SUNSHINE. As a meteorological element sunshine requires some conventional definition. There is uninterrupted continu ance of gradation from the burning sunshine of a tropical noon to the pale luminosity that throws no shadow, but just identifies the shape of the sun through the thin clouds of northern skies.
The Campbell-Stokes Sunshine Recorder.—In the British Isles the sun is allowed to be its own timekeeper and the scorch of a specially prepared card used as the criterion for bright sun shine. The practice arose out of the use of the sunshine recorder which depends upon the scorching effect of a glass sphere in the sun's rays. The original form of the instrument was suggested by J. F. Campbell of Islay in 1857. He used a glass sphere within a hemispherical bowl of wood.
The scorching of the wood along successive lines of the bowl as the sun alters its declination from solstice to solstice leaves a rugged monument of the duration and intensity of the sunshine during the half-year, but does not lend itself to numerical measurement.
The design of a metal frame to carry movable cards and thus give a decipherable record of each day's sunshine is due to Sir G. G. Stokes. The excursions of the sun to the north and south of the equator are limited by the tropical circles, and the solar record on the hemispherical bowl will be confined within a belt 23° 27' north and south of the plane through the centre parallel to the equator or perpendicular to the polar axis. Thus a belt 54' in angular width will be suitable for a sunshine recorder for any part of the world. What ever place be chosen for the observation the same belt will do if it is set up perpendicular to the earth's polar axis. As examples of the cutting of the belt for different latitudes we may put side by side the recorder as used in temperate latitudes (fig. I) and the special form designed in the Meteorological Office, London, for use on the National Antarctic Expedition, 19or-19o4 (fig. 2). A belt cut for a particular latitude is serviceable for some io° on either side of that latitude if the cards are not trimmed too closely to the cutting of the belt. The belt must always be adjusted round the parallel to the polar axis. If the cut of the belt is too oblique for the latitude of the place where it is exposed, and the cards are cut strictly to the belt, the northern side of the cut will be below the horizon and the southern side above it, some sunshine may be lost near sunrise or sunset in the winter because there is no card to receive it. The part projecting above the horizon in summer will partly shadow the globe, and faint sunshine may be lost, for at most only half the globe can be solarized at sunset. But the loss due to this cause is unimportant. Stokes designed the complete belt to use successively three cards of different shape for different times of the year. The equinoctial
card forms a portion of a cylinder round the polar axis for spring and autumn, the summer card and the winter card each forms a part of a cone making an angle of 16° with the polar axis.
Measurement of the Sun shine Record.—It was men tioned that the Campbell-Stokes recorder involves a conventional definition of sunshine. The re corded day of sunshine is less than the actual time during which the sun is above the horizon by about twenty minutes at sunrise and sunset on account of the want of burning power of a very low sun. Some further convention is necessary in order to obtain a tabulation of the records which will serve as the basis of a comparison of results for climatological purposes. The spot which is scorched on the card by the sun is not quite limited to the image of the sun, and a few seconds of really strong sunshine will produce a circular burn which is hardly distinguishable in size from that produced by a minute's record. Consequently with intermittent sunshine exaggeration of the actual duration of burning is very probable.
Other Types of Sunshine Recorder.—There are, however, various other conventions as to sunshine which are used as the basis of recorders of quite different types. The Jordan recorder uses ferro-cyanide paper and the sun keeps the time of its own record by the traverse of a spot of light over the sensitive paper, arranged as a cylinder about a line parallel to the polar axis. The effect thereby recorded is a photochemical one, and the composite character of the sun's radiation, modified by the selective absorp tion of the atmosphere, makes the relation of the record to that of the sun's scorching power dependent upon atmospheric con ditions, so that the two records give different aspects of the solar influence on different occasions. Other recorders prefer to use the thermal or photographic effects of the sun's rays and record duration by a clock instead of allowing the sun to keep its own time. In the Marvin sunshine recorders of the United States weather bureau an electrical contact is made by the thermal effect of the sun and the duration of the contact is recorded. An instru ment which gives a corresponding result is described by W. H. Dines (Quart. Journ. Roy. Met. Soc. xxvi. 243). These define sunshine by the effect necessary to produce or maintain a certain thermal effect, but the definition once accepted there is no uncer tainty as to the record. The Callendar sunshine recorder gives a record of the difference of temperature of two wires, one solarized and the other not, and it is therefore a continuous record of the thermal effect of solar and terrestrial radiation.