METEOROLOGY, that branch of sci ence which observes, registers, classifies, and compares the various and varying phenomena of our atmosphere. It re marks, at the same time, the connection of those phenomena with heavenly bodies, and with the solid and liquid materials of the earth, in reference to their recip rocal and combined influence in deter mining the character of different climates, and with the view of learning the mete oric history of every region of our globe, of ultimately investigating the laws of atmospheric change and the plan of mete oric action; the theory, in fact, of mete orological phenomena, on which depends essentially the fitness of the various por tions of the earth's surface of the pro duction of different vegetable and other substances, and for the support of animal life.
It was not till the discovery of the barometer, in 1643, that the first great step was made toward a knowledge of the nature of our atmosphere. We were then, by its help, enabled to ascertain the weight and pressure of the great aerial ocean which surrounds us, and to learn when and where it was in a state of calm or storm. The invention of the ther mometer, shortly afterward, gave the means of determining its temperature. The hygrometer for showing the amount of moisture it contained, and the ane mometer for giving the direction and force of the wind, are also instruments of great importance to the meteorologist. The indications of these instruments, combined with the careful observation of atmospheric appearances, interpreted by the results of former observations, will enable the individual observer generally to predict the kind of weather that may be expected in his immediate locality for a day or sometimes longer in advance. A strip of seaweed forms a very useful hygrometer for practical purposes, pro vided it be not kept in a room warmed artificially. In fine weather it will keep dry and have a somewhat dusty feeling, but with an increase of moisture in the air will become limp and sticky, indicat ing a probable change of weather in the shape of rain.
Since the discovery of the barometer, the science of weather forecagting has made much progress in its details, but, for the individual observer, the method remains much the same now as before. The principal rule in use for forecasting the weather at present may be briefly stated as follows: A rising barometer usually foretells less wind or rain. and a falling barometer more wind or rain, or both; a high barometer fine weather, and a low one the contrary. If the barome ter has been about its ordinary height at the sea-level, and is steady or rising, while the thermometer falls and the air becomes drier, N. W., N, or N. E. wind, or less wind, may be expected; and, on the contrary, if a fall takes place with rising thermometer and increasinc dampness, wind and rain may be looked for from the S. E., S., or S. W.; a fall of
the barometer, with low thermometer, foretells snow, with the barometer below its ordinary height a rise foretells less wind, or change in the direction toward the N., or less wet; but when the ba rometer has been low, the first rising usually precedes strong wind or heavy squalls from the N, W., N. or N. E., and continued rising foretells improving weather. If the barometer falls and warmth continues, the wind will prob ably back, and more S., or S. W. winds will follow.
At the present day, by the help of the telegraph, the meteorologist can obtain from as many stations as he desires the height of the barometer, direction and force of wind, etc., data which will in form him of the condition and move ments of the aerial ocean at a definite time. He then marks on a map the height of the barometer at each place, and, drawing lines through all the places where the quicksilver stands at the same height, at any convenient interval he ob tains a series of lines of equal pressure or weight, called shortly isobars, which show the height or depression at those places as the contour lines on a map show the different altitudes of the moun tains and valleys. The thermometer readings, treated in the same way, are called isotherms. To make these syn optic charts (as they are called) com plete, the force and direction of the wind, the amount of humidity, character of clouds, and other weather signs are also marked down, so that the chart may fur nish a view of the weather at that par ticular time over the area from which reports have been obtained. Supposing now that at the same time the next day a new set of data are received and marked on another chart, a comparison of the two will show the nature and di rection of the change going on, and en able the meteorologist to predict, to a certain extent, what will be the immedi ately coming weather. This is a general description of the way in which the fore casts of weather, printed in the daily papers, are made.
The World War brought about great activities in meteorological work, espe cially in relation to the atmosphere of aeronautics. Meteorological work was added to the routine activities of armies and navies with the duty of studying the atmosphere and the practical applica Vol.
tion of the results to aeronautics. There was established in July, 1919, in Brus sels, an International Union of Geodesy and Geophysics, one section of which is devoted to meteorology. New meteoro logical instruments include an improved pole star recorder, used for registering cloudiness of night. An improved snow sampler was also devised, as well as a new continuous recorder of atmospheric pollution. See WEATHER BUREAU.