Sunshine

Sunshine. As a meteorological element sunshine requires some conventional definition. There is uninterrupted continuance of gradation from the burning sunshine of a tropical noon to the pale luminosity that throws no shadow, but just identifies the position and shape of the sun through the thin cloud 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 sunshine. 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 0 27' north and south of the plane through the centre parallel to the equator or perpendicular to the polar axis. Thus a belt 46° 54' in angular width will be suitable for a sunshine recorder for any part of the world. Whatever place be chosen for the observation the same belt will do if it is set up perpendicular to the earth's polar axis. But there can be no record if the sun is below the horizon; hence any part of the belt projecting above the horizon is not only useless for recording but is liable to shadow a part of the belt where there might be a record. Hence to meet the requirements of a particular locality the belt as set up round the polar axis should be cut in two by a horizontal plane through the centre and the half projecting above the horizontal removed. Reversed it makes a half belt, exactly similar to what is left, and thus each complete belt is cut by a horizontal plane through the centre into two frames suitable for sunshine recorders for the particular locality.

The cutting of the belt may, of course, vary between the direct transverse cut along the polar axis which gives a half-ring belt to be set vertical in order to receive the record for a point on the equator, and the cut perpendicular to the polar axis which 1 These claims to early origin are mere fables, like the claim of the Oweisi order to spring from Oweis, one of the oldest traditionalists, and so forth.

2 For the dervish orders see Dervish.

divides the belt into two similar rings suitable for recording the sunshine at the poles. Clearly, when the belt is so cut that two complete rings are formed, a continuous record of sunshine throughout the twenty-four hours may be expected, so that for the polar circles the cut will run diagonally between opposite points of the extreme circles of the sun's records. 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. 1) and FIG. I. - Campbell-Stokes Sunshine Recorder.

the special form designed in the Meteorological Office, London, for use on the National Antarctic Expedition,1901-1904(fig. 2). A belt cut for a particular latitude is serviceable for some Io Antarctic Sunshine Recorder, to carry 24-hour record.

FIG. 2. - Antarctic Sunshine Recorder, to carry 12-hour record.

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, regard to the equinoctial card. The section of the supporting surface by a plane through the polar axis is to be as in fig. 3.

The Sphere

The material for the sphere must be " crown glass, colourless, or of a very pale yellow tint. The diameter 4 in. The weight between 2.92 and 3.02 lb. The focal length from the centre of the sphere to the geometrical focus for parallel rays should be between 2.96 in. and 2.99 in.

Measurement of the Sunshine Record

It was mentioned that the Campbell-Stokes recorder involves a conventional definition of sunshine. The recorded 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 of a minute's record. (See fig. 4.) Consequently with intermittent sunshine exaggeration of the actual duration of burning is very probable. Strictly speaking measurements ought to be between the diameters of the circular ends of the burns, but the practice of measuring all the trace that can be distinctly recognized as scorched has become almost universal in Great Britain, and appears to give a working basis of comparisons.

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 (From the Observer's Handbook, by permission of the Controller of H.M. Stationery Office.) FIG. 3.

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 a vertical angle of 16° with the polar axis as indicated in fig. 3. Adjustments. - The adjustments of the instrument are to set the belt so that its axis is parallel to the polar axis and symmetrically adjusted with reference to the meridian of the place, and to set the sphere so that its centre coin cides precisely with the centre of the belt. No one of the three adjustments is easy to make or totest because neither the centre of the sphere nor 3' the centre (nor indeed the g 4? axis) of the belt can be easily identified. For an instrument for testing these adjustments 02t see Quart. Journ. Roy. Met. fi'R?P Soc. xxxii. 2 49.0'S Instruments differ according to the means provided for mounting or adjusting the positions of the belt or sphere, from in that known as the Whipple Casella instrument the fixed belt is replaced by a movable card holder. The chief advantage of Stokes's specification is the simplicity of the use of the instrument when once it has been properly adjusted and fixed.

It is essential that the glass sphere s and refractive index to give an image of the sun on the prepared card or within the 20th of an inch of it nearer the centre. It is also essential that the cards used should not only be of suitable material but also of the right dimensions for the bowl. The colour and material of the cards were selected by Stokes in consultation with Warren De la Rue, who was at that time his colleague on the Meteorological Council, and the cards used by the meteorological office are still supplied by Messrs De la Rue & Co. Accuracy in the comparative measurements of sunshine by this method depends upon the proper adjustment of the dimensions of the different constituent parts of the recorder and accordingly the following specification of standard dimensions has been adopted by the meteorological office.

The Time Scale

On the time scale of the equinoctial card twelve hours are represented by 9.00 in.

