1902 Encyclopedia > Rainband

Rainband




RAINBAND. Every transparent substance is perfectly opaque to some particular kinds of light. A certain shade of orange light. is absorbed by the vapour of water, and, when sunlight which has traversed a stratum containing this vapour is decomposed in a spectroscope, the blank caused by the missing rays appears as a black band or group of fine lines. This is called the rainband, because from its intensity the amount of moisture in the atmo-sphere may be guessed at, and the occurrence of rain predicted with considerable certainty. It has long been known that the spectrum of sunlight shows lines of telluric as well as of solar origin. The former constitute the absorption-spectrum of the atmosphere (see SPECTROSCOPY) ; some are produced by the permanent gases, others by aqueous vapour, which is always present, though in variable amount. The absorption-spectrum of water-vapour has been minutely studied and carefully mapped by Janssen and by Cornu. In Angstrom's table of normal solar spectra there are numerous groups of lines which appeared most conspicuous when the sun was close to the horizon, and many of these are coincident with the absorp-tion-lines of water-vapour. They are found principally in the red and yellow, and the main group is seen a little to the red side of the D line. In small spectroscopes the water-vapour lines appear fused together into a band on the red side of D, or even as a mere widening of that line. There are several variable bands in the spectrum which come to a maximum of intensity when the sun is on the horizon. These are due to absorption by the permanent gases; the one which in a small instrument appears to separate the yellow from the green is frequently mistaken for the water-vapour band, and this is the cause of many incorrect " rainband predictions." In 1872 Professor Piazzi Smyth noticed a change in the water-vapour lines before and after a sirocco in Palermo, and the same phenomenon was brought before him very strikingly in France before great rains in 1875 (Edin. Ast. Obs., vol. xiv.), when he named the main group of water-vapour lines the " rainband."





The rainband may be observed with any spectroscope, but direct-vision instruments of medium size are most convenient. It is im-portant that the spectroscope should have as great dispersion as possible and good definition, especially at the red end. To make an observation the slit, which must be kept perfectly clean, should be narrowed down until the spectral lines are sharp and clear. The instrument should be carefully focused to get the maximum absorp-tion effect; but all observers lay stress on noting whether the intensity of the band decreases rapidly or gradually as the altitude is increased. When a dark band is observed at the horizon and at the zenith, heavy rain is almost certain to follow immediately. It is of little importance whether the sky be clear or covered with high clouds at the point of observation; low clouds or haze make the result untrustworthy by shortening the line of sight, thus reducing the strength of the band and equalizing it in all directions. The utility of the spectroscope in meteorology depends on its power of investigating the hygrometrical conditions of the whole slice of atmosphere looked through, and it affords a means of ascertaining the difference of humidity in different directions. The hygrometer only indicates the state, as regards moisture, of the few yards of air surrounding it. The great difficulty in the way of obtaining accurate results with the rainband spectroscope has been the mental scale of comparison employed by most observers. Although some have found it easy to estimate the intensity of the band from 0 to 20, it is beyond the power of many to describe it in figures even from 0 to 5. Professor Piazzi Smyth noted its strength relatively to that of the dry-air band between the green and yellow. The fixed solar lines E, b, and F have been used as a closer approxi-mation to a scale in instruments not powerful enough to separate the rainband from the D line. This compound line appears to vary from something less intense than E in very dry weather to something considerably darker than F when rain is imminent. Seven degrees can be discerned, and these may be represented by using the sign = to mean "of equal intensity with," > to mean "darker than," and < "less dark than," as, <E, = E,>E<b, = b, > b < F, = F, and > F. When the thin solar lines in the green are seen very distinctly there is less probability of rain falling than when they are indistinct or invisible.

The following table gives an idea of the intensities corresponding to rain-probabilities at Edinburgh.

== TABLE ==





It appears that the average percentage of fulfilments of predictions of "rain" and "no rain," made for a period of twelve hours after one observation in the morning, which may be expected in Scotland is about 75. In less variable climates, such as those of the south of Europe and parts of the United States, a much higher degree of accuracy has been attained. The precise strength of rainband which corresponds to the probability of a fall of rain within a definite time depends on the temperature and also on the place of observation ; in every case it must be determined by the observer for himself. Very dark rainbands are found to precede rain in, more than 95 per cent, of the cases everywhere ; entire, or almost entire, absence of the band presages a dry day with equal prob-ability. With a spectroscope powerful enough to split 1) a mental numerical scale must be used in default of a suitable micrometer, and by practice the observer will be able to draw up a table for its conversion into probabilities of rain.

The production of an artificial absorption-line the intensity of which could be varied by known degrees suggested itself to more than one observer as the principle for a rainband-mierometer. A wedge of didymium glass comes very near success in this direction, but the absorption-line is awkwardly situated. Professor Cook of Dartmouth College, the leading meteorological spectroscopist in the United States, has constructed a micrometer for a spectroscope of sufficient power to separate the rainband into lines. A silk fibre is fixed to a frame capable of being moved to and fro in the tube of the spectroscope by a micrometer screw. When brought into focus the fibre appears as two sharp lines, which become fainter and wider as it is withdrawn. When the lines appear of equal intensity with the most prominent line of the rainband (a of the I) group in Janssen's map) the micrometer is read ; forty shades of intensity may be indicated by it. The unit proposed for graduating such micrometers on a uniform scale is the intensity of the faintest and least refrangible of a group of three lines in the red (w. I. 6207) of the spectrum produced by a 1 centimetre column of the gas given off by heated lead nitrate (mixture of nitrogen peroxide and oxygen) at 25° C. and 760 mm. The spectroscope when reinforced by this micrometer has been found to indicate with unfailing accu-racy the existence in certain parts of the sky of banks of invisible cloud, which become visible when a fall of temperature and other necessary conditions allow the vapour to condense. In settled weather such masses of vapour are not to be found, the micrometer readings being the same in all directions at the same altitude. Sometimes the rainband grows gradually darker for several days before a period of steady and long-continued rain, while sudden violent showers may give very short notice.

The spectroscopic history of a thunderstorm observed on 9th June 1884 by Professor Cook is extremely interesting. During its approach the water-vapour line observed at 10° north of the zenith towards the storm was darker by ten degrees of the scale than that observed 10° south of the zenith, although to the eye the clouds presented exactly the same appearance at both places. On this occasion the strength of the line varied as follows :—

== TABLE ==

Some relation has been traced between the variations of the rain-
band and the appearance of aurorse, but this matter is not yet
fully investigated. (H. B. M.)


The above article was written by: H. R. Mill, B.Sc.



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