# Handbook of Meteorology/Sunshine Recorders

THE MEASUREMENT OF SUNSHINE: SUNSHINE
RECORDERS
Recording the daily duration of sunshine is a part of the work of all Weather Bureau stations; it is carried on also at agricultural experiment stations, at university laboratories, and at many aviation fields. The objective information may be different in the various cases; the methods of measurement are usually the same.
Sunshine measurements are not required of cooperative observers; their reports, however, include an estimate of cloudiness; less than one-third of cloudiness, clear; one-third to two-thirds, partly cloudy; and two-thirds or more, cloudy. A day with a sky full of broken clouds is necessarily recorded as cloudy; nevertheless the registered sunshine may be almost continuous. At times the cloud film of a sky completely overcast may be so thin that a sunshine recorder of any sort will register a considerable part of the day. On the other hand, a light dust haze may interfere materially with registration, although to the sense of sight, the light seems normal. The amount of sunshine, therefore, cannot be reckoned from a record of cloudiness. Even with recording instruments having the best possible adjustments, the record of sunshine for a given period is an approximate only; there is no such refinement in sunshine measurements as exists in measurements of pressure or of temperature.
Sunshine Recording Instruments.—The various sunshine recording instruments may be reduced to three types: the burning-glass type, such as the Campbell-Stokes recorder; the photographic type, such as the Jordan recorder, for many years used in the United States Weather Bureau; and the thermoelectric type; of which the Marvin recorder is practically the only one.
​ The Campbell-Stokes recorder consists of a sphere of colorless or slightly yellow glass of a high degree of transparency. It is mounted in a frame in front of a concave surface set at focal distance from the glass sphere. The central line of the recording chart must lie in the plane of the true meridian. The frame itself is adjustable to the sun’s altitude. The recording chart is graduated to hour intervals. The focal rays, shifting with the position of the sun, char a line along the chart. The charred line represents the duration of the sunshine. When the sunshine has occurred at short and irregular intervals the aggregate duaration may be found most quickly by placing the edge of a
Campbell-Stokes sunshine recorder.
sheet of paper along the record and marking thereon lengths equal to the lengths of the successive charrings. By sliding the paper along the trace, the lengths form a continuous line. Their aggregate then may be measured along the graduations of the paper.
​ The intensity of the charring varies according to the condition of the air as to cleanliness and freedom from moisture. If the air is clear and dry, the focal rays burn a long gap in the sheet. On the other hand, a moist or a polluted air may absorb
Sunshine differential thermometer, electrically connected with the recording apparatus.
so much heat that the surface of the recording chart is merely discolored. The record for the first half hour or more after sunrise is usually indistinct; this is usually true of the time just before sunset. If a cloud obscures the sun, even for a few minutes, the burning process is arrested.
The charts used in this type of sunshine recorder may be changed at any time between sunset and sunrise of the following ​day. Each chart, therefore, carries the record of a full daily period of sunshine.
The Campbell-Stokes recorder is simple in construction and inexpensive. It requires neither clockwork nor electrical mechanism in its operation. The records are unsightly, but they are ineffaceable and permanent. As a piece of mechanism it is practically fool-proof. In the laboratories of Europe its use is general.
Photographic recorders depend on the action of sunlight on sensitive paper. The record sheet is placed within a camera of circular section. A minute aperture permits a spot of light to enter the camera and fall on the record sheet. In one form there are two apertures, one for the period from sunrise till noon, the other from noon to sunset. The changing position of the sun causes the spot of light to traverse the record sheet in an opposite direction. After exposure the sheets are developed and fixed by ordinary photographic processes. Silver paper gives the most legible charts; and when a bit of blue glass is used as a light filter, the line of record is more sharply drawn and clearer. Silver paper is expensive, however, and ordinary blueprint paper is more commonly used.
The time and effort required to prepare the sensitive paper, and to develop and fix the record sheets is the great objection to photographic recorders. In some respects they are more accurate in time measurement than any other form; and in this particular they have possibilities not possessed by any other recorders.
The Marvin thermo-electric recorder is used at United States Weather Bureau stations and in most meteorological laboratories. It consists of a differential thermometer in a vacuum tube and a recording apparatus. The expansion of a volume of air in a blackened tube pushes a column of mercury between two platinum points, the ends of which pierce the tube, thereby making an electric circuit possible in the recording apparatus. The air volume within the blackened tube is exceeding sensitive to heat. Even in the coldest weather, the heat of direct sunshine is sufficient to push the mercury to the circuit-making points; with the absence of sunshine the mercury drops below them. Inclining the tube takes some of the weight off the air chamber and causes the mercury to be more easily lifted. In
Sunshine in the United States. Figures show the per cent of possible sunshine.
order to overcome the friction of the mercury against the glass, it is lubricated with alcohol. The upper part of the tube, which is not blackened, also contains air. A dextrous shaking of the tube will transfer bubbles qf air from one end to the other, thereby holding the column of mercury to any desired height, or to any desired distance from the circuit points.
