Climate (Gr. Fr. climat), the condition of any portion of the earth's surface in respect to temperature, moisture, and other atmospheric phenomena. The Greek original (from , to slope or incline) was employed to signify the obliquity of the sphere, to which the inequality of the length of the day was due, the word primarily signifying an inclined plane or slope; hence applied to a belt or zone of the earth's surface differing, for the reason above given, from another or contiguous belt in the length of its day. The geographer Ptolemy was the first to establish these belts and to define them as climates; dividing the surface from the equator to the 58th parallel into 25 such climates, differing a quarter of an hour each in the length of the longest day; next from the 58th to the 63d parallel making four climates of one half hour's difference each, and from the 63d to the 66th parallel three climates of one hour each. From the polar circle to the pole there were six climates of one month's difference each. At the equator the first belt was 4° 15' in width, diminishing gradually till at the 45th parallel it was 1° 50' in width, and at the 57th parallel 30' only. This irregular and fanciful division long remained, in consequence of the difficulty of attaining to any positive basis of distinction on astronomical or purely geographical grounds.

As discoveries extended in all directions, and particularly to western Europe, distinctions founded on latitude or length of the day were found to be inapplicable and valueless, and they fell into disuse long before observation began with the instruments now in use to measure the heat, the weight of the air, etc. With the invention of these instruments the word climate came to signify the aggregate of atmospheric conditions affecting organic existence, animal and vegetable; and particularly the degree of heat, the winds, the rain, the changes in atmospheric weight, and all like sensible conditions. The barometer, developed from the Torricellian vacuum in 1643, was the first instrument suited to exact observation; soon afterward expansible fluids were used as measures of heat, and good spirit thermometers were in use in 1680. But it was not till 1730 that mercury was substituted for spirit in part, and permanent series of observations began to be recorded; the first in America being at Charleston by the learned Dr. Lining in 1738. Great activity existed near the close of the 17th century, when these instruments were novel, and when positive and even mathematical precision was hoped for through records of their observation; and the earlier "Philosophical Transactions" of the royal society are full of papers and discussions on the subject.

But it was soon found that there was no parallel in this to other departments of physics, and interest in it greatly declined until the new and broad generalizations of Humboldt were put forth. To Sir William Herschel and Alexander von Humboldt the world is chiefly indebted for the rapid progress made in the science of climatology within the present century. Humboldt in 1817 published his celebrated treatise entitled Des lignes iso-thermes, et de la distribution de la chaleur sur le globe, in which he showed that the decrease of heat with the increase of latitude takes place more slowly on the west coasts of the old world than on the east of the new. He connected places having an average amount of temperature during the year by isothermal lines, the convex summits of which fell near the west coast of the old world, and their concave near the east coast of the new. By combining the decrease of temperature by increasing elevation with its decrease by increasing latitude, he represented the intersection of isothermal surfaces with a vertical plane cutting the surface of the earth along a meridian, and showed that if the examination of places of equal summer heat and equal winter cold were conducted in a similar manner by drawing isothermal and isochimal lines, the difference between a sea and a continental climate would be included in the general view.

These isothermal lines differ materially from the parallels of latitude. It is not necessary to enter into detail as to how the annual rotation and oblique motion of the earth in relation to the sun fix the tropical limits of the sun's apparent declination south and north of the equator, and produce alternate winter and summer on either side of the line, as it will be evident that the mean annual temperature obtained at different latitudes must decrease from the equator to the poles. Had the whole surface of the earth been uniform, presenting the like relations to radiant heat, unaffected by the unequal action of disturbing causes, the mean temperature of every point would have been in proportion to the radius of the parallel of latitude. But the mean temperature of places in the same lines of latitude differs very materially. The isothermal line, for example, of 59° F. traverses latitude 42° in Europe, but descends to latitude 35° in America. Hence it appears that the mean annual temperature in the latitude of Beaufort, N. C., 34° 41', is nearly the same as that of Rome in latitude 41° 54'; and it consequently follows that other causes are in operation in both of these different localities affecting the mean temperature than nearness to or remoteness from the equator.

