Gary Archibald, Weathercaster

Here’s another in our weather devices/instruments series – this one is unique and not well known… sunshine recorders. Check it out…

Sunshine Recorders are used to indicate the amount of sunshine at a given location. The results are used to help provide information on the climate of an area and some of the fields it is of importance to are science, agriculture and tourism.

Traditionally, sunshine recorders are divided into two groups. In the first group the time of the occurrence of the event is provided by the sun itself and in the second a clock type device is used to provide the time scale.

The older type of recorders required the interpretation of the results by an observer and these may have differed from one person to another. Today, with the use of electronics and computers it is possible to record the sunshine duration that does not rely on an observers interpretation. At the same time the newer recorders can also measure the global and diffuse radiation.

One such type of sunshine recorder (and perhaps the most widely used to this day) is…

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. 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.

Campbell-Stokes recorder

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.

Sunshine recorders…interesting don’t you think? Next time around, I will discuss the unique device that was designed to measure the height of the base of a cloud…

Cheers!

GA