CHAPTER XI
HOW CLIMATE IS MODIFIED AND CONTROLLED
If the surface of the earth were all land, and the axis of the earth’s rotation were perpendicular to the plane of the earth’s orbit, the day and the night would be equal everywhere, and there would be no seasons. There would be no wind, for the friction of the air against the rotating earth would soon cause all levels of the atmosphere to take up the exact easterly velocity of the solid body below. The atmosphere would be contracted by cold and drawn downward so as to have less depth at the poles than at any place having latitude, and it would be deepest at the equator, where the direct rays of the sun would expand it to an altitude of probably twice what it could have at the poles. Centrifugal force—the force that causes mud to fly off the rim of a swiftly rotating wagon wheel—would further lower the height of the atmosphere at the poles and cause it still more to extend outward at the equator. The atmosphere would soon adjust itself to these constant conditions and forces and thereafter remain at rest relative to the earth. There would be no life, for there would be no vaporous atmosphere if the surface all were without water. There would be extremely little heat to disturb the atmosphere with motion, for the dry gases of the atmosphere are practically diathermanous, and the heat of the sun would pass out by radiation from the burnt and parched surface of the earth during daytime without imparting more than a minute fraction of its energy to the atmosphere; and at night the thin surface of the top soil that had been heated to a furnace temperature during sunshine would be quickly locked in the fastnesses of intense cold—probably 200° below zero.
If we now incline the axis of our imaginary earth 23½°, we shall introduce seasons whose only change, the one from the other, will be in the duration of sunlight, as there is no water vapor to absorb and utilize the sun’s rays in the initiation of motion and the creation of storms. We are assuming that there would be enough heat absorbed to prevent the atmosphere from liquefying, which it would do at any temperature lower than 312° below zero. If the temperature were to fall below the liquefying point of air, we would have the singular phenomenon of the air expanding to a gas during daylight and condensing to a liquid during nighttime, and, of course, that would mean motion and winds, but of such a nature that one would hardly dare speculate as to their peculiarities.
We introduce these hypothetical cases for the purpose of conveying a clearer idea of the overlapping of conditions and the combinations of forces that influence and control the seasons, the climate, and the weather of the earth.
If the surface of the earth were all water and its axis perpendicular to the plane of its orbit, the day and the night would everywhere be equal and there would be no seasons. With a water surface there would be an atmosphere nearly if not quite saturated with vapor of water, in other words, of practically one hundred per cent. relative humidity. It is doubtful if either animal or vegetable life could exist; the first would die of internal heat, because a saturated air would permit of no cooling by evaporation from the pores of the skin, or from the tongue and mouth of animals that do not perspire; and the second could not grow without the chemical action of sunshine, which is a necessary part of the laboratory of the leaf of every growing plant, the sunshine acting upon the green granular matter which constitutes the chlorophyll of the plant. There would be little difference between the temperature of day and of night—probably not more than one degree. As the earth would everywhere and at all times be covered with a deep stratum of cloud there would be little loss of heat to space by radiation and the temperature would be excessive, rising in the tropics to near the boiling point. We will assume that the atmosphere would reach a stable and unchanged condition of great heat and humidity and be without motion or precipitation.
If now we incline the axis of this water-covered earth and introduce the complication of seasons, we shall not only have variation in the hours of sunshine, increasing as we go from the equator toward the poles, but, the capacity of air for moisture being less and less with falling temperature, we shall have downpours of rain as the summer slowly merges into fall and the latter into winter. Although the air will be saturated, there probably will be no rainfall from the time when the temperature begins to rise after midwinter until it reaches and passes the maximum heat of summer. It is fair to assume that during the rainy period there will be cyclonic storms with torrential precipitation, and that the anti-cyclones that are a necessary concomitant of cyclones, while they may cause a temporary cessation of precipitation in the area that they cover, by the dynamic heating of the air in their downward motions, will be ineffective in fully clearing away the clouds from a water-covered earth. It is doubtful if such an earth would be suitable for life,—certainly not for man.
The Real Earth of Land, Water, and Inclined Axis. The different manner in which land and water surfaces absorb, radiate, and reflect the heat from the sun has a profound influence on climate, which also is modified by latitude, elevation above sea level, elevation above a valley or above a surrounding plane, direction of wind, height and trend of direction of hills and mountains, the position of lakes and inland seas, the relative position and magnitude of continents and oceans, storm tracks, and ocean currents.
