CHAPTER X
CLIMATE
CHANGE OF SOLAR RAYS INTO LIGHT, HEAT, AND OTHER FORMS OF ENERGY AS THEY ARE ABSORBED BY OUR ATMOSPHERE OR AS THEY ENCOUNTER THE EARTH—TEMPERATURES OF WATER, EARTH, AND AIR—HOW SANITARY HOMES MAY BE CHEAPLY CONSTRUCTED BELOW GROUND, COOL IN SUMMER AND WARM IN WINTER
Difference between Climate and Weather. One may speak of the weather of to-day or of some time that is past, but not of the climate of to-day, or of any day, month, or year that is gone: for the climate of a place is determined by a study of its weather records for a long period of years. Climate changes so slowly that we speak of the movement as a mutation rather than as a change. The time that has elapsed since the discovery of the barometer and the thermometer—about two and a half centuries—is so short as to show little if any change in climate, while the weather changes from day to day.
The Sun Our Only Source of Appreciable Heat. Each one of the stars visible to the eye and many of the millions that are not visible, are suns accompanied by planets. Their conditions are similar to those of our sun, except that most of them are larger than our sun, some a million times larger. But their distance is so great that they exercise little or no influence in the heating of the earth. Light travels at about the rate of 186,400 miles per second, and yet these stars are so distant that if the nearest one had been created at the time of the signing of the Declaration of Independence we still would be in ignorance of its existence, for its first rays of light would not reach us for many years yet to come; and light from some of the remote suns that we call stars requires thousands of years to come. It is apparent therefore that we depend exclusively upon our own luminary for the heat that warms our atmosphere and gives life to the surface of the earth.
Different Temperatures with the Same Quantity of Solar Heat. On the same day of each year at the same place practically the same amount of heat falls upon and into the earth’s atmosphere from the sun, but rarely does the same temperature and weather occur, and often there is wide variation in the weather of the same day of two different years. The first of July may be cold enough to wear an overcoat at midday, or the first of January may be so temperate as to permit the donning of summer habiliments, while, according to the amount of heat received from the sun, there would have occurred the usual seasonal conditions on the days named had there been no other influence than the direct action of the sun’s heat. The cause of these seeming inconsistencies is due to the motions of the atmosphere in a stratum only five to seven miles in depth, air cooling by expansion as it ascends in cyclonic whirls and heating as it descends in anti-cyclonic movements. Condensation, in the form of cloud or rain or snow, also introduces complications, usually producing a cooling effect in summer and a warming in winter. In other words: interference in the uniform and gradual change in temperature, of the lower stratum of air in which we live, from the heat of summer to the cold of winter, and then the reverse process, is due entirely to the heating and the cooling of the lower air by its upward and downward motions.
Fig. 23.—Winter Solstice, December 21. North Pole is dark now instead of light, as at Summer Solstice. Pole leans in same direction but Earth being on opposite side of its orbit rays come from opposite direction. Refer to Figure 24.
If the earth’s axis were vertical to the plane of its orbit all places on its surface always would have days of twelve hours each and the nights would be of the same length; sunshine would just touch both poles (Figure 21) throughout the entire course of the earth around the sun and there would be no seasons. One would need to change one’s location on the earth in order to get a change of weather, which would be monotonous and quite different from the active conditions of the atmosphere that we now enjoy. The whole conditions of life would be altered for the worse. You have seen a top tilt over to one side as it spun on the floor. In the same way the earth spins on its axis as it pursues its course around the sun without changing the direction towards which its axis points, as shown by Figure 24.
Fig. 24.—Note that direction of axis does not change as Earth moves around Sun. This causes variation in area of surface illuminated. If axis were perpendicular to plane of orbit there would be no seasons.
Fig. 25.—As angle of incidence decreases from 90° to 10° the heat received on upper end of blocks is spread over greater area at bottom, and its temperature diminished. (Abbe.)
The intensity of the sun’s rays at sunrise and at sunset is less than at midday because the quantity of heat received at the outer limits of the atmosphere on a given area, as for instance at the area of the upper ends of the blocks in Figure 25, passes through a deeper stratum of air the lower the angle of incidence, and because it is distributed over a larger area when it reaches the surface of the earth.
As the heat of day increases from morning until midday and then decreases, so does the heat of the year increase from midwinter to midsummer and then decrease, and for the same reason: change in obliquity of the sun’s rays, to which must be added change in distance from the central luminary. Figure 26 shows that the sun reaches its greatest midday altitude on June 21st and its least on December 21st.
Fig. 26.—Observer at center of picture at latitude 45°. Showing altitude attained by the Sun at midday and length of its track above the horizon at the Summer and Winter Solstices and at the two Equinoxes.
