Arid climate

“ The general climate is hot and arid, as is evidenced by the vegetation, although in the southern regions there are districts which have regular summer rains. As a consequence of the arid conditions the surface water is scarce, and is limited to isolated waterfalls or to springs from which small streams sometime flow for a short distance, and then sink into the earth. ”

“ At the present time, the arid regions of the old world are still largely suffering from having been overrun by the nomadic Turanians, whose original habitat—Mongolia and Turkestan—while also arid, does not permit of the ready realization of the advantages above outlined, on account of the rigorous climate brought about by altitude. ”

“ It is, upon the whole, not probable that so small an extent of forest, so situated, could produce a sensible influence on the general climate of the globe, though it might appreciably affect the local action of all climatic elements. The total annual amount of solar heat absorbed and radiated by the earth, and the sum of terrestrial evaporation and atmospheric precipitation, must be supposed constant; but the distribution of heat and of humidity is exposed to disturbance in both time and place by a multitude of local causes, among which the presence or absence of the forest is doubtless one. ”

“ The waters of lakes, therefore, and rivers, would be much hotter in winter, and would be never chilled in summer by melted snow and ice. A remarkable uniformity of climate would prevail amid the archipelagoes of the temperate and polar oceans, where the tepid waters of equatorial currents would freely circulate. The general humidity of the atmosphere would far exceed that of the present period, for increased heat would promote evaporation in all parts of the globe. ”

“ As a whole, the portion of the continent embraced in the temperate zone is characterized by its pronounced seasonal changes, including wide extremes of heat and cold over large areas, and by its frequently sudden and strongly marked weather changes during short periods of time. It is a highly suggestive fact that of all the great climatic zones the one having the most changeable climate, the greatest extremes of heat and cold, and the most frequent storms should be the one in which man has reached the highest development both of body and mind. ”

“ A region of deficient rainfall, once thickly settled and prosperous, may readily become an apparently hopeless desert, even without the intervention of war and pestilence, if man allows the climate to master him. In many cases the reports of increasing dryness really concern only the decrease in the water supply from rivers and springs, and it is well known that a change in the cultivation of the soil, or in the extent of the forests, may bring about marked changes in the flow of springs and rivers without any essential change in the actual amount of rainfall. ”

“ Surely these are serious obstacles to the reception of the hypothesis of change of ancient climate, by altered position of land and sea, on the ground of its being sufficient to meet the phenomena. In general, perhaps, we may venture to remark that it is unsafe to push the opinion of the possible average change of temperature in extra-tropical regions, beyond the extremes now observed therein. America, with little north tropical and wide north polar land, gives us a case of extreme refrigeration from the pole toward the equator ”

“ But although some species might thus be preserved, every great change of climate must be fatal to many which can find no place of retreat when their original habitations become unfit for them. For if the general temperature be on the rise, then there is no cooler region whither the polar species can take refuge; if it be on the decline, then the animals and plants previously established between the tropics have no resource. Suppose the general heat of the atmosphere to increase, so that even the arctic region became too warm for the musk-ox, and rein-deer, it is clear that they must perish ”

“ In contrast to the monotonous extremes of climate in the hot and cold zones, temperate lands are characterized by the intermediate degrees of annual temperature and marked seasonal diversity which are so favorable to human development. In Arctic lands labor is paralyzed by cold as it is by heat in the enervating and overproductive Tropics. In one, the growing season is too short and ill-favored; in the other, too long to stimulate man to sustained industry. ”

“ The dense clouds could store considerable quantities of heat in the equatorial belt through evaporation in their upper layers and the violent wind storms above the clouds would carry the aqueous vapours to colder regions where the heat would be liberated through new cloud formations. Currents in the oceans now largely attend to this heat transportation and give for instance to the coast of Norway, and indeed to the whole of Western Europe, its remarkably mild, and to life and civilization, propitious climate, but in the carboniferous age humid air currents fulfilled the same task. ”

“ The opposite extremity, the dry desert climate, is far more pronounced in the present time. This condition is well known in all continents except Europe, where we hardly93 can claim a desert but instead have steppes, with a vegetation abundant after the spring rains but fast disappearing with the arrival of the burning summer heat. A particular type of plant life has adapted itself to this periodic change from rain to drought, from bitter cold during the winter to parching sun during the summer. Perennial plants, and particularly trees, can rarely endure the rigors of such climatic upheavals. ”

“ The rate at which the edge of the advancing ice is wasted depends largely on the climate. If, while the rate of advance remains constant, the climate becomes warmer, melting will be more rapid, and the ratio between melting and advance will be increased. The edge of the ice will therefore recede. The same result will follow, if, while temperature remains constant, the atmosphere becomes drier, since this will increase wastage by evaporation. Were the climate to become warmer and drier at the same time, the rate of recession of the ice would be greater than if but one of these changes occurred. ”

“ The variation of temperature between day and night is considerable, but if the new-comer exercises a little care, he will not be unpleasantly affected by it. There are coast fogs, but these are not chilling and raw. Why it is that with the hydrometer showing a considerable humidity in the air the general effect of the climate is that of dryness, scientists must explain. The constant exchange of desert airs with the ocean air may account for the anomaly, and the actual dryness of the soil, even on the coast, is put forward as another explanation. ”