Influence of Continents and Oceans on Climate. Charts 1 and 2 (pages 99 and 100), constructed from observations taken on ships and on land, for a long series of years, show certain Highs and Lows of vast extent, sometimes called “Centers of Action”, because they do not travel across continents and oceans, as do the migrating Highs and Lows that cause weather. Rather do they slowly reverse their relative positions between winter and summer. Continents cool by radiation in winter more rapidly than do oceans; the air contracts, settles down and grows denser and air flows in at the top from the oceans and outward at the surface of the earth toward the oceans; thus is built up the winter Highs, or centers of action, on continents. Continents heat up by absorption in summer more rapidly than do oceans; the air expands, rises, and flows away in the upper levels to oceans and flows in at the bottom from the oceans; and thus are the Lows, or centers of action, established on continents in summer. It is apparent that these processes must be reversed for the oceans, and that the Highs will be found there in the summer and the Lows in the winter. Carefully follow the illustrations of these principles by examining the whole region north of the equator on Charts 1 and 2.
In the Southern Hemisphere there is not such a pronounced shifting of the Highs and the Lows from oceans to continents and back again, with change in the seasons, as there is in the Northern Hemisphere, because of the small area of land in comparison with that of water; but in the midst of the southern summer, which occurs in January (Chart 2), Lows are shown over South America, Africa, and Australia. Note how the winds blow out of all the Highs and into all the Lows. Also observe that the winds generally blow from about latitude 30° north and south towards the equator, due to the great heat of the tropics, which causes the air to rise and in the high levels to flow northward and southward, settling down to the earth again through the belts of high pressure that irregularly encircle the earth at latitudes 30° north and south.
In the interior of continents the temperature falls lower at night and rises higher during the day, and falls lower in winter and rises higher in summer than on any of their coasts. On the coast of central California, for instance, the ocean is so cool in summer and the winds blow so steadily from it that the thermometer ranges between 55° and 70°, even when there are temperatures of over 100° but a few hundred miles away in the great interior valleys, or on the broad plateaus of the mountains. New York and Boston, in nearly the same latitude, also have their summer temperatures modified by ocean influence, but they are on the east side of a broad continent, where the prevailing westerly winds give to them more the character of a continental climate than one marine; but occasionally the east wind, for a short time, gives to them the modifying influence of the ocean. In the winter the influence of the oceans is to modify the extremes of cold, the same as they do the excessive heat of summer.
Chart 8 (page 129) showing the lowest temperatures ever recorded at Weather Bureau stations, and Chart 12, presenting the average of the highest daily temperatures of July, graphically show, clearer than any text can describe, the influence of continents and oceans on climate. On the Atlantic the average maximum of day varies from 70° on the Maine coast to 85° on the coast of North Carolina; while on the Pacific, where the marine influence is stronger, the average is from 65° on the Washington coast to 80° on the coast of southern California. But near the center of the United States where the continental influence predominates, the average of the highest daily temperatures varies from 85° to 90°. On Chart 8, showing the lowest temperatures, the line of 20° below zero passes through Boston, southwest to Chattanooga, west to Flagstaff, Arizona, and then irregularly north to Seattle, showing the influence of both oceans in carrying the line northward.
Because of the vast extent of the Eurasian (Europe and Asia) continent the difference between continental and marine climates is more marked than in the Western Hemisphere. Huntington and Cushing, in their splendid work on “Principles of Human Geography”,[3] make a comparison between the southern Lofoten Islands, off the coast of Norway, and Verkhoyansk in Siberia, which probably furnish the greatest contrast to be found anywhere between places of the same latitude. Although both are inside the Arctic Circle, the influence of the Atlantic Ocean with its warm-water currents coming all the way from the tropics (Chart 13) protects the Lofoten Islands from the extreme cold that otherwise would come to them; vegetation remains green and cattle are pastured every month in the year. But the ocean retains nearly the same temperature in summer as in winter, and as a result the Islands are too cold to grow trees or many crops. Verkhoyansk is so different that one can scarcely believe that both places are in the same latitude. At the Siberian town the winter temperature falls to 70° or 80° below zero every winter, and has been known to register 90° below zero. It is said that steel skates often will not “take hold” but slip sideways as well as forward on the surface of the excessively cold ice. This doubtless is due to the fact that under ordinary winter cold the weight of the skater melts a thin film of water under the edge of the skate, which freezes instantly when the skate passes and relieves the pressure. But here the cold is so intense that the weight of no skater is sufficient to lubricate his movements with water molecules. Remarkable to relate, the summer at Verkhoyansk is warmer than in the islands off the Norwegian coast, due to the rapidity with which the land surface warms up under the action of the solar rays in the midst of a continental area remote from water, 75° to 80° frequently being recorded during the long summer days. The ground never thaws for more than a foot or so, but a number of crops are successfully grown.
