STUDY OF SOLAR RADIATION

AT THE SHIRA RESORT

A. A. VOROB'EV and D. N. POPOV

Introduction

The Sun is the principal source of the energy reaching the Earth from outer space. Beyond the limits of the atmosphere, in the form of radiant energy, the Sun delivers approximately 1.95 calories per minute to 1 sq. cm. This quantity is called the solar constant. The flux of radiant energy partly passes through the thickness of the atmosphere, undergoing some scattering and absorption by the atmosphere, and is partly reflected back into outer space. The part that reaches the Earth’s surface is determined by the formula

\[ I = I_0 p^m, \]

where \(I_0\) is the solar constant, \(p\) is the coefficient of transparency of the atmosphere, and \(m\) is the thickness of the atmospheric layer traversed by the ray (taking as unity the path length of a ray incident vertically).

The quantity of radiant energy delivered by the Sun to the Earth’s surface is called insolation, or radiation. Sometimes radiation is distinguished from insolation, the latter being understood as the quantity of heat falling on a horizontal surface in proportion to the sine of the Sun’s altitude.

The branch of science concerned with studies of the Sun’s radiant energy is commonly called actinometry. The beginning of strictly scientific actinometry was laid by O. D. Khvolson with his work carried out in Slutsk in 1890. However, actinometric measurements for a long time did not receive sufficiently broad development. Twenty-five years ago systematic measurements of solar radiation were carried out at only a few points on the globe. In the USSR a network of standard actinometric stations has been built.

This paper presents the results of investigations¹ of an actinometric station organized in 1940 by the Department of Physics of the Tomsk Polytechnic Institute named after S. M. Kirov, to support the heliotherapeutic activities of the solarium at the Shira resort. The results obtained may provide some material for the actinometric characterization of the microclimate of the Shira resort.

Apparatus and method of investigation

For measuring direct solar radiation we used an Ångström pyrheliometer, made in the workshop of the Department of Physics, and thermopiles with certificates. Scattered solar radiation was intended to be measured with photoelements. For counting cloudy and sunny hours there was—

a heliograph. In addition to these instruments, the actinometric station had barographs, a hygrograph, an anemometer, and various thermometers.

The thermopile was installed in the tube of a theodolite after the optics had been removed from the latter. This arrangement, first, made it possible to carry out measurements of the intensity of direct solar radiation quickly, with full assurance that the rays were falling normally on the thermopile, and, second, made it possible to determine the height of the sun above the horizon with great accuracy.

By comparing the readings of the thermopile and of Ångström’s compensation pyrheliometer, when both instruments were simultaneously irradiated by solar radiation, the pyrheliometer constant was determined. The basis for the determination was the certified values of the thermopile readings.

Measurements were carried out daily from sunrise to sunset from June 24 to July 14. Every 15 minutes the following were determined: 1) air temperature (in the shade), 2) ground temperature (at a depth of 5 cm), 3) wind speed and, if the sun was not obscured by clouds, the intensity of direct solar radiation.

Every three hours, at the meteorological station of the resort, the absolute humidity was determined with an August psychrometer.

The barographs and hygrographs provided a continuous record of barometric pressure and relative humidity.

The compensation pyrheliometer was used once or twice a day for periodic checking of the readings of the galvanometer connected in the thermopile circuit. The measurements obtained under simultaneous irradiation of the pyrheliometer and the thermopile agreed within the limits of observational error.

It was not possible to organize systematic measurements of diffuse solar radiation because of the insufficient sensitivity of the available galvanometers.

Variation of the Intensity of Direct Solar Radiation Depending on Various Factors

The amount of solar radiant energy reaching the earth’s surface depends: 1) on the radiating capacity of the sun, i.e., on the above-mentioned solar constant; 2) on changes in the distance between the earth and the sun; 3) on changes in the transparency of the atmosphere; 4) on the angle of incidence of the radiation.

The values of the solar constant found by various investigators are in close agreement. However, the value of 1.95 cal/cm² min mentioned by us in the introduction should be regarded only as the mean value of a quantity that undergoes certain fluctuations. The causes that produce fluctuations of the solar constant have not yet been precisely established. One of the causes—the presence of sunspots—can produce a decrease of the solar constant by as much as 5%. The combination of the remaining causes produces much smaller fluctuations.

The change in the distance between the earth and the sun leads to a periodic (annual) change in the solar constant, which can always be computed in advance. If the value of the solar constant for the mean distance is taken as unity, then for January 1 solar radiation decreases by 3.4%, and for July 1 it increases by 3.5%.

The transparency of the atmosphere depends on the length of the path traversed by the rays in the atmosphere; more precisely, on the “mass” of gases through which the rays have passed, on the humidity of the atmosphere, and on the contamination of the atmosphere by dust. The “mass” of the atmosphere traversed by a solar ray is determined mainly by the hei-

of the sun above the horizon and, to some extent, barometric pressure. The relation between the altitude of the sun and the path of the ray in the atmosphere, from purely geometrical considerations, may be expressed by Table 1.

Table 1

The longer the path of the ray in the atmosphere, the greater the percentage of the passing radiant energy that is absorbed or scattered.

