Meteorology : comprising a description of the atmosphere and its phenomena, the laws of climate in general, and especially the climatic features peculiar to the region of the United States; with some remarks upon the climates of the ancient world, as based on fossil geology / by Samuel Forry.
- Forry, Samuel, 1811-1844.
- Date:
- [1843]
Licence: Public Domain Mark
Credit: Meteorology : comprising a description of the atmosphere and its phenomena, the laws of climate in general, and especially the climatic features peculiar to the region of the United States; with some remarks upon the climates of the ancient world, as based on fossil geology / by Samuel Forry. Source: Wellcome Collection.
Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)
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![of rain diminishes from the equator to the poles, is a general law ; but upward of 300 inches of rain, according to the British army statistics, the quantity varies so much, from local or other causes, that the ex- have frequently fallen during the wet season alone. Upon the sea, ceptions are more numerous than the instances of the correctness of the a less quantity falls than upon the land, as the former has no elevations rule. A much larger quantity, however, must fall in the equatorial than around which clouds may be attracted. As the augmented quantity in in the polar regions, of which evidence is afforded in the magnitude of tropical countries falls at a particular season and in a shorter space of the rivers within the tropics; lor as rivers are the conduits along which time than in colder regions, the annua) number of dry da>s is propor- a certain portion of the precipitated water is transported to the sea, their tionally increased. Hence the LiHmidiiy of any climate cannot be size may be regarded as an index of the mean annual quantity of rain, inferred from the annual quantity of rain, inasmuch as the number of The following table as given by Prout, notwithstanding the progression rainy days is generally least where the fall of rain is greatest. A region exhibits great irregularity, fully establishes the general law of the de- remarkable for a low annual ratio of rain, may be involved in almost crease of rain with the increase of distance from the equator:— Inches. 13.5 - 16,17.5 19-9 *20.7, f22.2, ±25.2 Uleaborg, Petersburg, Paris, London, Edinburgh, 22 , 24.5, §26.4 Mean of Carlsruhe, Manheim, Stuttgart, Wurtzburg, Augsburg, and Regensburg, (Schow) - - 25.1 Epping, ...... 27,0 Bristol, ...... 29.2 England, (Dalton's mean.) ... 31.3 Liverpool, ...... 34.1 Manchester, - - - - - 36.1 Rome, - - - - - - 39 0 Lancaster, ..... 39.7 Geneva, ...... 42.6 Penzance, *----- 44.7 Kendal, ------ 53.9 Mean of 20 places in the lower valleys at the base of the Alps, - - - Great St. Bernard, - Vera Cruz, - Keswick, Calcutta, Bombay, Ceylon, Adam's Peak, - Coast of Malabar, Leogane, St. Domingo, 58 5 63.1 63.8 67.5 81.0 82.0 84.3 100.0 1235 11150.0 The annual quantity of rain, it thus appears, diminishes with the lati- perpetual fogs, or have many days of drizzling rain. i On seacoasts, rain falls more abundantly than in inland localities, , notwithstanding they may both have the same physical features. This I has been explained on the ground that the atmosphere in the vicinity of : the sea has greater humidity ; and hence it is more liable to have its i moisture precipitated. The quantity of rain is also much influenced by the vicinity «f moun- j tains. Mountains, says Higgins, when acted upon by the sun, J heat the air which is in contact with tin mi, even in the cold regions of ; the upper atmospheric strata. These heated masses of air absorb the i moisture from the colder*/iolumns around ; but, meeting with humid j masses of lower temperature, or cooled by the constant abstraction of their heat, the humidity becomes too great for the U mperature, and 1 rain is produced. The difference between plains and mountainous countries is so great, that, at'Paris, the annual quantity is only twenty inches, while at Geneva, it is 42^ inches, and on the Great St. Bernard, j the highest meteorological station in Europe, it is 63 inches. Again, in I the Julian Alps, the annaai fall is estimated at 100 inches, while in the valley of Lombardy, it does not exceed 35 inches ; and so of Demerara, in the swamps of Guiana, and of the lofty island of Granada, the for- mer being 97 and the latter 126 inches. In most tropical countries, rain falls only at particular periods of the year. At Bombay, for example, June, July, August, September, and October, are the rainy months, an unclouded sky being presented the rest of the year ; but on the opposite side of Hindostan, on the Coro- mandel coast, the period of the rainy season is reversed,—a result attri- buted to the high intervening table-land, which is supposed to influence the atmospheric currents. Wherever the atmosphere experiences a periodical change in th« direction of its currents, periodical rains occur. During the steady prevalence of the trade-winds, rain is a very uncom- mon occurrence ; for, on account of the uniformity of temperature, no tude ; but there are powerful local modifying causes, which will be J condensation can take place, the aqueous vapor being carried upward presently brought under notice. ; and steadily moved onward. But no sooner do these great currents, It has been computed by Sir John Leslie, that if all the aqueous vapor following the course of the sun, commence to shift their direction, than which the whole atmosphere is capable at any time of holding in solu- heavy rains begin to fall; for as currents of different temperatuics now tion, were precipitated on the earth at once in the form of rain, it would become intermingled, condensations of moisture, commensurate with not exceed five inches in depth. Now, as the mean annual fall of rain the high temperature, are produced. It is during the shifting of the ever the globe is not less than seven times this quantity, it follows that monsoons, that the heavy rains of India fall. Even in the temperate it must be replenished as many times by evaporation. An important climate of England, it has been long observed that a dry season is question is presented in the relative mean evaporation and condensa- always accompanied by a wind of remarkable uniformity, while a tion. As regards the mean annual evaporation, some interesting ex- variable and an unsteady motion of the atmosphere