In the interior of a continent like that of Siberia or of North America not only the changes from season to season but from day to night are extreme; while in mid-ocean the diurnal and the annual range of temperature is small, day and night, winter and summer being much the same. A place is influenced by the ocean in proportion to its distance from the sea, the presence or the absence of hills or mountains between the place and the water, and by the fact that the prevailing winds come from or go to the ocean. Cities as far inland as Baltimore and Philadelphia have their extremes of temperature somewhat modified by the Atlantic Ocean, and if it were not for the Coastal and the Sierra Nevada Mountains the influence of the Pacific Ocean would be felt at least as far inland as Denver, and the great Rocky Mountain plateau would be one of the garden plots of the world. The influence of the Pacific would reach inland farther than now does the Atlantic because of the prevailing westward drift of the atmosphere in all middle latitudes.
Exaggeration of the Forest Influence on Climate. Chapter XIII, on Change of Climate, shows more in detail the process whereby the sun lifts up the water vapor from the Gulf of Mexico and the Atlantic Ocean and how cyclonic storms draw this vaporous atmosphere northwestward far into the interior of the continent, the Alleghany Mountains not being high enough to offer serious obstruction.
The writer would again caution the reader not to be misled by any pseudoscientist, no matter how worthy his purpose may be, who would teach that the operations of men in changing forest areas to cultivated fields, gardens, villages, and cities, has in the slightest degree harmfully affected the climate, or augmented floods or intensified droughts. A field of grass, of wheat, of corn; an orchard of fruit; a highway bordered with towering, majestic oaks and elms; or a grove of cultivated trees about a prosperous home is just as beneficial to the climate as the thickest and most impenetrable forest and far more pleasing to the eye and helpful to mankind. Forests should be protected, conserved, and grown because we need timber, not because a lot of foolish people are writing nonsense about them.
Influence of Lakes and Rivers. With the exception of contributing to the formation of occasional fogs over their surfaces and the adjacent low lands, through the rising of warm water vapor into the cold air that often collects at the bottom of valleys during nighttime, rivers exercise little influence on climate. Lakes exert a modifying influence on the temperature of places near their shores but only for a few miles therefrom, and they are too small to exert any appreciable influence on rainfall. If one examine charts showing the average rainfall for the United States by seasons, he will observe that the amount gradually shades off as the distance from the Gulf or Ocean increases, without any relation whatever to the five Great Lakes. Deserts exist on either side of the Caspian Sea, although it slightly increases the rain of the Elburz Mountains to the south. If these great bodies of water do not influence the rainfall, how ridiculous to assume that the changing of forest areas to other forms of vegetation possibly can affect precipitation or influence droughts. Stress is laid on the fact that some land is left bare and then is eroded into deep gullies. This is true, but the fault is one that may be corrected by a proper system of plowing and cultivation. And at most the area so eroded is so infinitesimal in comparison to the vast regions changed from forests to growing crops as to be negligible.
Influence of Ocean Currents on Climate. Climates are markedly influenced by the currents of oceans. Charts 15 and 16 show the normal wind circulations of the globe; note that the centers of the great swirls are coincident with the location of the High and the Low centers of action located on Charts 1 and 2. Next observe Chart 13, showing the ocean currents, and it will be seen at once how closely the circulation of the great ocean currents follows that of the winds, due to the friction of the air upon the water, and to the interposition of bodies of land that turn about or deflect the currents.