Accurate and long continued observations are necessary to give value to meteorological facts. These have been pursued of late with much zeal. Kamtz and Mahlraan have carefully represented annual isothermal lines, but Professor Dove was the first- to reduce these to monthly temperatures. The observations of Professor Dove were first published in the "Transactions of the Royal Academy of Sciences" in Berlin, in 1846, and have since appeared in the "Transactions of the British Association." These observations, which are of great value, and place their author among the first of living meteorologists, fully sustain the views of Humboldt, and naturally lead to an examination of the physical formation of the earth, as a cause of this great discrepancy in climate in the same parallels of latitude. According to Malte-Brun, climate is the assemblage of all those external physical and natural circumstances connected with each particular locality, which have a bearing upon the modifications of its temperature. This definition, adopted by M. Virey in the Dictionnaire des sciences medicales, and by M. Foissac in his work entitled De l'influence des climats sur l'homme, is susceptible of embracing all those phenomena involved in the complex study of physical climate, which are so varied in character, and frequently so little allied to each other, as to forbid their exact and rigorous classification. - The actual climates of the earth, as practically experienced by its living forms, animal and vegetable, are greatly diversified and highly irregular.

The averages or fixed quantities for different latitudes and localities are widely variable, and the non-periodic range of the leading conditions is even more extreme. The causes of this marked exception to the otherwise well defined character of all movements of matter, and the reason why this department of physics cannot, in the present state of knowledge at least, and probably not at any time, be the subject of precise analysis, may be seen from the following general statement. It is already admitted as certain that the various climates cannot be defined by any astronomical conditions, or by the form and movements of the earth relatively to the sun. Yet the heat received from the sun is the fundamental condition and controlling cause in all cases, and it acts both directly and through the secondary agencies of aeriform fluids and all mobile forms of matter. That is, the heat of the sun's rays is one part of the supply, as those rays fall direct on the surface, while air, atmospheric vapor, an,d water are also powerful agents in receiving and diffusing heat, bringing the direct product of the sun's power, as received in the tropics, far northward to soften the climates which cannot receive sufficient heat directly. The power of this tropical heat is enormous.

Not only are animal and vegetable forms stimulated by it to their largest growth, but the whole inorganic mass capable of motion is put in active circulation. By this heat the air is rarefied and given rapidity of circulation, at the same time that vast volumes of water are converted into vapor, capable of absorption and diffusion almost throughout the entire aerial mass. This system of air and water circulation causes, as one general result, the evaporation and subsequent precipitation in rain of a sheet of water that may be stated as being ten feet deep at the equator, six feet deep on the average in the tropics, three feet deep at the 45th parallel, and one foot deep or less at the Arctic circle. This vapor, being raised by heat, is precipitated in rain as the heat is dissipated; and this absorption and diffusion are also concurrent with a systematic movement of the aeriform mass, a movement which in the tropics is against the rotary movement of the solid mass of the earth, and probably is due in part to simple retardation of these fluid volumes, relatively to this solid portion.

An inward and upward movement within the tropics, consequent upon this heat and saturation, originates the great system of atmospheric circulation, and distributes to the temperate latitudes most of the moisture there deposited. A surplus of heat is also so diffused, and through these joint agencies the actual climates of a large portion of the earth's surface are established. And as the middle or temperate latitudes are in the direct path of the returning atmospheric current, the more striking general phenomena these climates present are easily explained in this connection. In passing northward and descending to the surface, these volumes of heated and saturated air lose their surplus of heat and most of their contained moisture; and in moving from west to east in these latitudes this circulation brings mild temperatures and local humidity from any ocean surface traversed; hence the profuse rains and moderate heats of the western coasts of both continents. In the interior, the same circulation continuing, there are alternations of both conditions; the heat being greater in the season of greatest heat and less in the cold season than on the coasts, and the rains being alternately profuse and deficient.

The degree of these alternations is increased until the immediate eastern boundary of the continents is reached, when a limited and local maritime influence appears to neutralize the agencies prevailing on the interior surface. Climates distinctly maritime, or directly controlled by adjacent water surfaces, exist in but few positions on the eastern borders of the continents. At Norfolk, Newport, Nova Scotia, and Newfoundland on the Atlantic coast, there are quite decisive maritime influences - particularly at or near Newport, and over most of the Nova Scotian peninsula; but these are due to unusual exposure, and close proximity to the Gulf stream. - Next to the atmospheric circulation in its influence on the actual distribution of heat is the movement of great sea currents. These currents are caused by the same tropical excess of heat, with its consequent rarefaction of the fluid mass of the equatorial latitudes. A movement westward within the tropics is arrested by the continental masses of both America and Asia, and deflected in the gulf of Mexico and on the coast of China, forming the Gulf stream of the Atlantic and the Euro Siwo of Japan and the Pacific. These great currents move northward on the surface of vast seas, diffusing their excess of heat, and greatly modifying the climate of the land surfaces within their reach.

