Note 18 — ON STEPPES AND DESERTS.

p. 7.—“A diversity of causes diminishes the dryness and heat of the New Continent.”

I have endeavoured to compress the various causes of the humidity and lesser heat of America into one general category. It will of course be understood, that I can only have reference here to the general hygroscopic condition of the atmosphere, and the temperature of the whole continent; for in considering individual regions, as for instance, the island of Margarita, or the coasts of Cumana and Coro, it will be found that these exhibit an equal degree of dryness and heat with any portion of Africa.

The maximum of heat, at certain hours of a summer’s day, considered with reference to a long series of years, has been found to be almost the same in all regions of the earth, whether on the Neva, the Senegal, the Ganges, or the Orinoco, namely, between 93° and 104° Fahr., and on the whole not higher; provided that the observation be made in the shade, far from solid radiating bodies, and not in an atmosphere filled with heated dust or granules of sand, and not with spirit-thermometers, which absorb light. The fine grains of sand (forming centres of radiant heat) which float in the air, were probably the cause of the fearful heat (122° to 133° Fahr. in the shade) in the Oasis of Mourzouk to which my unhappy friend Ritchie, who perished there, and Captain Lyon, were exposed for weeks. The most remarkable instance of a high temperature, in an air probably free from dust, is mentioned by an observer who well knew how to arrange and correct all his instruments with the greatest accuracy. Rüppel found the temperature 110°.6 Fahr. at Ambukol, in Abyssinia, with a cloudy sky, a strong south-west wind, and an approaching thunder-storm. 97The mean annual temperature of the tropics, or the actual climate of the region of palms, is on the main land between 78°.2 and 85°.5 Fahr., without any sensible difference between the observations made in Senegal, Pondichery, and Surinam^[DV].

The great coolness, one might almost say coldness, which prevails during a great portion of the year in the tropics, on the coast of Peru, and which causes the mercury to fall to 59° Fahr., is, as I hope to show in another place, not to be attributed to the effect of neighbouring mountains covered with snow, but rather to the mist (garua) which obscures the sun’s disk, and to a current of cold sea-water commencing in the antarctic regions, and which coming from the south-west, strikes the coast of Chili near Valdivia and Concepcion, and is thence propelled with violence, in a northerly direction, to Cape Pariña. On the coast of Lima, the temperature of the Pacific is 60°.2 Fahr., whilst it is 79°.2 Fahr. under the same parallel of latitude when outside the current. It is singular, that so remarkable a fact should have remained unnoticed, until my residence on the coast of the Pacific, in October, 1802.

The variations of temperature, of many parts of the earth, depend principally on the character of the bottom of the aërial ocean, or in other words, on the nature of the solid or fluid (continental or oceanic) base on which the atmosphere rests. Seas, traversed in various directions by currents of warm and cold water (oceanic rivers), exert a different action from articulated or inarticulated continental masses or islands, which may be regarded as the shoals in the aërial ocean, and which, notwithstanding their small dimensions, exercise, even to great distances, a remarkable degree of influence on the climate of the sea. In continental masses, we must distinguish between barren sandy deserts, savannahs, (grassy plains,) and forest districts. In Upper Egypt and in South America, Nouet and myself found, at noon, the temperature of the ground, which was composed of granitic sand, 154° and 141° Fahr. Numerous careful observations instituted at Paris, have given, according to Arago, 122° and 126°.5 Fahr.^[DW] The Savannahs, which, between the Missouri and the Mississippi, are called Prairies, and which appear in the south at 98the Llanos of Venezuela and the Pampas of Buenos Ayres, are covered with small monocotyledons, belonging to the family of the Cyperaceæ, and with grasses, whose dry pointed stalks, and whose delicate, lanceolate leaves radiate towards the unclouded sky, and possess an extraordinary power of emission. Wells and Daniell^[DX] have even seen in our latitude, where the atmosphere has a much less considerable degree of transparency, the thermometer fall to 14°.5, or 18° Fahr. on being placed on the grass. Melloni has most ably shown^[DY] that in a calm, which is a necessary condition of a powerful radiation, and of the formation of dew, the cooling of the stratum of grass is promoted by the falling to the ground of the cooler particles of air, as being the heavier.

In the vicinity of the equator, under the cloudy sky of the Upper Orinoco, the Rio Negro and the Amazon, the plains are covered with dense primeval forests; but to the north and south of this woody district, there extend, from the zone of palms and of tall dicotyledonous trees in the northern hemisphere, the Llanos of the Lower Orinoco, the Meta, and Guaviare; and in the south, the Pampas of the Rio de la Plata and of Patagonia. The area thus covered by grassy plains, or Savannahs, in South America, is at least nine times greater than that of France.