The Bowl. - The diameter of the bowl, measured between the centres of the 6 o'clock marks on a metal equi noctial card of thickness 0.02 in. when FIG. in its place, is to be 5.73 in. (t o. 01 in.). The distance between the exposure edges of the upper winter flange and the lower summer flange must not be less than 2.45 in., nor exceed 2.50 in. The distances from the middle line on the equinoctial card to the middle lines on the summer and winter cards are to be 070 in. (* 0.02 in.). The inclination of the summer card, in place, to the winter card, in place, is to be 32° 1°, s y mmetrically arranged with 5. - Sunshine Record (June 19 and 20, 1908).

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 elective absorption of the atmosphere makes the relation of the record to that of the sun's scorching power dependent upon atmospheric conditions and therefore on different occasions, so that the two records give different aspects of the solar influence. Other recorders use the thermal or photographic effect FIG. 4. - Records obtained by intervals varying rom one second burns increases from right to left o exposing a Campbell-Stokes Sunshine Recorder for measured to thirty minutes. The duration of the exposure of the separate f the diagram.

Loulu ye of the proper size traverse of a spot of itgnt over the sensitive paper, arrangea as a
10 II Noon				I 2 3 4 5 6789 10 II Mid. I 2 3 4 5 6 7 8 9

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 of the sun's rays and record duration by a clock instead of allowingexclusively local, and indeed the possible duration of sunshine at the sun to keep its own time. In the Marvin sunshine recorders of I any station is a local characteristic which it is desirable to know.

Forenoon, (a.m.), (p. m.) Consequently as evidence of the peculiarity of Noon Afternoon 6 8 10 12246 g the site the recorded sunshine might be referred to the total possible with a free horizon. On the other hand, taking the record of sunshine as an indication of the clearness of the sky for the purposes of general meteorology, the screening of the sun by hills must be regarded simply as limiting the time during which observation is possible and the duration of the sunshine recorded should be referred to the possible duration at the particular site. It would, therefore, be desirable in publishing records of the duration of sunshine recorded to note also the possible amount for the instrument as exposed (see Hourly Means at Five Observatories under the Meteorological Council, 1891, No. 113, p. io). The table shows the number of hours the sun is above the horizon during each month in the latitude of the British Isles.

By way of exhibiting the results obtained from sunshine records we reproduce (fig. 7) the sunshine map of the British Isles taken from the annual summary of the Monthly " Weather Report," 1908 (British Meteorological Year-Book, pt. ii.). Corresponding maps embodying data from over 130 stations are prepared each month; fig. 8 shows the variation in the distribution of ° sunshine that may take place in different months.

Further, fig. 9 represents the average weekly distribution of sunshine in different sections of the British Isles according to the average of the United States weather bureau an electrical contact is made by twenty-five years.

4 January February March April May June July August Septembe October Nouember December 25 24 9 10 FIG. 6. - Monthly Average Duration of bright Sunshine for each at Valencia (Ireland).

the thermal effect of the sun and the duration of the contact is recorded. An instrument 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 uncertainty 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. It is vastly more detailed than that of other instruments (see fig. 5), but the interpretation of the record in terms suitable for meteorological or climatological purposes is a special study, which has not yet been attempted. In a somewhat similar way information about the duration and intensity of sunshine with an abundance of detail can be obtained from the record upon photographic paper passing under an aperture in a drum which revolves with the sun, as in the Lander recorder, but the study of such details has not been begun.

Sunshine Records for the British Isles

The interest in the use of sunshine recorders is more widely extended in the British Isles than elsewhere, and it is, so far as the public are concerned, the most important meteorological element, but it is singular that up to the present a knowledge of the total amount of sunshine recorded during the day, the week, the month or the year is all that is apparently required. Except for the observatories in connexion with the meteorological office and a few others the distribution of sunshine during the day is not taken out, so that we are still some distance from attacking the problems presented by the finer details of solar records. Fig. 6 shows the average duration of bright sunshine for each hour of the day for each month at Valencia. The expectation of sunshine is greatest at 1 p.m. and 2 p.m. in May, while there is a well-marked secondary maximum in September.

Exposure

We now consider what the daily sunshine record for a particular station means. An ideal exposure has an uninterrupted view of those parts of the horizon in which the sun rises or sets; and elsewhere the view of the sun must not be obstructed by the ground, buildings, trees or any other obstacle; but ideal exposures are not always to be obtained. In mountainous districts particularly it may be impossible to find a site in which the sun is not obstructed for an appreciable part of the day. In these circumstances it becomes a question whether the amount of sunshine recorded should be referred to the maximum possible for an uninterrupted horizon or the maximum possible for the particular exposure. The answer to the question really depends upon the purpose for which the information is wanted. As a climatological factor of the locality the shadow cast by the surrounding hills is of importance, it is part of the difference between the fertility of the southern and northern slopes of hill country. This importance is, of course, in many respects 1 Brit. Assoc. Report (1900), p. 44.

hour of the day FIG. 7. - Sunshine in the British Isles in 1908.