WEATHER, the condition of the atmos phere, at any time m respect to heat, moisture, wind, rain, cloud and electricity; a change of weather implies a change in one or more of these elements. For the possibility and mode of making real predictions of the weather, see Smelt; for the system of weather forecast, see Mermeotoce.
The great changes of solar heating attend ing the alteration of day and night, and the annual march of the seasons, produce well marked and predictable changes of weather in all parts of the world. There are indeed many tropical and sub-tropical regions of the earth where there is so much of regularity in several or all of the weather elements that the experi ence of preceding years forms a trustworthy guide by means of which the weather may usu ally be closely forecasted for a considerable time in advance. For example, the daily march of the barometric pressure at many tropical stations is so regular that the time of day can be told pretty closely by reading the barometer. But on the other hand, large irregular fluctuations of barometric pressure, rainfall and temper ature prevail over a great portion of the earth, and in these regions there is as yet no sound system of local weather prediction for more than two days in advance.
The proximity of western Europe to the Atlantic makes it impossible in that region to predict the weather beyond a day or two at the utmost, except in the case of considerable storms leaving the American coast and lffrely, if not certain, to strike widely on the coast of Europe. In Norway and the Baltic. and places toward eastern Europe, the weather may be predicted for a longer time, owing to the more easterly situation. In America also where storms advance chiefly from west to east, gales and unsettled weather are predicted for on the seaboard in the east some days sites ea before though the course that a storm may take, whether more to the north or to the south, maim be certainly foretold.
A forecast, conjectural indeed. yet often useful. may be made by observing the barome ter, the winds and the face of the sky—espe cially the cirrus cloud, most elevated and deli cate of the clouds; but some previous knowl edge of the general features of storms (q.v.) n indispensable: these specially — (I) Storms have a circular area, and (2) advance in an eawsrly direction, bearing a low barometric with them. (3) Winds blow from a to a low barometer—the observer, stand ing with his back to the wind, having always the low barometer to his left in the northern hemisphere —and (4) with a force propor booed to the difference of the pressure, or to the steepness of the barometric gradient. (5) Storms are noticed first in the upper regions of the atmosphere, or in the region of the Cirrus timid. (6) In front of the storm the air is wants and humid; in the rear of it, cold, or cod and dry. With such observations, requir wg only a barometer intelligently interpreted, parucalarly if hills form part of the landscape, the character of the weather may be foreseen for one day, or even on occasions longer. In enteral a falling barometer is not followed certainly by storm; but a rising barometer in had weather indicates the passing away of the storm though, in the colder season, possibly sot without a change to higher winds and awe..
To the agriculturist and horticulturist, not high winds but hails, heavy rains, frosts and fine weather are what are required to be 'mown. Such forecasts were begun in the United States by General Albert J. Myer; also ai France by Leverrier shortly before his death; and they are gradually being introduced vier the continent of Europe.
The weather prognostics of every language embrace much that is shrewd and of consider able value, but more that is vague and absurd. The changes of the moon long were, and in many minds still are, regarded as supplying the elements of prediction; but when brought to the test of accurate examination and figures, the supposed influence of the moon's changes on the coming weather is found to be a delu non This was shown especially by an exami nation of 50 years' observations at Greenwich, and similar old records of the weather. For Thomas du Boulay predicted the character of the weather of each summer the weather conditions which prevailed (hiring the of the spring equinox preced ing. supposing that the general character of the 'leather of t r. next six months is then settled, requiring only skillful reading. For a few years he speculated in grain on the faith of these predictions, which turned out correct on the whole; hut later his predictions did not come true. Many persons forget that a great ember of indiscriminately chosen instances are required for the verification of any theory weather forecasting; and that more than half of these, when impartially considered, mast be favorable to the soundness of the theory, eke the result is attributable to chance alone Within the past few years progress has been made in two beanches of experimental work, which may soon lead to marked advance in the line of forecasting. In the first place, the exploration of the upper air by means of kites and captive balloons carrying registering meteorological apparatus is yieldaqg much knowledge of the distribution of atmospheric temperatures, currents and electrical conditions, which can hardly fail to promote the accuracy of storm forecasts.
In the second place, the work of the Smith sonian Astrophysical Observatory, begun under the direction of the late Secretary S. P. Lang ley, has tended strongly to show that the radia tioq of the sun (on which, directly or indirectly, all weather elements depend) is not constant, but varies often through a range of 10 or even 15 per cent in a few months. The connection between the temperature of the earth and the radiation it absorbs from the sun is such that a permanent decrease of 10 per cent in the latter would produce about 7' Centigrade F.) fall of the earth's mean temperature. But owing to the short period of the observed solar changes, and to the great capacity for heat of the oceans, the actual effect of the changes of solar radiation would generally be much less than this. Stations near the oceans will be only slightly affected, while stations in the interior of the great continents are apt to feel more nearly the full effect above estimated. On account of the secondary effects of changes of solar radiation upon cloudiness, and the reaction of cloudiness on temperature, the tem peratures of different stations will be differently influenced. Owing to the immense sue of the sun a considerable time must elapse before con r iutt (igloos affectin its radiation can reverse, and thus it seems * y probable that when the exact connection ween the changes of solar radiation and the weather has been determined, it will be possible to predict the prevailing char acteristics of a season, whether cold or hot, wet or dry, for some weeks, and perhaps one or two months in advance. See CLIMATIL ; Itrnoxpouicv; Raturau