The stand should be mounted in a locality that is not shaded. The tube should be in the plane of the true meridian, and face the south. The angle of inclination should be roughly about 45 degrees. In summer it should be a little nearer to the vertical; in winter a little more to the horizontal. The angle depends partly on the amount of air in the lower bulb and partly on the position that gives it the maximum of insolation. In general, the results are best when the top of the mercury is from half an inch to an inch above the circuit points during the warmest part of the day.
The recording device is attached to that of the anemometer, using the same sheet but a different recording pen. When the sun is shining, a contact of the clock completes the circuit. The movement of the armature operates a jigger, which moves the pen once every minute. The jigger and the progressive movement of the drum cause the pen to make steps in series of five, back and forth. Each series represents five minutes of time. These continue while the sun is shining. When the sun is not shining, the mercury in the thermometer drops away from the platinum circuit points. The pen then draws a straight line.
Even when carefully adjusted, the recorder will not begin to register for some time after sunrise; it ceases to register a short time before sunset. These periods must be measured from time to time by the observer, taking the time of sunrise and sunset from a reputable almanac for the approximate latitude of the station. These intervals are the twilight corrections.
The morning twilight correction is usually somewhat greater than that of evening. Both vary slightly between winter and summer. In localities where city smoke and floating dust do not contribute to instrumental sluggishness, the morning correction should not exceed one hour; the evening correction should not be more than half as much. The presence of smoke, ​dust and haze may extend this correction to more than one and one-half hours.
Measurement of Sunshine.—The adjustment for registration practised by the Weather Bureau is based on experience and is reasonable. The tube holding the thermometer should be inclined so that the recorder will register when the actual disk of the sun—not a shapeless blotch of light—can be discerned through the clouds of an overcast sky. It is better to make the adjustment when the sun is about two hours high, by inclining the tube. The observer must wait for such a day, and perhaps several trials may be necessary.
The computation of the total actual hours of sunshine may be made by any system which the observer finds convenient. If the total sunshine is not more than a few hours, it is perhaps most easily counted in the manner suggested in a previous paragraph—that is, measurement along the edge of a strip of paper. When the obscuration by cloudiness is slight, or is absent, the following method may be followed:
From the total possible hours for the day deduct:
The calculations may be made in detail on the back of the record sheet. On the face of the sheet these should be entered in the proper place:
By this, or by a similar method, the computation for the month is finished on the last day of the month. In Weather Bureau practise, minutes of time are reckoned in decimals of an hour, in the measurement of sunshine.
Sunshine records are approximate only; close calculation, however, will bring reasonably accurate results. At times ​the judgment of a careful observer may be more trustworthy than an imperfectly adjusted instrument. At times, too, there may be momentary periods of sunshine which are not registered at all. Even in the absence of all measuring instruments, an observer whose record consists merely of the total of overcast days is gathering information of great value.
Concrete Results of Sunshine Records.—It is well to bear in mind that the mere gathering and tabulating of monthly statistics of sunshine is not an end, but merely a means to an end. Knowledge of any sort possesses but little value unless it can be applied to the betterment of humanity. In agriculture the results may be applied so as to obtain more definite knowledge concerning the growth and maturity of plants—essentially the minimum amount of sunshine necessary to fructification. In almost every department of agriculture the total of sunshine has a direct bearing on the amount of evaporation.
In climatology, much more information concerning the relation of sunshine to public health is required. The healing value of sunlight is not overestimated; its value in therapy of the mind is underestimated. The mental depression following prolonged spells of overcast skies is marked.
In commerce and transportation, the effects of obscuration on visibility is becoming a matter of systematic study and investigation. The impairment of visibility costs more than money; its toll of human life is heavy.
In military and naval strategy, helio-signaling depends on sunshine; so also, sunshine is the key to many problems involving visibility.
The efforts of a single observer may not solve general problems, but they will go a long way in solving the specific cases of his own bailiwick. The observer may determine whether or not specific times of obscuration—daily or seasonal—prevail. In many parts of the country good beginnings have been made already by volunteer observers. Incidentally, there is no station, permanent or transient, from which additional information would not prove of value.
The area in which the per cent of sunshine is less than 40 is very small, and but little of it is crop-growing land. The region of greatest sunshine, for the greater part, is deficient in rainfall. Irrigated lands, however, produce crops that are ​extraordinary in quantity and unsurpassed in quality. The Lake region is below the average in sunshine, but the deficiency does not impair the quality of the fruit crop. The region of greatest cotton production receives from 60 to 70 per cent; and a comparison of this region with that of a lower per cent shows that the higher per cent is essential. Practically every part of the United States receives enough sunshine for a fair crop production, and a very great part receives enough for maximum production.
In the latitude of New Orleans, June days are about 14 hours long; in the latitude of Minneapolis they are nearly 16 hours. The aggregate warmth is about the same in each case. The oblique rays of the sun and their lower heating power are balanced by greater duration in time.