This agency is especially powerful on the western coast and islands of Europe, deflecting the atmospheric circulation itself, charging the air with humidity, and rendering the general climate exceptional in the highest degree. Also in the Pacific ocean the agency of the Japan current is very great in bearing heat and moisture to higher latitudes than they otherwise would go. The western coast of America, from San Francisco to Behring strait, is bathed in this warm and softening influence, receiving much more of both heat and its attendant conditions than the atmospheric circulation alone would bring. In the southern hemisphere a similar but less powerful movement of the sea currents attends the atmospheric circulation, with consequences generally similar in modifying the climate of the land surfaces, but less striking and important. There are also in both hemispheres returning cold currents of much local importance, affecting the climate of certain tracts on the immediate coasts. The most important of these is the cold deep-sea current of the Pacific ocean, which, returning southeastward from the northern areas of that sea beneath a warm surface current, impinges on the W. coast of North America, and rises to the surface from the mouth of the Columbia river to Monterey, giving rise to the extraordinary cold day winds of summer at San Francisco, and over a long line of that coast.

This very striking fact finds no solution in any general theory of distribution of heat, either as received direct from the sun or as modified by atmospheric circulation, with its attendant standards of maritime and continental influences. It is due solely to the aqueous circulation, in a phase unknown to the eastern continent. As the result of this great diversity of controlling influences, there can be no determination of the leading conditions of climate otherwise than by observation. Even with certain standards for the equatorial belt and for the poles, as has sometimes been assumed, there can be no formula for interpolating the measures of heat or of humidity for the intervening latitudes; and observation must be continued for a period sufficient to cover the average of non-periodic variations above and below a true mean. These non-periodic changes are very great, and both in their degree and origin are in the present state of knowledge to a great extent inexplicable. An average for ten years is, however, nearly sufficient to give a true mean or fixed quantity in any of the conditions: the temperature, the fall of rain, and the date of significant changes, as the freezing or opening of rivers, the opening and closing of the season for growth of plants, etc.

Observation of the leading conditions of climate has made great progress within the past half century, and a remarkable degree of continuous and faithful attention to the record of such observations has been shown by citizens and public institutions in the United States since 1820, while valuable and almost continuous series exist for some localities for a long time previous to that date, the earliest having been begun in 1738. In Europe the records of instrumental observation are but litthe more extended, the earliest being about 1720. - Anterior to the discovery of instruments suited to measure the degree of heat, the weight of the atmosphere, etc., the tone of speculation and discussion of climatologi-cal laws was very loose, and full of highly imaginative views and conjectures. With the invention of the thermometer and barometer a new impetus was given to the study of clima-tological physics, and a vigorous attempt was made to apply positive measurements and to discover absolute laws. But it soon became apparent that the range of non-periodic phenomena was too great, and that the differences were too striking which were disclosed in the comparison of localities which should, so far as could be seen, very nearly agree, to admit of calculation on recognized physical laws.

A long period then elapsed in mere waiting for the requisite basis of observed facts on which a legitimate and safe induction could be founded, and this period had not closed when Humboldt published his striking and effective generalizations, a system of illustration of the dis-tribution of heat by isothermal lines. Time has not yet been afforded for the accumulation of observations of sufficiently extended periods over more than a small portion of the earth's surface, and it is not possible yet to fix the dis-tribution of heat with accuracy over any con-siderable part of the southern hemisphere. But the general outline of a positive climatology put forth in Humboldt's treatise has been greatly extended and filled up by subsequent labors, until it has attained a fixed position among scientific determinations. The leading condition determined is the mean temperature; next to that is the average rainfall, and next the resultant or average direction of the winds. The barometric measurements have been equally well established; and also the range or measurement of departure of each of these conditions from the mean, both periodic and non-periodic. The hygrometric condition, or the proportion of suspended moisture in the air, has been less generally observed, but it is recognized as a necessity in the proper definition of climates.

For all these the periods of observation in many parts of Europe and of the United States are ample, covering, as has already been said, more than a century at a few stations. But the necessity for a wide distribution of observed positions delays the most important of the uses to which these observations may be put. We turn, therefore, from the discussion of the ultimate laws concerning the principal conditions of climate, to the consideration of the remarkable facts of actual climate in the countries where determinations are recent. In the eastern part of the United States the facts have been long known, but in the interior and the west much is new. While the refrigeration originating with the mere presence of great continental areas is great, and develops a continual series of violent alternations of temperature over much of the surface in the entire valley of the Mississippi, there is a novel change apparent in going westward to the foot of the Rocky mountains, the degree of heat increasing not only relatively but positively, being greater at considerable altitudes of the great central plateaus than on the Atlantic coast in the same latitudes.

