Note 18 — STEPPES AND DESERTS.

p. 8.—“The causes which lessen both heat and dryness in the New World.”

I have tried to bring together in a brief and compendious manner the various causes which produce greater moisture and a less degree of heat in America; it will of course be understood that the question respects the general hygrometric state of the atmosphere, and the temperature of the New Continent as a whole. Single districts, such as the island of Margarita, the Coasts of Cumana and Coro, are as hot and as dry as any part of Africa. It must also be remarked that the maximum of heat at certain hours of a summer’s day has been found, on a series of years, to be almost equal at very different parts of the earth’s surface, on the Neva, the Senegal, the Ganges, and the Orinoco; being approximately between 27° and 32° Reaumur (93° and 104° Fahrenheit), and generally not higher,—providing the observation be made in the shade, at a distance from all solid bodies which could radiate heat to the thermometer, not in an air filled with hot particles of dust or sand, and not with spirit thermometers, which absorb the light. It is probably to fine grains of sand floating in the air, and forming centres of radiant heat, that we must ascribe the dreadful temperature of 40° to 44°.8 Reaumur (122° to 133° Fah.) in the shade, to which my unhappy friend Ritchie, who perished there, and Captain Lyon, were exposed for weeks in the Oasis of Mourzouk. The most remarkable instance of very high temperature, in an air probably free from dust, has been recorded by an observer who knew well how to place and to correct all his instruments with the greatest degree of accuracy. Rüppell found 37°.6 Reaumur, (110°.6 Fahrenheit,) at Ambukol in Abyssinia, with a clouded sky, strong south-west wind, and an approaching thunderstorm. The mean annual temperature of the tropics, or of the proper climate of palms, is, on land, between 20°.5 and 23°.8 Reaumur (or 78°.2 and 85°.5 Fahrenheit) without any considerable difference between the observations collected in Senegal, Pondichery, and Surinam. (Humboldt, Mémoire sur les lignes isothermes, 1817, p. 54. Asie Centrale, T. iii. Mahlmann, Table iv.)

The great coolness, I might almost say cold, which prevails for a considerable part of the year within the tropics on the coast of Peru, causing the thermometer to sink to 12° Reaumur (59° Fahrenheit), is, as I have noticed elsewhere, by no means to be ascribed to the vicinity of the snow-covered Andes, but rather to the fogs (garua) which veil the solar disk, and to a cold sea current which, commencing in the antarctic regions and coming from the south-west, strikes the coast of Chili near Valdivia and Conçeption, and thence streams rapidly along the coast to the northward, as far as Cape Pariña. On the coast, near Lima, the temperature of the Pacific is 12°.5 Reaumur (60°.2 Fahr.), whilst in the same latitude out of the current it is 21° R. (79°.2 Fahr.) It is singular that so striking a fact should have remained unnoticed until my visit to the shores of the Pacific, in October 1802.

The variations of temperature of different regions depend in a great degree on the character of the bottom of the “aerial ocean,” or on the nature of the floor or base, whether land or sea, continental or oceanic, on which the atmosphere rests. Seas, often traversed by currents of warmer or colder water, (oceanic rivers), have an effect very different from that of continental masses, whether unbroken or articulated, or of islands, which latter may be regarded as shallows in the aerial ocean, and which, notwithstanding their small dimensions, exert, often to a great distance, a notable influence on the climate of the sea. In continental masses we must distinguish between sandy deserts devoid of vegetation, savannahs or grassy plains, and forest-covered districts. In Upper Egypt and in South America, Nouet in the former, and myself in the latter, found respectively at noon the temperature of the ground composed of granitic sand 54°.2 and 48°.4 Reaumur (154° and 141° Fahr.) Many careful observations in Paris have given, according to Arago, 40° and 42° Reaumur, 122° and 126°.5 Fahrenheit. (Asie Centrale, T. iii. p. 176.) The Savannahs, which between the Missouri and the Mississipi are called Prairies, and which appear in South America as the Llanos of Venezuela and the Pampas of Buenos Ayres, are covered with small monocotyledonous plants of the family of Cyperaceæ, and with grasses of which the thin pointed stalks or ears, and the delicate lanceolate leaves or blades, radiate towards the unclouded sky, and possess an extraordinary power of “emission.” Wells and Daniell (Meteor. Essays, 1827, p. 230 and 278) have even seen in our latitude, where the atmosphere has so much less transparency, the thermometer sink 6°.5 or 8° of Reaumur (14°.5 or 18° Fahrenheit), on being placed on the grass. Melloni, in a memoir, “Sull abassamento di temperatura durante le notti placide e serene,” 1847, p. 47 and 53, has shewn how in a calm state of the atmosphere, which is a necessary condition of strong radiation and of the formation of dew, the cooling of the grassy surface is also promoted by the particles of air which are already cooled sinking to the ground as being the heaviest. In the vicinity of the equator, under the clouded sky of the Upper Orinoco, the Rio Negro, and the Amazons River, the plains are clothed with dense primeval forests; but to the north and south of this wooded region there extend from the zone of palms and lofty dicotyledonous trees, in the northern hemisphere, the Llanos of the Lower Orinoco the Meta and the Guaviare, and in the southern hemisphere the Pampas of the Rio de la Plata and of Patagonia. The space thus occupied by Savannahs or grassy plains in South America is at least nine times as great as the area of France.

