1911 Encyclopædia Britannica/Sunshine

by William Napier Shaw

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° 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.

Fig. 1.—Campbell-Stokes Sunshine Recorder.

Antarctic Sunshine Recorder, to carry 24-hour record.

Antarctic Sunshine Recorder, to carry 12-hour record.

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 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 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 10° on either side of that latitude if the cards are not trimmed too must always be closely to the cutting of the belt. The belt 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 polarized 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 coincides precisely with the centre of the belt. No one of the three adjustments is easy to make or to test because neither the centre of the sphere nor the centre (nor indeed the axis) of the belt can be easily identified. For an instrument for testing these adjustments see Quart. Journ. Roy. Met. Soc. xxxii. 249.

Instruments differ according to the means provided for mounting or adjusting the positions of the belt or sphere, and 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 off the use of the instrument when once it has been properly adjusted and fixed.

It is essential that the glass sphere should be of the proper size 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-oo in.

The Bowl.~»The diameter of the

bowl, measured between the centres of the 6 o'clock marks on a metal equinoctial card of thickness 0-02 in. when

in its place, is to be 5-73 in. (1 0°0I in.). The distance between the exposure edges of the upper winter flange and the lower summer fiange 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 0-70 in. (=i= o-02 in.). The inclination of the summer card, in place, to the winter card, in place, is to be 32° = %°, symmetrically arranged with FIG.

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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 astlre" basis of a comparison of results for climatological purposes., The spot winch is scorched on the card by the sun is not quite limited to-the image of 'the sun, anda few seconds of really strong sunshine will produce a circular” burn which is hardly distinguishable in size from that of/a niinute'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, butfthe 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.

FIG. 4.-Records obtained by exposing a Campbell-Stokes Sunshine Recorder for measured intervals varying from one second to thirty minutes. The duration of the exposure of the separate burns increases from right to left of the diagram.

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

Fig. 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 of the sun's rays and record duration by a clock instead of allowing exclusively local, and indeed the possible duration of sunshine at

the sun to keep its own time. In the Marvin sunshine recorders of an station is a local characteristic which it is desirable to know. Consequently as evidence of the peculiarity of 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

indication of the clearness of the sky for the purposes of general meteorology, the screening larch 2 of the sunh by hillsdmust be feglarded simply as limiting the time uring w ic observation is A 'H A I - possible and the duration of the sunshine recorded 3, oz shou ld be referred to the possible duration at the 5 particular site. It would, therefore, be desirable W9 0 k in publishing records of the duration of sunshine 0 ° ie M N M is M 5° 5° °° 63 °° '°' recorded to note also the possible amount for the /-me I instrument as exposed (see Hourly Means at " '° '° '° "' ° °° " "3 °» Five Observatories under the Meteorologicaé Council, J, F 1891 No. 113 p. 10). The table s ows the My 000 io so as so ~aa -an 40 on 42 43 sz so as -ra -on Humber of hOLI'l'S the Sun 1S above the hOl'1Z01l A" A ' during each month in the latitude of the British g °° i 1 21 S1  »# 42 42 as 42 so as -as ' -as Isles* . By way of exhibiting the results obtained Septcmbrr L, 4 from sunshine records we reproduce (fig. 7) the -02 ' 34 41 .44 .45 .45 .41 . . r . . . ° W '° W '°° sunshine map of the British Isles taken from the oem", A annual summary of the Monthly “ Weather -in ' 1 -34 as - . .1 4 A1 as' V ~os Report, ” 1908 (British Meteorological Year-Book, Noucmber ' ' pt. ii.). Corresponding maps embodying data 3, 3, 3, 3, 2, os from over 130 stations are prepared each month; Hg. 8 shows the variation in the distribution of DGOIMMF . “ V l sunshine that may take place in different months. 97 ' | '28 '25 '26 '18 '10

FIG. 6.-Monthly Average Duration of bright Sunshine for each at Valencia (Ireland).

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 instrument which gives a corresponding result is described by W. H. Dines (Quart. Journ. Roy. Met. Soc. xxvi. 243). These define sunshine by the eliect 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 polarized 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 I slcs.—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 I p.m. and 2 p.m. in May, while there is a well-marked secondary maximum in September.

ExPOS14f¢.-We now consider what the daily sunshine record for; particular station means. An ideal exposure has an uninterrupted view of those parts of the

horizon in which the sun rises or sets; Further, fig. 9 represents the average weekly distribution of sunshine in different sections of the British Isles according to the average of twenty-five years.