“ The 11-year period is plainly evident through most of the record and for some centuries is the predominant cycle, but for long periods other slightly differing cycles, such as 10 years, 12.6 years, or 13 years, are more evident. It is as yet impossible to say whether at these times there was a real change in the sunspot period, whether some subordinate period is operating in the sun, or whether only local conditions of some kind are the controlling factor.
The yellow pines of northern Arizona are dry-climate trees. They have a modified winter precipitation of the temperate zone. ”

“ In addition to the rainfall, the evaporation, and the temperature, there is another important climatic factor which should be taken into account, but which can not be stated in accurate terms, i.e., air-currents. It is a matter of common experience that one rarely observes a calm day on the desert, but that usually the wind, which is often of considerable force, is found to be blowing. This is unquestionably an important factor in raising the total of evaporation and therefore in increasing the arid conditions of this region. ”

“ But when the land-masses became more and more consolidated, when owing to the growth of the continents the warm ocean-currents found less ready access to arctic regions, then the temperature — 379 — of those regions was gradually lowered, until eventually the seas became frost-bound, and the lands were covered with snow and ice. But while the chief determining cause of climate has been the relative distribution of land and water, it is impossible to doubt that during periods of high eccentricity of the orbit, the climate must have been modified to a greater or less extent. ”

“ The chief effect of a varied relief is a varied climate. This changes with altitude in much the same way as with latitude. Heat and absolute humidity diminish, generally speaking, as height increases, while rainfall becomes greater up to a certain level. The effect of ascending and descending currents of air is to diminish the range of temperature on mountain slopes and produce rather an oceanic type of climate. The larger and more uniform a climatic district, the more conspicuously do even slight elevations form climatic islands, like the Harz Mountains in the North German lowlands. ”

“ But evaporation is profoundly influenced by the velocity of the wind; and the wind, which in the Tropics represents the strength of the atmospheric circulation, depends on the thermal gradient between the equator and the poles; since there is no evidence of any appreciable change of temperature over the Tropics as a whole, while there was a very great fall in cold temperate and polar regions, the thermal gradient, and therefore, ultimately, the tropical and sub-tropical, rain and snowfall must have been very greatly increased. ”

“ If we suppose the sun to have been hotter than it is at present to account for the vegetation which produced coal, then we ought to assume the sun to be colder than it is now to account for the glacial period. It is not reasonable to attribute such phenomena to fluctuations in the radiation from the sun. The glacial periods prove that we cannot appeal to geology in aid of the doctrine that a secular cooling of the sun is now in progress. The geological variations of climate may have been caused by changes in the earth itself, or by changes in its actual orbit ”

“ The land receives heat from them, but, instead of having the capacity of water for retaining it, it imparts it straightway to the air; and thus the proper climate, because it is the climate which the Creator has, for his own wise purposes, allotted to this portion of the earth, is maintained until the marine caldron of Cape St. Roque and the tropics is again heated and brought into the state for supplying the vapour and the heat to maintain the needful temperature in Europe during the absence of the sun in the other hemisphere. Thus the equable climates of Western Europe are accounted for. ”

“ In all these cases, as in those where the strata are vertical, much will depend on the thickness and resistance of the layers and on the strength of the currents concerned.
The Influence of Climate.
Climate has both a direct and an indirect effect on erosion. Its direct influence is through precipitation, evaporation, changes of temperature, and wind; its indirect, through vegetation. Like declivity and rock structure, climate does not affect all elements of erosion equally. ”

“ The moist chilling fogs, and drizzling weather, which prevail almost perpetually along the coast, must necessarily render the parts adjacent very unfit for all the purposes of agriculture.
It is natural to suppose, that the severity of the climate must be in due proportion to the general sterility of the soil, of which it is probably the cause. The first time we saw this country was in the beginning of May, 1779, when the whole face of it was covered with snow, from six to eight feet deep. On the 6th we had snow, with the wind from the north-east. ”

“ During much of the time smoke from forest and prairie fires (in the dry season) , dust (in deserts) , and water-vapour give the sky a pale whitish appearance. In the heart of the trade-wind belts at sea the sky is of a deeper blue. Twilight within the tropics is shorter than in higher latitudes, but the coming on of night is less sudden than is generally assumed.
Climatic Subdivisions.—The rational basis for a classification of the larger climatic provinces of the torrid zone is found in the general wind systems, and in their control over rainfall. ”

“ However, should there be sufficient moisture in the soil to permit absorption by roots, provided a slow rate was adequate to replace the transpiration loss, which rate was made lower by a more moist air, a slight rain would be of great significance, even if it did not penetrate to any appreciable depth. On the intensely arid desert such slight modifications of the water relations as the lowering of the temperature as winter approaches, causing decreased evaporation or rains, although actually small in amount, may be of large moment to plants. ”

“ We well remember how abruptly the temperature changes in the course of the day in the dry desert climate, while corresponding variation is comparatively slight in humid climates (compare page 86) . This is the result of the ability of water vapour to arrest the radiation from the Earth. Carbon dioxide is about evenly distributed over the globe—although somewhat149 sparser over highlands—and its heat-conserving and equalizing influence is, therefore, not so manifest as that of moisture. Only by the most accurate investigations has this influence been demonstrated. ”