Cosmos: A Sketch of the Physical Description of the Universe, Vol. 1

by Alexander von Humboldt

In enumerating the causes which produce disturbances in the form of the isothermal lines, I would distinguish between those which 'raise' and those which 'lower' the temperature. To the first class belong the proximity of a western coast in the temperate zone; the divided configuration of a continent into peninsulas, with deeply-indented bays and inland seas; the aspect of the position of a portion of the land with reference either to a sea of ice spreading far into the polar circle, or to a mass of continental land of considerable extent, lying in the same meridian, either under the equator, or, at least, within a portion of the tropical zone; the prevalence of southerly or westerly winds on the western shore of a continent in the temperate northern zone; chains of mountains acting as protecting salls against the winds coming from colder regions; the infrequency of swamps, which, in the spring and beginning of summer, long remain covered with ice, and the absence of woods in a dry, sandy soil; finally the constant serenity of the sky in the summer months, and the vicinity of an oceanic current, bringing water which is of a higher temperature than that of the surrounding sea.

Among the causes which tend to 'lower' the mean annual temperature I include the following: elevation above the level of the sea, when not forming part of an extended plain; the p 320 vicinity of an eastern coast in high and middle latitudes; the compact configuration of a continent having no littoral curvatures or bays; the extension of land toward the poles into the region of perpetual ice, without the intervention of a sea remaining open in the winter; a geographical position, in which the equatorial and tropical regions are occupied by the sea, and consequently, the absence, under the same meridian, of a continental tropical land having a strong capacity for the absorption and radiation of heat; mountain chains, whose mural form and direction impede the access of warm winds, the vicinity of isolated peaks, occasioning the descent of cold currents of air down their declivities; extensive woods, which hinder the isolation of the soil by the vital activity of their foliage, which produces great evaporation, owing to the extension of these organs, and increases the surface that is cooled by radiation, acting consequently in a three-fold manner, by shade, evaporation, and radiation; the frequency of swamps or marshes, which in the north form a kind of subterranean glacier in the plains, lasting till the middle of the summer; a cloudy summer sky, which weakens the action of the solar rays; and, finally, a very clear winter sky, favoring the radiation of heat.*

[footnote] *Humboldt, 'Recherches sur les Causes des Inflexions des Lignes Isothermes', in 'Asie Centr.', t. iii., p. 103-114, 118, 122, 188.

The simultaneous action of these disturbing causes, whether productive of an increase or decrease of heat, determines, as the total effect, the inflection of the isothermal lines, especially with relation to the expansion and configuration of solid continental masses, as compared with the liquid oceanic. These perturbations give rise to convex and concave summits of the isothermal curves. There are, however, different orders of disturbing causes, and each one must, therefore, be considered separately, in order that their total effect may afterward be investigated with reference to the motion (direction, local curvature) of the isothermal lines, and the actions by which they are connected together, modified, destroyed, or increased in intensity, as manifested in the contact and intersection of small oscillatory movements. Such is the method by which, I hope, it may some day be possible to connect together, by empirical and numerically expressed laws, vast series of apparently isolated facts, and to exhibit the mutual dependence which must necessarily exist among them.

The trade winds — easterly winds blowing within the tropics — give rise, in both temperate zones, to the west, or west-southwest p 321 sinds which prevail in those regions, and which are land winds to eastern coasts, and sea winds to western coasts, estending over a space which, from the great mass and the sinking of its cooled particles, is not capable of any considerable degree of cooling, and hence it follows that the east winds of the Continent must be cooler than the west winds, where their temperature is not affected by the occurrence of oceanic currents near the shore. Cook's young companion on his second voyage of circumnavigation, the intelligent George Forster, to whom I am indebted for the lively interest which prompted me to undertake distant travels, was the first who drew attention, in a definite manner, to the climatic differences of temperature existing in the eastern and western coasts of both continents, and to the similarity of temperature of the western coast of North America in the middle latitudes, with that of Western Europe.*

[footnote] *George Forster, 'Klein Schriften', th. iii., 1794, s. 87; Dove, in Schumacher's 'Jahrbuch fur', s. 289; Kämtz, 'Meteorologie', bd. ii., s. 41, 43, 67, and 96; Arago, in the 'Comptes Rendus', t. i., p. 268.

Even in northern latitudes exact observations show a striking difference between the 'mean annual temperature' of the east and west coasts of America. The mean annual temperature of Nain, in (lat. 57 degrees 10'), is fully 6.8 degrees 'below' the freezing point, while on the northwest coast, at New Archangel, in Russian America (lat. 57 degrees 3'), it is 12.4 degrees 'above' this point. At the first-named place, the mean summer temperature hardly amounts to 43 degrees, while at the latter place it is 57 degrees. Pekin (39 degrees 54'), on the eastern coast of Asia, has a mean annual tempeerature of 52.8 degrees, which is 9 degrees below that of Naples, situated somewhat further to the north. The mean winter temperature of Pekin is at least 5.4 degrees below the freezing point, while in Western Europe, even at Paris (48 degrees 50'), it is nearly 6 degrees above the freezing point. Pekin has also a mean winter cold which is 4.5 degrees lower than that of Copenhagen, lying 17 degrees further to the north.

We have already seen the slowness with which the great mass of the ocean follows the variations of temperature in the atmosphere, and how the sea acts in equalizing temperatures, moderating simultaneously the severity of winter and the heat of summer. Hence arises a second more important contrast — that, namely, between insular and littoral climates enjoyed by all articulated continents having deeply indented bays and peninsulas, and between the climate of the interior of great masses of solid land. This remarkable contrast has been fully p 322 developed by Leopold von Buch in all its various phenomena, both with respect to its influence on vegetation and agriculrure, on the transparency of the atmosphere, the radiation of the soil, and the elevation of the line of perpetual snow. In the interior of the Asiatic Continent, Tobolsk, Barnaul on the Oby, and Irkutsk, have the same mean summer heat as Berlin, Munster, and Cherbourg in Normandy, the thermometer sometimes remaining for weeks together at 86 degrees or 88 degrees, while the mean winter temperature is, during the coldest month, as low as -0.4 degrees to -4 degrees. These continental climates have therefore justly been termed 'excessive' by the great mathematician and physicist Buffon; and the inhabitants who live in countries having such 'excessive' climates seem almost condemned, as Dante expresses himself, "A sofferir tormenti caldi e geli."*

[fiitbite] *Dante, 'Divina Commedia, Purgatorio', canto iii.

In no portion of the earth, neither in the Canary Islands, in Spain, nor in the south of France, have I ever seen more luxuriant fruit, especially grapes, than in Astrachan, near the shores of the Caspian Sea (46 degrees 21'). Although the mean annual temperature is about 48ºdegrees, the mean summer heat rises to 70ºdegrees, as at Bordeaux, while not only there, but also further to the south, as at Kislar on the mouth of the Terek (in the latitude of Avignon and Rimini), the thermometer sinks in the winter to -13 degrees or -22 degrees.