The importance of sunshine among the elements of weather and climate is evidenced by the fact that at least two States of the Union, South Dakota and California, contend for the title of “the Sunshine State”—which does not properly belong to either of them. Arizona is the sunniest State of all, and the whole Southwest is sunnier than South Dakota.
Devices for registering the duration of sunshine are called sunshine recorders. One type (the Campbell-Stokes) works on the burning-glass principle; in others the sun’s rays trace a record on photographic paper. The instrument used by the Weather Bureau consists of an air thermometer having a bulb at each end, one bulb being coated with lampblack. There is a small column of mercury between the two inclosed masses of air. The thermometer is inclosed in a sheath of glass, from which the air is exhausted. When the sun shines on this instrument, the air in the black bulb warms and expands, and the mercury is forced toward the other bulb until it comes in contact with a pair of electrodes, thus closing an electrical circuit. While the circuit is closed, the registering apparatus connected with the instrument makes a step-shaped mark once every minute. When the sun stops shining, the mercury drops back, the circuit is broken, and the recording pen merely traces a straight line.
ELECTRICAL SUNSHINE RECORDER
At the larger stations of the United States Weather Bureau the direction and speed of the wind, the rainfall and the duration of sunshine are all recorded on a single sheet of paper, wound around a large cylinder, which is turned by clockwork. The paper is ruled with lines to denote the hours and minutes of the day, and a fresh sheet is put on the cylinder every day at noon. This complex registering device, sometimes called in book language a meteorograph, but colloquially referred to by weather men as the “triple register,” is entitled to high rank among labor-saving machines; for, with hardly any attention, except for a few minutes at noon, it does the work of a staff of trained meteorologists on duty day and night.
BRITISH TYPE OF SUNSHINE RECORDER
(Campbell-Stokes Pattern)
We have now enumerated the elements of weather most commonly observed at meteorological stations, and the principal types of meteorological instruments, with special reference to those used in the United States. In nearly every civilized country there are certain stations at which regular observations are maintained of a number of phenomena not mentioned in the foregoing paragraphs, such as the intensity of solar radiation (measured with the pyrheliometer), evaporation (measured with atmometers or evaporimeters), and the temperature of the soil; and the number of stations is rapidly growing at which the winds and weather far aloft in the atmosphere are observed by means of kites and balloons. Meteorologists of the Old World use a great many types of apparatus that are rarely seen in this country, and some of our instruments are but little known abroad.