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FIG. 7.1-Sunshine in the British Isles in 1908. Possible Duration of Bright Sunshine in the 'Latitude'of the British I sles. and elsewhere the view of the sun ' M Lnqiginot be obstructed b};;he li?-0u;;d jan, Feb. March April May June July Aug. Sept. Oct. N ov. Dec. UI In S, trees OI' any ot el' 0 SICEIC C; but ideil exposures are not always to 50° 262 278 288 365 410 473 482 485 442 373 327 266 246 be obtained. In mountainous districts 51° 257 276 286 365 411 477 487 489 444 373 -325 262 241 particularly it may be impossible to 52° 251 273 284 365 412 481 491 494 446 374 324 258 236 find a site in which the sun is not 53° 247 271 281 364 414 486 498 499 450 375 323 254 231 obstructed for an appreciable part of 54° 243 268 279 363 417 490 503 505 453 375 322 251 225 the day. In these circumstances it 55° 237 265 276 363 418 494 510 511 456 376 319 245 218 becomes a question whether the amount 56° 232 263 273 362 420 499 516 516 459 376 316 239 2 1 1 of sunshine recorded should be referred 57° 226 260 270 362 423 504 524 523 463 377 314 236 205 to the maximum possible for an un- 58° 219 257 267 361 426 510 532 530 467 378 312 232 197 interrupted horizon or the maximum 59° 211 253 263 361 429 517 541 538 471 379- 309 225 187 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 Brit. Assoc. Report (1900), p. 44.

Sunshine in the Antarctic Regions.-It is clear that so far as concerns the zone from 50° to 60° N. in this particular region, the annual amount of sunshine diminishes as one goes northward. It would, however, not be safe to conclude that this diminution in the aggref gate duration of sunshine during the year goes on without interruption as one proceeds northward. At least the corresponding statement would not be true of the southern hemisphere. No doubt the frequency of cloud and the consequent loss of duration of sunshine would increase for corresponding latitudes from the tropical anticyclone southward, but beyond the region of minimum (pressure at the winter quarters of the “ Discovery ” in latitude 77 51' S., longitude 166° 45' E., the amount of bright sunshine recorded during the two years 1902 and 1903 was remarkably large. The total for 1903 equalled that for Scilly, and in December of that year an The one question of the kind to which' the sunshine recorder gives an absolutely controvertible answer is as to the effect of the smoke of great citie s in diminishing the sunshine in the immediate average of 16 hours per day was registered. Ieahe/s are shou/nfnr 9111 750,200,250, 300 and 350 hrS. 3 200 I 10| -11 o . "

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O s 111 QL? 0 I V 11? W FIG. 8.-Sunshine in the British Isles in May and June 1909. Sunshine Results for Other Ports of the World.-Maps showing the average annual distribution of sunshine over Europe and North America are given in Bartholomew's Phymkol Atlas, vol. iii. Atlas of Meteorology. Over Europe the largest totals, over 2750 hours per annum, are shown over central Spain. In North America, values exceed 3250 hours per annum in the New Mexico region. For other parts of the world the information available is not sufficiently extensive for the construction of charts. Eject upon Sunshine Records of the Smoke of Great Cities.—Much discussion has taken place from time to time as to whether the climate of a locality can be altered by artificial means. Questions have been raised as to the effect of forests upon rainfall, as to the indirect effect of irrigation or the converse process, the obliteration of natural irrigation by blown sand, and as to the possibility of producing, arresting or modifying rainfall by the discharge of explosives. 1 I0 is 20 25 30 ds 40g 45 50 3 3 News Spring Summer ' Autumn Winter i4g. I- I, I al l I0 20 no — s Extreme North 60- 1 so- 40.1 1 20 m I0 - . Extreme South 40 ., 30~ 20 10-f Western Section Z I l 40- 30- 20— I0 - . Eastern Sectzon FIG. 9.-Average Duration of bright Sunshine in the British Isles for each week. neighbourhood. This may be illustrated by the figures for sunshine during the winter months 05 Bunhill Row, E.C., in the middle of London, Westminster, Kew and Cambridge. Monthly Average Duration of Bright Sunshine derived from Observations extending over Twenty Years. Station. November. December. January. February. Bunhill Row . 22-8 7~5 14-1 30-6 V/est minster . . 27-7 13-1 18-4 32-8 Kew . . . . 50-8 38-1 40-3 54-6 Cambridge . 61-0 40-6 48-9 73-8 This is not a question which comes out merely by taking averages. The answer can be seen directly by comparing the daily cards (see fig. IO, Sunshine Cards for Cambridge, Westminster and Bunhill Row for December 1904). Thus it appears that the direct effect of the local contamination of the London atmosphere results in the diminution of the recorded sunshine for the whole year by 37%, and it is clear that the contamination extends in some degree as far as Kew, where the loss amounts to about 10%. There is evidence of various kinds to show that the effect of the smoke cloud of cities Cambridge. Westminster. Bunhill Row.

IG. 10.

can be traced sometimes for great distances, and in special conditions of weather with easterly winds the effect is sometimes remarkably persistent.  (W. N. S.)