as constantly periments were performed by Dr. Dalton and Mr. Hoyle, at Manchester,! attends a wel season. In tropical climates, the phennmenon of a pen- England, in 1796, 1797, and 1798. A cylindrical vessel of tinned iron, odical descent of rain is even often produced duily by the land and sea three feet deep and ten inches in diameter, having one pipe near the breezes While masses of water which afford a large evaporatinsr sur- bottom and another one inch from the top, was filled with soil; and in face, cause a great abundance of rain in certain regions j there are, on this condition it was left for a year, when it had become covered with the other hand, as already remarked, certain situations in which there grass. Bottles being now attached to the end of each pipe, the surplus is an absence of all prec *. in the Sahara of Africa, on the water which soaked through the earth, and that which ran off at the low coast of Egypt, and a portion of the coast of Peru. As a uniform upper pipe, wene both collected. On commencing the experiment, the wind must produce constant precipitation or no rain at all, the perma- soil was soaked with water. A rain-gauge of dimensions similar to that nency of the result will depend on the permanency of the cause. As of the cylinder being placed near it, the amount of evaporation was the prevailing wind, for example, on that part of the coast of Peru just estimated by subtracting the quantity which passed into the bottles from adverted to, passes from a colder into a warmer region, there can be the whole rain. Thus in— j no precipitation, because its capacity for moisture continually aug- 1796. 1797. 1798. ments. Rain .... 30.629 inches. 38 791 inches. 31 259 inches. | As regards the seasons, the greatest amount of rain Tails when the Evaporation. . 23.725 27.857 23.862 ] mean monthly temperature is highest; but, although the quantity is giving 25.148 inches for the mean annual evaporation, and 33.559 inches greater in summer than in winter, the latter exhibits the greater num- for the annual condensation, at Manchester ; but to this, in the opinion ber of rainy days. Rain falls in greater abundance during the day than of Dr. 1 ialton, there ought to be added five inches for the annual dew, the night. thus making the I otal annual evaporation about thirty inches. As the The drops of falling rain vary in size from l-25th to l-3rd of an inch in mean annual evaporation is generally estimated at thirty-four inches, diameter : and their ultimate velocity, it has been calculated, is in the the whole mass of water raised by this process from the surface of the duplicate ratio of their diameters. To ascertain the quantity of rain earth is equal to 105,614 cubic miles, which is the estimated quantity that falls in any place, various kinds of instruments, known by the annually precipitated on our globe. These calculations, however, are names of udometers, ombrometers or rain-gauges, have been employed, nothing more than rude approximations. In different countries, al- It is not, however, deemed necessary to give a description of them. In though the relative proportion of water evaporated, and of the water regard to the proper position of a rain-gauge, some practical difficulties condensed, must necessarily vary exceedingly, yet it is probable that are presented, in consequence of the fact now repeatedly verified, that in the same country, these proportions exhibit little variation. Asa the mean annual quantity of rain is less in proportion as the receiving large proportion of water condensed on the land, must have been evapo- vessel is elevated above the surface of the earth. This result, as it fol- rated from neighboring seas, it follows that in countries which dis- lows uniformly, cannot be referred to accidental circumstances. The charge superfluous waters into the ocean, condensation must exceed experiments made at the Royal Observatory, at Paris, during a period of evaporation. fourteen successive years, as well as at Yorkshire, England, estab!i«hes There are many causes which exert a powerful influence upon the! the fact beyond all doubt. The mean annual quantity falling at Paris, annual fall of rain ; such as, its position in relation to the equator, its: jn the court of the Observatory, is 56 centimetres, while at an eleva- proximity to the sea and its elevation above its level, as well as the ex- ' ti«n 0f 28 metres above this point, on the roof of the building, the mean posure of the place, and the mountains, woods, i\rc. in the vicinity. | quantity is only 50 centimetres. It was satisfactorily ascertained that It has been already shown that the mean animal quantity decreases even a difference of only five or six feet, affected the annual result in a from the equator to the poles. _ The following table, according to Hum- sensible degree. These differences, which are fully confirmed by the experiments at Yorkshire, may be attributed, in a great measure, to the circumstance that each drop of rain, in its passage through the atmo- sphere, augments, on the principle of condensation, in proportion as it approaches the surface of the earth. boldt. exhibits the proportional quantity of rain in different latitudes Latitude. Mean annual quantity of rain. 0° 96 inches. 19 80 45 29 69 17 Although in the table given above we find the annual quantity, from Ulea- borg, Lapland, to St. Domingo, varies from 13 5-10 to 150 imches, yet there is no regular average throughout a parallel. While Do. some places it seldom or never rams, as in the Great Desert of Africa, and on the arid shores of Peru between the 15° and 30° of lat.; there are, on It is necessary to determine not only the mean annua/, but also the mean ntontlilt/ quantities of rain, inasmuch as the latter results have a more direct relation with vegetation. The directions at our military posts, al all of which the conical rain-gauge is used, are as follows:—'It will be kept remote from all elevated structures, to a distance at least equal to their height, and still further off, where it can be conveniently done. It is nded in the other hand, regions in which rain is al nost cons'antly falling. mj|a circular opening made in a board, which is to be fixed to a post, eight the British Possessions on the western coast of Africa, for example, jfeot from the ground ; the openinc iches diameter, and bev- * Dalton. + Daniell. t Howard. <i> Adic. 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