Thus at Fort Benton, Virginia City, Laramie, and Denver, points at various altitudes from 2,500 to 6,000 ft. above the sea, at the eastern foot of the Rocky mountains, the climate is milder than at sea level on the Atlantic coast in the same latitudes. At Fort Benton, on the upper Missouri, lat. 47° 50', and 2,700 ft. above the sea, the mean is 10° warmer than at St. John's, Newfoundland, nearly in the same latitude. At Fort Laramie, 4,500 ft. above the sea, it is warmer by 2° or 3° in the annual mean than at Boston, nearly in the same latitude. Denver, at 6,000 ft. above the sea, is as warm as Baltimore, which is in the same latitude. These are representative positions for a large mass of interior surface, on which the climate is in marked contrast with the usual standards for like elevations, and where also the special modifying influences of the Pacific do not reach. A like anomaly exists on the great plateaus of central Asia, which has been noticed by Humboldt, and was recognized by him as establishing a new form of continental influence, or that of increased heat due to the presence of great elevated areas.

Again, of new and striking developments in the climates of the western coast of North America, there are illustrations in the abrupt contrasts of the immediate coast of the Pacific with the interior, especially in summer. The peculiar cold winds of that coast in summer are met with off the coast before reaching the 30th parallel, and they are severe and almost violent from lat. 35° to 45° N., or almost to the mouth of the Columbia. They are due, as has been stated, to the vast mass of cold water moving from the northwest as a deep-sea current, and rising to the surface on the immediate coast. The highly heated plains and basins of the interior cause great rarefaction of the air in those basins, toward which the colder mass of the coast is drawn with force during the day, to subside in calm at night when radiation has cooled the temperature and restored the equilibrium. Further north the sea is warmer, both positively and relatively, and no contrast exists with the temperature of the land. The summer is very warm, and the climate most prolific in both animal and vegetable life over all Alaska and the broad valley of the Yukon, to lat. 65° or 08° N. Here again are marked contrasts of the Yukon and Mackenzie river valleys with the great plains eastward to Hudson bay.

On this tract, and still more eastward and northward of this bay, the maximum of continental refrigeration exists, and a summer is scarcely known. The extreme severity of the climate of this district, down to the lower border of Labrador, is in marked contrast with the climate of like areas of Asia, rendering it less capable of occupation than any other area of the earth's surface. At Fort Hope, Repulse bay, there are but two months, July and August, in which the mean temperature is above freezing; while at Yakutsk, East Siberia, in nearly the same latitude, there are five months with the mean above freezing, and the summer mean is 58.5°, compared with 39.7° at Repulse bay. - In the northeast of Ireland, says Humboldt, lying under the same parallel of latitude as Konigsberg in Prussia, the myrtle blooms as luxuriantly as in Portugal. The mean temperature of the month of August, which in Hungary rises to 70°, scarcely reaches 61° at Dublin, which is situated in the isothermal line of 49°; the mean winter temperature, which falls to about 28° at Pesth, is 40° in Dublin (whose mean annual temperature is not more than 49°), 3.6° higher than that of Milan, Pavia, Padua, and the whole of Lombardy, where the mean annual temperature is upward of 55°. At Stromness in the Orkneys, scarcely half a degree further south than Stockholm, the winter temperature is 39°, and consequently higher than that of Paris, and nearly as high as that of London. Even in the Faroe islands, at latitude G2°, the inland waters never freeze, owing to the favorable influence of the west winds and of the sea.

On the charming coast of Devonshire, near Sal-combe bay, which has been termed on account of the mildness of its climate the Montpellier of the north, the agave Americana has been seen to blossom in the open air, while orange trees trained against espaliers, and only slightly protected by matting, are found to bear fruit. There, as well as at Penzance and Gosport, and at Cherbourg on the coast of Normandy, the mean winter temperature exceeds 42°, falling short by only 2.4° of the mean winter temperature of Montpellier and Florence. In comparison with the western coast of Europe, it is believed that the western coast of America is somewhat colder, although more equable than the eastern part of the western continent. - A. Keith Johnston says: "The climates of the Asiatic coast correspond with those of America along the Atlantic; and thoso of Columbia, Vancouver, and Washington are duplicates of those of western Europe and the British islands. The climate of California resembles that of Spain; the sandy plains and rainless region of Lower California reminding one of Africa with its deserts between the same parallels."The elastic atmosphere and bracing effect of the Pacific climates," says Blodget, "constitute a striking difference from those of the eastern states.