Water has a greater capacity for heat than nearly any other substance. It requires ten times the quantity of heat to raise a pound of water one degree that it does a pound of iron. The oceans therefore store up vast quantities of the heat of the sun and, unlike the continents, distribute this heat northward and southward without regard to latitude. Much of the heat of the tropics is thus transported far northward and southward from the equator. The extensive eddy-like circulation of the south half of the North Atlantic Ocean sends currents northward along the coast of the United States which set eastward at latitude 40°. A part of these reach the coast of Spain and then turn south; the greater part spread out in mid-ocean and move northeast, bathing the coasts of the British Islands, Iceland, and Norway. They still retain some of the heat that they absorbed from a tropical sun, and they therefore give to the coasts that they reach a higher temperature than they would have if the ocean waters were moving from the north, or than they would have if there were no currents at all. On Chart 14 note how the isothermal lines are carried northward by these currents as they cross the Atlantic Ocean. The Gulf Stream mingles with these northeast currents but adds little to their temperatures, for the general ocean circulation would produce practically the same effects if there were no Gulf Stream.
Follow the currents down the coast of Spain and of northeast Africa; then note on Chart 14 the southward trend of the lines of equal temperature, as the currents bring colder water southward to cool the air. Next examine the currents of the Pacific and the isothermals. The currents moving northward towards the equator along the west coast of South America, and those moving southward, also toward the equator, along the west coast of the United States and Mexico cause a bulging of the isothermal lines from the positions that they would occupy if there were no currents coming from colder regions.
Influence of the Gulf Stream on Climate. From either side of the equator the surface winds (Charts 15 and 16) blow the water westward, causing what are known as the “Equatorial Currents” (Chart 13). The eastward projection of the coast of South America divides the Atlantic equatorial current into two parts; one goes south along the coast of South America and sets up the circulation in the South Atlantic, which sweeps north along the southwest coast of Africa. The other passes to the northwest, a part setting up the North Atlantic circulation and the remainder sweeping through the Windward Islands and storing itself in the Gulf of Mexico, whence it is driven out at a velocity of some five miles per hour through the narrow channel between Key West and Cuba. Here it has a depth of half a mile and a width of forty miles. Its velocity is accelerated because it enters the Gulf in a broad sweep and passes out through a constricted channel. It retains its individuality as a warm river passing through the ocean because of its greater velocity and higher temperature than the waters in which it finds itself soon after it leaves the Gulf; but it gradually merges with the great Atlantic circulation as it passes to the middle of the ocean. It is the opinion of the writer that its influence on climate has been exaggerated, that the warming of Europe that is credited to the Gulf Stream is accomplished by the mere presence of the ocean to the westward and to the general circulation of that ocean without regard to the wonderful phenomenon known as the Gulf Stream.
Effect of Valleys on Day and Night Temperatures. Valleys affect temperatures in proportion to their depth and width. A deep, narrow valley might have the effect illustrated by Figure 27, if the time were summer and the sky clear. During the daytime radiation would warm the interior so that the bottom of the valley would have a much higher temperature than the free air at the top of the valley, and the movement of the air would be sluggishly down the center and up the sides of the depression. During nighttime all the conditions would be reversed. Vegetation, losing heat by radiation much faster than the air, would cool the latter as it came in contact with the sides of the valley. The air would slowly descend along the sides through gain in specific gravity and collect at the bottom with a temperature much lower than it had when it started its descent.
| Summer day temperature in a narrow valley. | Summer night temperature in the same valley. |
Effect of Mountains on Climate. The rarity of the atmosphere of mountains readily allows the rays of the sun to pass through it and thus the surface of mountains is quickly warmed, but the same conditions permit a rapid radiation at night, so that there are considerable extremes of temperature. Air cooled by contact with a mountain may flow down its sides at night and collect in depressions below, often causing frost on still nights where the temperature higher up is much above freezing. Mountains may be more cloudy and rainy than plains, for the currents of air that cross them must rise, and in rising they cool by expansion and often reach the dew point of the air, moisture being precipitated in the form of clouds, rain, or snow. Often a peak is constantly capped with a crown of clouds. Mountains may intercept vapor-bearing winds from oceans, force them to such an elevation that their vapor is largely precipitated on the windward side of the mountain, and receive them on the leeward side as dry, rainless winds. Vast desert areas are often the result. A good example is presented in the case of the Pacific coast mountains and the desert plateau to the east.
Mountain peaks may be covered with snow, even though they be located in the tropics, if their elevation be sufficient. This is because the absorption of both incoming and outgoing radiation is so much greater in the lower reaches of the atmosphere, where the water vapor is densest. Wherever observations have been made they have shown that the temperature of the air on high mountain peaks and crests and for a distance of one to three hundred feet above them is cooler than adjacent free air of the same height, due to upward deflection of air currents and their cooling by expansion, and to radiation from the peak.