The forest region acts in a threefold manner, by the coolness induced by its shade, by evaporation, and by the cooling process of radiation. Forests uniformly composed in our temperate zone of “social” plants, belonging to the families of the Coniferæ or Amentaceæ (the oak, beech, and birch), and under the tropics composed of plants not living socially, protect the ground from direct insolation, evaporate the fluids they have themselves produced, and cool the contiguous strata of air by the radiation of heat from their leafy appendicular organs. The leaves are by no means all parallel to one another, and present different inclinations towards the horizon; and according to the laws established by Leslie and Fourier, the influence of this inclination on the quantity of heat emitted by radiation is such, that the radiating power of a given measured surface a, having a given oblique direction, is equal to the radiating power of a leaf of the size of a projected on a horizontal 99plane. In the initial condition of radiation of all the leaves which form the summit of a tree, and which partially cover each other, those which are directly presented towards the unclouded sky, will be first cooled.

This production of cold (or the exhaustion of heat by emission) will be the more considerable in proportion to the thinness of the leaves. A second stratum of leaves has its upper surface turned to the under surface of the former, and will give out more heat by radiation towards that stratum than it can receive from it. The result of this unequal exchange will then be a diminution of temperature for the second stratum also. A similar action will extend from stratum to stratum, till all the leaves of the tree, by their greater or less radiation, as modified by their difference of position, have passed into a condition of stable equilibrium, of which the law may be deduced by mathematical analysis. In this manner, in the serene and long nights of the equinoctial zone, the forest air, which is contained in the interstices between the strata of leaves, becomes cooled by the process of radiation; for a tree, a horizontal section of whose summit would hardly measure 2000 square feet, would, in consequence of the great number of its appendicular organs (the leaves), produce as great a diminution in the temperature of the air as a space of bare land or turf many thousand times greater than 2000 square feet.^[DZ] I have thus sought to develope somewhat fully the complicated relations which the action of great forest regions exerts on the atmosphere, because they have so often been touched upon in connection with the important question of the climate of ancient Germany and Gaul.

As in the old continent, European civilization has had its principal seat on the western coast, it could not fail to be early remarked that under equal degrees of latitude the opposite eastern littoral region of the United States of North America was several degrees colder, in mean annual temperature, than Europe, which is, as it were, a western peninsula of Asia, and bears much the same relation to it as Brittany does to the rest of France. The fact, however, escaped notice that these differences decrease from the higher to the lower latitudes, and that they are hardly perceptible below 30°. For the west coast of the New Continent exact observations 100of the temperature are still almost entirely wanting; but the mildness of the winter in New California shows that in reference to their mean annual temperature, the west coasts of America and Europe under the same parallels, scarcely present any differences. The annexed table gives the mean annual temperatures, which correspond to the same geographical latitudes, on the eastern coast of the New Continent and the western coast of Europe:—

Similar Degrees of Latitude. Eastern Coast of America. Western Coast of Europe. Mean Temperature of the Year, of Winter, and Summer. Difference between the annual Temperature of Eastern America and Western Europe.

- 0°.4

57° 10′ Nain 25°.7

45°.7

20°.7

31°.5

57° 41′ Gottenburg 46°.4

62°.4

23°

47° 34′ St. John’s 38°.1

54°

31°.1

47° 30′ Buda 50°.5 13°.6

69°.8

37°.8

48° 50′ Paris 51°.7

64°.6

24°.1

44° 39′ Halifax 43°.5

63°.0

13°.7

42°.8

44° 50′ Bordeaux 57°.2

71°.1

101 32°.2

40° 43′ New York 52°.5

72°.9

32°.2

39° 57′ Philadelphia 52°.2

72°.7

36°.0

38° 53′ Washington 54°.9 9°.3

71°.1

49°.5

40° 51′ Naples 61°.0

74°.9

52°.2

38° 52′ Lisbon 61°.5

71°.1

59°.5

29° 48′ St. Agustin 72°.3

81°.5

0°.5

58°.5

30° 2′ Cairo 71°.8

84°.6

In the preceding table the number placed before the fraction represents the mean annual temperature, the numerator of the fraction, the mean winter temperature, and the denominator the mean summer temperature. Besides the more marked difference between the mean annual temperatures, there is also a very striking contrast between the opposite coasts in respect to the distribution of heat over the different seasons of the year; and it is indeed this distribution which exerts the greatest influence on our bodily feelings and on the process of vegetation. Dove^[EA] makes the general remark, that 102the summer temperature of America is lower under equal degrees of latitude than that of Europe. The climate of St. Petersburgh (lat. 59° 56′), or to speak more correctly, the mean annual temperature of that city, is found on the eastern coast of America, in lat. 47° 30′, or 12° 30′ more to the south; and in like manner we find the climate of Königsberg (lat. 54° 43′) at Halifax in lat. 44° 39′. Toulouse (lat. 43° 36′) corresponds in its thermic relations to Washington.

It is very hazardous to attempt to obtain any general results respecting the distribution of heat in the United States of North America, since there are three regions to be distinguished—1, the region of the Atlantic States, east of the Alleghanys; 2, the Western States, in the wide basin between the Alleghanys and the Rocky Mountains, watered by the Mississippi, the Ohio, the Arkansas, and the Missouri; and 3, the elevated plains between the Rocky Mountains and the Coast Range of New California, through which the Oregon or Columbia river wends its course. Since the commendable establishment by John Calhoun of uninterrupted observations of temperature, made on a uniform plan, at thirty-five military stations, and reduced to diurnal, mensal, and annual means, we have attained more correct climatic views than were generally held in the time of Jefferson, Barton, and Volney. These meteorological stations extend from the point of Florida and Thompson’s Island (West Key), lat. 24° 33′, to the Council Bluffs on the Missouri; and if we reckon Fort Vancouver (lat. 45° 37′), among them, they include a space extending over forty degrees of longitude.