The wooded region acts in a threefold manner in diminishing the temperature by cooling shade, by evaporation, and by radiation. Forests,—which in our temperate zone consist of trees living together in “society,” i. e., many individuals of one, or of a few kinds, of the families of Coniferæ or Amentaceæ, oaks, beeches, and birches, but in the tropics, of an immense variety of trees living separately or “unsocially,”—protect the ground from the direct rays of the sun, evaporate fluids elaborated by the trees themselves, and cool the strata of air in immediate contact with them by the radiation of heat from their appendicular organs or leaves. The latter are far from being all parallel with each other; they are, on the contrary, variously inclined to the horizon, and, according to the law developed by Leslie and Fourier, the influence of this inclination upon the quantity of heat emitted by radiation is such, that the power of radiation (pouvoir rayonnant) of a measured surface a, having a given oblique direction, is equal to the “pouvoir rayonnant” which would belong to a surface of the size of a, projected on a horizontal plane. Now in the initial condition of radiation, of all the leaves which form the summit of a tree and partly cover each other, those are first cooled which are directed without any intervening screen towards the unclouded sky. The cooling result (or the exhaustion of heat by emission) will be the more considerable the greater the thinness of the leaves. A second stratum of leaves has its upper surface turned to the under surface of the first stratum, and will give out more heat by radiation towards that stratum than it can receive by radiation from it. The result of this unequal exchange will thus be a loss of temperature for the second stratum of leaves also. A similar operation will continue from stratum to stratum until all the leaves of the tree, by greater or less radiation as modified by their diversity of position, have passed into a state of stable equilibrium of which the law can be deduced by mathematical analysis. In this manner, in the long and clear nights of the equinoctial zone, the forest air contained in the intervals between the strata of leaves becomes cooled by the process of radiation; and by reason of the great quantity of its thin appendicular organs or leaves, a tree, the horizontal section of whose summit would measure for example 2000 square feet, would act in diminishing the temperature of the air equivalently to a space of bare or turf-covered ground several thousand times greater than 2000 square feet (Asie Centrale, T. iii. p. 195–205). I have sought thus to develop in detail the complicated effects which make up the total action of extensive forests upon the atmosphere, because they have been so often touched upon in reference to the important question concerning the climates of ancient Germany and Gaul.

As in the old continent European civilization has had its principal seats on a western coast, it could not but be early remarked that, under equal degrees of latitude, the opposite eastern coast of the United States was several degrees colder in mean annual temperature than Europe, which is, as it were, a projecting western peninsula to Asia, as Brittany is to the rest of France. But in this remark it was forgotten that these differences decrease from the higher to the lower latitudes in such manner that they almost entirely disappear from 30° downwards. For the west coast of the new continent, exact thermometric observations are still almost entirely wanting; but the mildness of the winters in New California shews that the west coasts of America and Europe, under the same parallels of latitude, probably differ little from each other in mean annual temperature. The subjoined table shows what are the corresponding mean annual temperatures, in the same geographical latitudes, of the west coast of Europe and the east coast of the New Continent.

Similar degrees of latitude. East Coast of America. West Coast of Europe. Reaumur. Fahrenheit.

Mean temperature of the year, of winter, and of summer. Differences of annual temperature of East coast of America and West coast of Europe. Mean temperature of the year, of winter, and of summer. Differences of annual temperature of East coast of America and West coast of Europe.