The Himalayan Mountains exercise a profound effect on the climate of Asia. The monsoon (any wind that alternates annually in direction or force) of summer brings the moist air from the Bay of Bengal and precipitates torrential rains from it as it ascends to higher and higher elevations in passing over the great heights of the mountains. At a place four thousand feet above the sea and not distant from Calcutta, the annual rainfall is 466 inches, while the average for most of the region east of the Mississippi River is only forty inches. More than forty inches have been known to fall in one day in the Himalayan Mountains. As in the case of all very high mountains, the rainfall increases in these mountains up to a certain elevation and then decreases. North of the mountains the monsoon passes into the interior of Asia with withering dryness and vast deserts are the result.
Figure 28 graphically presents the average monthly temperature and rainfall of typical places in North America, and Figure 29 of places in the Old World. Here may be seen every phase of climate from tropical to temperate and to cold, and from marine to continental. By studying the winds on Charts 15 and 16 and the ocean currents on Chart 13, the reader should be able to find an explanation for the different conditions shown. For example: Mazatlan and Vera Cruz are both on the coast of Mexico, the first on the west and the latter on the east. Each has a rainy period in the summer, but at Vera Cruz the rain begins earlier and lasts later and is much heavier. The reason is that they both have north winds in winter (Charts 15 and 16), but in summer Vera Cruz receives winds direct from the Gulf of Mexico and at Mazatlan the winds continue to blow from the north, with but a slight inclination landward. Again, the explanation for the fact that Mazatlan has a monthly range of temperature from 60° in winter to 80° in summer, while Vera Cruz has a range of only 70° to 80° is found in the wind direction.
The City of Mexico is wonderfully favored by climate. Here a moderate rainfall occurs from May to September. The oceans are not far distant on either side, as distances are measured continentally, but its great elevation on a table-land relieves it of the torrential rains usual to the tropics; and yet it is close enough to marine influence so that its air has not the nerve-irritating dryness of the plateau of the Rocky Mountains, and it has a remarkable evenness of temperature between winter and summer, with a monthly range between 50° and 60°. Its range between day and night is sufficient to be stimulating.
Still looking at Figure 28, note the remarkable similarity between the climate of Pittsburgh and Toronto. Each has about the same rainfall and it is almost equally distributed throughout the months of the year. The only difference is that Toronto is a little colder. St. Paul and Kansas City, typical of the climate in the interior cities, have a small amount of precipitation in winter, considerable in summer, and a wide range of temperature; while the Pacific coast cities have dry summers, and winters that vary from three inches of rain at Los Angeles to fourteen inches at Astoria, with no excesses in temperature.
Temperatures Aloft in the Atmosphere. Kite and balloon observations have not been continued long enough, nor have they been made at a sufficient number of places, to give one the data from which the climate of any considerable altitude in the free air may be determined, but from a large number of free balloon observations made with self-recording instruments, in the middle latitudes of this and foreign countries, Figure 1 (page 12) has been constructed, which shows the manner in which the temperature decreases with elevation up to eighteen kilometers (eleven miles). Note how rapidly it falls with elevation up to eleven and a half kilometers (about seven miles). This depth of air measures the thickness of the turbulent stratum in which cyclones and anti-cyclones operate. At its top the temperature always is about 64° below zero in winter and 70° below in summer. And right here occurs a most wonderful phenomenon,—one of which scientists were entirely ignorant less than two decades ago. At first it was thought that there was something wrong with the recording thermometers, for they failed to register falling temperature with gaining altitude after the storm stratum was passed at seven miles. Then it was noted that all instruments displayed the same peculiarity, and the “Isothermal Stratum” (equally heated region) was discovered, in which the temperature maintains the same degree of intense cold so far as exploration had been made. From Mount Weather, under the direction of the writer, a balloon was flown to nineteen and one tenth miles before it exploded and sent a parachute gently down to earth with its precious record. This flight showed practically no change in temperature after the isothermal stratum was reached. (See Chapter III.) One is reasonably safe in assuming that there is no oxygen beyond an altitude of thirty miles and that at fifty miles the nitrogen becomes inappreciable, and that, therefore, the temperature must shade away to practically nothing when the void of outer space is reached, notwithstanding the presence of the newly-discovered isothermal stratum nearer the earth.