Water vapor absorbs chiefly the infrared part of solar radiation. The dependence of the intensity of absorption of solar radiation on the amount of water vapor has been investigated many times and is well established. According to observations obtained in Slutsk, for a solar altitude of 30° the dependence between the intensity of solar radiation that has passed through the atmosphere and the absolute humidity is presented in Table 2.

Table 2

For one and the same altitude of the sun, changes in the amount of water vapor present in the atmosphere can greatly change the amount of radiant energy reaching the earth’s surface. The influence of all the other factors capable of causing fluctuations in the radiation reaching us is less substantial.

Thus, the diurnal course of changes in solar radiation should be determined mainly by the length of the rays’ path in the atmosphere and by changes in absolute humidity.

Observations of the diurnal course of change in direct solar radiation

The diurnal course of change in solar radiation for 24 June is shown in Fig. 1. The decrease in the smooth course of the curve after noon is explained by the accumulation, in the lake basin of the resort, of water vapor brought by the wind toward evening. This explanation is fully confirmed by direct measurements of absolute humidity (at 12 o’clock—5.1 and at 15 o’clock—7.8). As is seen from the graph, direct solar radiation, with the rising of the sun above the horizon, increases rapidly and already by 7 o’clock exceeds 1 cal/cm²·min. The maximum radiation value of 1.2 cal is reached at noon.

The graphs in Figs. 2, 3, and 4 also illustrate the diurnal changes in solar radiation. Fig. 2 is for a calm, windless day, 28 June, cloudy in the first half of the day. Here the microclimatic peculiarity stands out very sharply—

the feature of the Shira resort, consisting in the fact that the transition from almost continuous dense cloud cover to completely clear weather occurs abruptly. From Figs. 2 and 8, on the same grounds as from Fig. 1, one may conclude

Fig. 1

that in the lake basin of the resort, under calm weather, water vapor readily accumulates.

The sharpness of the transition from clear weather to dense cloudiness and even rain is vividly shown in Figs. 3 and 5. On June 27 rain began 15

Fig. 2

minutes after the sky had been almost free of clouds for 4 hours. On July 3 rain began 20 minutes after the appearance of the first cloudiness.

Fig. 5 gives the characteristics of the diurnal course of solar radiation for June 30, a day with rare convective cloudiness. There were a majority of such days during the period of operation of the actinometric station. Only three days—June 24, July 10 and 11—were completely cloudless. During 5 days

Fig. 3

Fig. 4

Fig. 5

continuous cloudiness was observed; Fig. 6 illustrates one such day.

According to the heliograph readings, out of the 315 hours during which observations were made, for 139 hours, i.e. 44%, the sun was covered by clouds. According to the testimony of permanent local residents, such cloudy weather at this time of year is unusual. It follows from this that a complete characterization of the percentage number of hours of sunshine could be obtained only as a result of many years of systematic observations.

Fig. 6

Fig. 7

The maximum value of solar radiation, \(1.24\ \mathrm{cal}/\mathrm{cm}^2\mathrm{min}\), at normal incidence on the surface, was observed on an almost cloudless, slightly windy day, July 9, 1941 (Fig. 7). The height of the sun above the horizon was \(50^\circ 40'\), the absolute humidity 4.3. For almost the same solar altitude in Slutsk on July 1, over many years of observations, the max-

maximum intensity of solar radiation of \(1.31\ \mathrm{cal}/\mathrm{cm}^{2}\mathrm{min}\). The value of absolute humidity, unfortunately, is not indicated. It should be noted that the geographic latitude of the two points differs little. For Slutsk—\(53^\circ\), for the Shira resort—\(54^\circ 41'\); the agreement is quite close. It is possible that, with many years’ duration of observations,

Fig. 8

at the resort in July a maximum value may be obtained that somewhat exceeds the observed value of \(1.24\ \mathrm{cal}/\mathrm{cm}^{2}\mathrm{min}\).

During the time over which the observations were conducted, on sufficiently clear days, for times close to noon, the solar radiation most often had values in the interval \(1.12\)—\(1.22\ \mathrm{cal}/\mathrm{cm}^{2}\mathrm{min}\). The absolute humidity of the air was 4.8—7.5.

Conclusions

The experience of the actinometric station, despite the short duration of the observations, makes it possible to draw several conclusions.

1. The microclimate of the Shira resort is characterized above all by extremely abrupt changes in the weather. The transition from sunny weather to rainy weather and back occurs suddenly, within very short intervals of time. Equally abrupt and sudden are the transitions from calm to windy weather.

2. In sunny, calm weather, excess moisture readily accumulates in the lake basin of the resort, reducing the intensity of the incoming solar radiation.

3. Direct solar radiation in June and July at the Shira resort, over a long interval of time (from 8 to 16 hours), exceeds \(1\ \mathrm{cal}/\mathrm{cm}^{2}\mathrm{min}\) and reaches a maximum of \(1.24\ \mathrm{cal}/\mathrm{cm}^{2}\mathrm{min}\). It should be noted that at the Crimean resorts radiation values of the same order are observed.

In conclusion, we consider it a pleasant duty to express our deep gratitude to the scientific director of the Shira resort, Professor K. N. Zavadovsky, for assistance in organizing the work, and to Docent D. D. Saratovkin for assistance in processing the results obtained.

1. The measurements were carried out by D. N. Popov. ↩