. In the northern hemisphere the winter card is used from the 15th of October to the 29th of February, the equinox card from the 01st March to the 11th of April and the 03rd of September to 14th of October. The summer card is therefore used from the 12th of April to 02 of September. Each card is marked as to the hour, with local noon being in the centre, and is read in 10th's. The unit is set in a stand facing south to enable the maximum amount of sun to be recorded. Of course it is of great importance to set the unit in an area where the sun will not be blocked by buildings, trees or flagpoles.
A modification to the standard unit for Polar regions is the addition of a second, north facing, sphere and card, to record the sunlight during the summer when it remains in the sky for 24 hours.
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[edit] Advantages
The major advantage of this type of recorder is its simplicity and ease of use. There are no moving parts and it thus requires very little maintenance. The unit can be used anywhere in the world with little or no modification and is relatively inexpensive - they can be purchased for about $2000 USD.
[edit] 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. 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. However, the single biggest problem is in the reading of the cards. As the sun is 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.
[edit] References
- Department of Transport - Meteorological Branch (Canada) - Sunshine Recorders - Manual 81, Second Edition, 1966.
[edit] External links
- 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
- Photograph of an 1876 version of the Campbell recorder - National Maritime Museum, UK
- Photograph of the Campbell-Stokes recorder - Environment Canada
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