All residents concur in pronouncing it more favorable to physical and mental activity than any they have known" ("Climatology of the United States," p. 200.) According to observations made at West Point, N. Y., and Fort Trumbull, Conn., both in lat, 41° 22', and but 1 1/2° of longitude apart, the winters are 4.67° milder and the summers 1° cooler at Fort Trumbull than at West Point, the former being upon the sea, and the latter inland. This difference is still more manifest in that portion of the North American continent lying north of the boundary line of the United States. In Nova Scotia, which is nearly surrounded by water, the thermometer seldom indicates a temperature higher than 88° in summer, nor more than 8° below zero in winter; but in Canada, occupying the same parallels of latitude, the thermometer in summer rises as high as 97°, and occasionally 100°, while in winter a cold of 30° below zero is frequent, and the usual range of the temperature during the winter months is from 8° to 30° below zero. - Elevation above the level of the sea exercises a decided influence on climate.

The temperature of the atmosphere is found to decrease in successive and regular gradation as it leaves the sea's level, so that in the ascent of lofty mountains within the tropics the traveller experiences every change of weather, from the oppressive heat of the summer's sun on the plain below, to the piercing cold of eternal frost on the lofty summit above. The region of perpetual spring in the neighborhood of Potosi, in South America, is remembered with emotions of delight by every traveller who has ascended that ridge of the Andes. The declension of temperature has been found, with occasional variations, to equal 1° for every 300 ft. in temperate climates. This subsidence of temperature with elevation is doubtless dependent on the extreme rarity of the atmosphere at a distance from the earth, and the consequent facility with which it is permeated by heat, as well as the radiating power possessed by the earth, which enables it to return to the contiguous atmosphere a portion of the solar rays it had previously absorbed. The atmosphere is condensed in proportion to the force with which it is compressed, and expands in exact ratio to the diminution of that force.

It follows that, the superincumbent strata of air being compressed with greater force in its most dependent part, and that dependent part being nearest the earth's surface, its density will be there greatest, and this density will diminish in exact proportion to the ascent of the column of air. Now the atmosphere when under a certain compression has a certain capacity for latent heat, which is increased by a diminution of the compression, and diminished by its increase. If a column of air at a certain distance from the earth receive a certain number of the sun's rays, and then be brought suddenly down to occupy a denser medium, its particles becoming compressed, a portion of the latent heat becomes sensible and is given off to surrounding bodies. The following observations, made by Mr. Green during an aerial voyage, exhibit this declension of temperature. The thermometer at the earth's surface indicated a temperature of 74°; at an elevation of 2,952 ft., 72°; at 7,288 ft., 70°; at 9,993 ft., 69°; at 11,059 ft., 45°; at 11,293 ft., 38°; making a difference of 36° between the earth's surface and the highest altitude attained, or about 1° for every 311 ft.

Sir John Leslie and Humboldt believe that the diminution is much more marked near the surface of the earth than is here indicated. Mr. Glaisher, in his midday ascents, found an average fall in temperature of 1° in 223 ft. for the first 1,000 ft. with a cloudy sky, and in 162 ft. with a clear one; while above 10,000 ft. the decline was 1° in 455 and 417 ft. respectively, and above 20,000 ft. it was only 1° in nearly 1,000 ft. under both conditions. (See Aeronautics.) But this reduction does not apply to elevation of large areas of surface above the level of the sea. It is only applicable to actual distances from the surface, or elevations in the atmosphere itself from any general surface. - Moisture exercises very marked influence over climate. Taken as a whole, all the gentle slopes on the American continent descend eastwardly toward the Atlantic, while the abrupt ones rise on its western aspect. In this respect there is a manifest difference between this continent and that of Europe, which gradually declines westwardly toward the Atlantic. This general configuration necessarily gives to Europe a moister as well as a more temperate climate than that of America in the same parallels of latitude.

This would be much more obvious were it not for the admirable compensation made by the Gulf stream and the trade wind that accompanies it. From this source not only the Atlantic coast, but the Mississippi valley, which is exposed on the south to the gulf of Mexico, derives a large proportion of its moisture. (See Meteorology.)