57° 10′ Nain -2°.8 -14°.4 9°.2 25°.8 -0°.5 20°.7

—— ——

6°.1 45°.8

57° 41′ Gottenburg 6°.4 -0°.2 46°.4 -31°.5

—— ——

13°.5 62°.4

47° 34′ St. John’s 2°.7 -4°.0 5°.8 38°.0 23°.0 13°.0

—— ——

9°.8 54°.0

47° 30′ Ofen (or Buda) 8°.2 -0°.4 50°.5 31°.0

—— ——

16°.8 69°.8

48° 50′ Paris 8°.7 2°.6 51°.6 37°.8

—— ——

14°.5 64°.6

44° 39′ Halifax 5°.1 -3°.5 6°.1 43°.5 24°.2 13°.7

—— ——

13°.8 63°.0

44° 50′ Bordeaux 11°.2 4°.8 57°.2 42°.8

—— ——

17°.4 71°.2

40° 43′ New York 9°.1 0°.1 3°.4 52°.5 32°.2 8.0

—— ——

18°.2 73°.0

39° 57′ Philadelphia 9°.0 0°.1 52°.2 32°.2

—— ——

18°.1 72°.8

38° 53′ Washington 10°.2 1°.8 55°.0 36°.0

—— ——

17°.4 71°.2

40° 51′ Naples 12°.9 7°.8 61°.0 49°.5

—— ——

19°.1 75°.0

38° 52′ Lisbon 13°.1 9°.0 61°.5 52°.2

—— ——

17°.4 71°.2

29° 48′ St. Augustin 17°.9 12°.2 0.2 72°.2 59°.5 0°.4

—— ——

22°.0 81°.5

30° 2′ Cairo 17°.7 11°.8 71°.8 58°.5

—— ——

23°.4 84°.7

In the column of temperatures in the preceding table the first number represents the temperature of the year; that which stands in place of a numerator the mean winter temperature; and that which stands in the place of a denominator the mean summer temperature. Besides the great difference of mean annual temperature, there is also a striking difference between the two coasts in respect to the distribution of that temperature into the different seasons of the year, and it is this distribution which is most influential both on our feelings and on the processes of vegetation. Dove remarks generally, that the summer temperature of America is lower under equal degrees of latitude than that of Europe: (Temperatur tafeln nebst Bemerkungen über die Verbreitung der Wärme auf der Oberfläche der Erde, 1848, S. 95.) The climate of St. Petersburgh, (or to speak more correctly the mean annual temperature of that city which is in lat. 59° 56′), is found on the east coast of America in lat. 47½°, or 12½° more to the south; in like manner we find the climate of Konigsberg, (lat. 54° 43′), at Halifax, (lat. 44° 39′). The temperature of Toulouse, (lat. 43° 36′) corresponds to that of Washington (lat. 38° 53′).

It would be very hazardous to lay down any general statements respecting the temperature in the territory of the United States of America, as we must distinguish in that territory three regions:—1, the Atlantic States east of the Alleghanies; 2, the Western States in the wide basin between the Alleghanies and the Rocky Mountains, through which flow the Mississipi, the Ohio, the Arkansas, and the Missouri; 3, the high plains between the Rocky Mountains, and the Maritime Alps of New California through which the Oregon or Columbia River finds a passage. Since the highly honourable establishment, by John Calhoun, of uninterrupted observations of temperature, made on an uniform plan at 35 military posts, and reduced to daily, monthly, and annual means, we have arrived at more just climatic views than those which were so generally received in the time of Jefferson, Barton, and Volney. These meteorological stations or observatories extend from the point of Florida and Thompson’s Island, (Key West), lat. 24° 33′, to the Council Bluffs on the Missouri; and if we reckon amongst them Fort Vancouver, lat. 45° 37′, they include differences of longitude of 40°.

It cannot be affirmed that, on the whole, the mean annual temperature of the second or middle region is higher than that of the first or Atlantic region. The further advance of certain plants towards the north, on the west of the Alleghany mountains, depends partly on the nature of those plants, and partly on the different distribution of the same annual quantity of heat. The wide valley of the Mississipi enjoys at its northern and southern extremities the warming influence of the Canadian Lakes, and of the Mexican Gulf stream. The five lakes, (Superior, Michigan, Huron, Erie, and Ontario), occupy a space of 92,000 English square miles. The climate is much milder and more equable in the neighbourhood of the lakes; for example, at Niagara, (lat. 43° 15′), the mean winter temperature is only half a degree of Reaumur (1°.2 Fahrenheit) below the freezing point, while at a distance from the lakes, in lat. 44° 53′, at the confluence of the river St. Peter’s with the Mississipi, the mean winter temperature of Fort Snelling is -7°.2 Reaumur, or 15°.9 Fahrenheit (see Samuel Forry’s excellent Memoir on “the Climate of the United States,” 1842, p. 37, 39, and 102.) At this distance from the Canadian Lakes, (whose surface is from 500 to 600—530 to 640 English—feet above the level of the sea, whilst the bottom of the lakes Michigan and Huron is about five hundred feet below it), recent observations have shewn the climate of the country to possess a proper continental character, i. e., 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 of the Atlantic side.” At Fort Gibson, on the Arkansas River which falls into the Mississipi in lat. 35° 47′, with a mean annual temperature hardly equal to that of Gibraltar, the thermometer in the shade, and without any reflected heat from the ground, has been seen, in August 1834, to rise to 37°.7 Reaumur, or 117° Fahrenheit.