It cannot be affirmed that on the whole the second region has a higher mean annual temperature than the first, or Atlantic. The further advance towards the north of certain plants on the western side of the Alleghanys, depends partly on the nature of those plants and partly on the different distribution through the seasons of the year of the same annual amount of heat. The broad valley of the Mississippi enjoys, at its northern extremity, the warming influence of the Canadian lakes, and at the south, that of the Mexican Gulf-Stream. These five lakes (Lakes Superior, Michigan, Huron, Erie, and Ontario,) cover an area of 92,000 square miles. The climate is so much milder and more uniform in the vicinity of the lakes, that at Niagara, for instance (in 43° 15′ 103north lat.), the mean annual winter temperature is only half a degree below the freezing-point, whilst, at a distance from the lakes, in 44° 53′ north lat. at Fort Snelling, near the confluence of the river St. Peter with the Mississippi, the mean winter temperature is 15°.8 Fahr.^[EB] At this distance from the Canadian lakes, whose surface is from five to upwards of six hundred feet above the sea’s level, whilst the bottom of Lakes Michigan and Huron is five hundred feet below it, recent observations have shown that the climate of the country possesses the actual continental character of hotter summers and colder winters. “It is proved,” says Forry, “by our thermometrical data, that the climate west of the Alleghany chain is more excessive than that on the Atlantic side.” At Fort Gibson, on the Arkansas river, which falls into the Mississippi, in lat. 35° 47′, where the mean annual temperature hardly equals that of Gibraltar, the thermometer was observed, in August, 1834, to rise to 117° Fahr. when in the shade, and without any reflected heat from the ground.

The statements so frequently advanced, although unsupported by measurements, that since the first European settlements in New England, Pennsylvania, and Virginia, the destruction of many forests on both sides of the Alleghanys, has rendered the climate more equable,—making the winters milder and the summers cooler,—are now generally discredited. No series of thermometric observations worthy of confidence extend further back in the United States than seventy-eight years. We find from the Philadelphia observations that from 1771 to 1824, the mean annual heat has hardly risen 2°.7 Fahr.;—an increase that may fairly be ascribed to the extension of the town, its greater population, and to the numerous steam-engines. This annual increase of temperature may also be owing to accident, for in the same period I find that there was an increase of the mean winter temperature of 2° Fahr.; but with this exception the seasons had all become somewhat warmer. Thirty-three years’ observations at Salem in Massachusetts show scarcely any difference, the mean of each one oscillating within 1° of Fahrenheit, about the mean of the whole number; and the winters of Salem, instead of having been rendered more mild, as conjectured, from the eradication 104of the forests, have become colder by 4° Fahr. during the last thirty-three years.^[EC]

As the east coast of the United States may be compared, in equal latitudes, with the Siberian and Chinese eastern coasts of Europe, in respect to mean annual temperature, so the western coasts of Europe and America have also very justly been compared together. I will here only adduce a few instances from the western region of the Pacific, for two of which, viz., Sitka. (New Archangel,) in Russian America, and Fort George. (having the same latitudes respectively as Gottenburg and Geneva,) we are indebted to Admiral Lütke’s voyage of circumnavigation. Iluluk and Danzig are situated in about the same parallel of latitude, and although the mean temperature of Iluluk, owing to its insular climate and the cold sea current contiguous to it, is lower than that of Danzig, the winters of the former are milder than those of the Baltic city.

33°.3

Sitka Lat. 57° 3′ Long. 135° 16′ W. 44°.6

55°.0

31°.6

Gottenburg Lat. 57° 41′ Long. 11° 59′ E. 46°.4

62°.4

37°.8

Fort George Lat. 46° 18′ Long. 123° 58′ W. 50°.2

60.°0

33.°6

Geneva Lat. 46° 12′ Altitude 1298 feet 49°.8

63°.5

25°.0

Cherson Lat. 46° 38′ Long. 32° 39′ E. 53°.1

71°.0

Snow is hardly ever seen on the banks of the Oregon or Columbia river, and ice on the river lasts only a few days. The lowest temperature which Mr. Ball ever observed there (in 1838) was 18°.4 Fahr.^[ED] A cursory glance at the summer and 105winter temperatures given above, suffices to show that a true insular climate prevails on and near the western coasts; whilst the winter cold is less considerable than in the western part of the old continent, the summers are much cooler. This contrast is made most apparent when we compare the mouth of the Oregon with Forts Snelling and Howard, and the Council Bluffs in the interior of the Mississippi and Missouri basin, (44°–46° north lat.,) where, to speak with Buffon, we find an excessive or true continental climate,—a winter cold, which on some days is –32° or even –37° Fahr., followed by a mean summer’s heat, which rises to 69° and 71°.4 Fahr.