The statement so often repeated, although unsupported by any thermometric measurements, that since the first European settlements in New England, Pennsylvania, and Virginia, the eradication of many forests on both sides of the Alleghanies had rendered the climate more equable, (i. e., milder in winter and cooler in summer), is now generally doubted or disbelieved. Series of trustworthy thermometric observations in the United States hardly extend so far back as 78 years. We see in the Philadelphia observations, that from 1771 to 1824, the mean annual temperature has hardly increased 1°.2 Reaumur, (or 2°.8 Fahrenheit),—a difference which is attributed to the increased size of the town, to its greater population, and to the numerous steam-engines. The difference may possibly be merely accidental, for I find in the same period an increase of mean winter cold, amounting to 0°.9 Reaumur, or 2° Fahrenheit; the three other seasons had become somewhat warmer. Three-and-thirty years’ observations at Salem in Massachusetts shew no alteration at all: the annual means oscillate, within a degree of Fahrenheit, about the mean of the whole number of years; and the winters of Salem, instead of having become milder, as supposed from the destruction of the forests in the course of the thirty-three years, have become colder by 1°.8 Reaumur, or 4° Fahrenheit. (Forry, p. 97, 101, and 107.)

As the east coast of the United States is comparable in respect to mean annual temperature in equal latitudes to the Siberian and Chinese coasts of the old continent, so also the west coasts of Europe and America have been very properly compared together. I will only take a few examples from the western region on the shores of the Pacific, for two of which (Sitka in Russian America, and Fort George, in the same latitudes respectively as Gottenburg and Geneva) I am indebted to Admiral Lütke’s voyage of circumnavigation. Iluluk and Danzig are nearly on the same parallel, and although the mean temperature of Iluluk, owing to its insular climate and to a cold sea-current, is somewhat lower than that of Danzig, yet the winter temperature of the American station is milder than that of the port on the Baltic.

Latitude. Longitude. Reaumur. Fahrenheit.

Sitka 57° 3′ 135° 16′ W. 5°.6 0°.6 44°.5 33°.4

—— ——

10°.2 55°.0

Gottenburg 57° 41′ 11° 59′ E. 6°.4 -0°.2 46°.4 31°.5

—— ——

13°.5 62°.4

Fort George 46° 18′ 122° 58′ W. 8°.1 2°.6 50°.3 37°.9

—— ——

12°.4 60°.0

Geneva 46° 12′ (Alt. 1298 E. ft.) 7°.9 0°.7 49°.8 33°.6

—— ——

14°.0 63°.5

Kherson 46° 38′ 32° 39′ E. 9°.4 -3°.1 53°.2 25°.0

—— ——

17°.3 71°.0

Snow is hardly ever seen on the banks of the Oregon or Columbia river, and ice on the river lasts only a very few days. The lowest temperature which Mr. Ball once observed there in the winter of 1833 was 6½° of Reaumur below the freezing point, or 17.4° Fahrenheit (Message from the President of the United States to Congress, 1844, p. 160; and Forry, Clim. of the U. States, p. 49, 67, and 73). A cursory glance at the summer and winter temperatures above given, shews that on and near the west coast, a true insular climate prevails. The winter cold is less than in the western parts of the old continent, and the summers are much cooler. The most striking contrast is presented by comparing the mouth of the Oregon with Forts Snelling and Howard, and the Council Bluffs, in the interior of the Mississipi and Missouri basin (Lat. 44°-46°),—where, to speak in the language of Buffon, we find an excessive, or true continental climate,—a winter cold, on single days, of -28°.4 and -30°.6 Reaumur (-32° and -37° Fahr.), followed by mean summer temperatures of 16°.8 and 17°.5 Reaumur (69° and 71°.4 Fahr.)