Solar Rays Absorbed by the Atmosphere. The atmosphere of the earth absorbs about seventy-six per cent. of the solar rays that pass through it. About one half is absorbed by a cloudless atmosphere, and nearly all is absorbed or reflected away by a cloudy air. On the average about fifty-two per cent. of the earth’s surface is obscured by clouds all the time, which reduces the total amount of heat that reaches the earth to but twenty-four per cent. But in regions like the high plateau of the Rocky Mountains, where there is little cloudiness or moisture in the air, fully fifty per cent. reach the earth. At the equator, when the sun is in the zenith at noon, the rays strike the earth perpendicularly and reach the earth through the shortest air distance possible; but for latitudes far north or south of the equator, the rays are more oblique and must pass through an ever-increasing thickness of air as the latitude increases. Consequently the heat that reaches the earth at high latitudes decreases, not only on account of the greater obliquity of the sun’s rays, but also because of the longer path of atmosphere traversed, which causes a further loss by absorption.
The Lag of Earth Temperatures. The solar rays reach their greatest intensity on June 21st, in the Northern Hemisphere, when the sun attains the farthest point north, and the obliquity of its rays is the least, but the highest temperature of the air for the year does not occur on the average for a month or six weeks later, due to the capacity of the earth and air to absorb heat; and the maximum for the earth does not occur until still later. The sun is the farthest south on December 21st, but the minimum air temperature of the year, on the average, does not occur until a month later, and at a later period in the earth. At Munich, Bavaria, at a depth of four feet, the minimum annual temperature occurs on the 2d of March, and the maximum on the 24th of August. For each increase of four feet in depth the time of occurrence of either maximum or minimum temperature is retarded twenty-one days, the minimum not occurring until the 23d of May at a depth of 20.2°, and the maximum being retarded until the 17th of November.
Annual Range in Air Temperature. The difference in temperature between winter and summer increases from the equator northward and from all oceans toward the interior of continents, and is greater in the middle latitudes on the eastern side of large bodies of land than on their western side. Yakutsk, Siberia, has experienced 80° below zero in January and 102° above in July, making a range of 182°. Dawson, Canada, has a record of 68° below for winter and 94° above for summer, making a range of 162°. In marked contrast with these large differences, shown in the northern interior of continents, is the annual range at Samoa, from a maximum of 92° to a minimum of 62°, a range for the year of only 30° for this island of the Pacific, located near the equator.
Reversal of the Seasons in the Two Hemispheres. The summer is shorter in the Southern Hemisphere than in the Northern and the winter is longer, but the Southern Hemisphere is nearer to the sun in the summer and farther away in winter, conditions that tend to add to the extremes of both seasons. Because of the slowness of the earth in passing through one half of its orbit, the northern summer lasts ninety-three days, while that of the Southern Hemisphere lasts but eighty-nine days. The result is that during like seasons and during the whole year the two hemispheres receive exactly the same quantity of heat.
Only Water Vapor Protects the Earth from Death by Freezing. In Chapter IV you are told that the earth is surrounded by four atmospheres that conduct themselves each quite independently of the others, and that water vapor (aqueous vapor) is one of them. Water vapor plays the most important part in absorbing incoming rays and in absorbing and reflecting back outgoing heat rays from the earth. Without the vaporous atmosphere the sun’s rays would be but slightly absorbed as they entered and radiation from the earth would readily escape through the atmosphere to outer space. No matter how fiercely the sun might shine, life on the earth would be entirely destroyed by cold.
When water vapor, clouds, or dust motes intercept certain portions of the sun’s rays, they change them from vibrations in ether to the motions of molecules, and the motions of these molecules are expressed in a rise in temperature in the vapor, cloud, or dust. Earth radiations of heat, having longer and slower wave lengths than those that come from the sun, are more readily absorbed by the atmosphere.
One of the principal functions of the atmosphere is to protect the earth from the intense cold of outer space, which must be near or at absolute zero—459° below the zero mark.
Why Should Not Mountain Peaks Be Warm? They Are Nearer the Sun. The absorption by the atmosphere of both solar and terrestrial radiation is greater in the lower levels of the air, where water vapor, cloud, and dust are the densest, while the transmission of both incoming and outgoing radiation is more rapid through the pure air aloft. Thus we account for the coolness of all mountain peaks, and the perpetual freezing temperatures of some, even though they be located in the tropics, and though their tops occupy positions several miles nearer the sun than the bases from which they rise.
How the Earth Cools at Night. Radiation from the earth goes on day and night, winter and summer. During daylight the gain of heat is greater than the loss, while at night the reverse is true. After sunset both the earth and the air continue to cool by radiation unchecked by the incoming heat of the daytime. The earth loses heat, even under a clear sky, more freely than the air, with the result that the surface of the ground and of vegetation may fall to a temperature ten to fifteen degrees lower than that of the air at a few hundred feet elevation. This condition is called “temperature inversion.” The greater difference will occur when there is little wind to mix the air. On a clear night the radiation outward will be rapid; then, if the wind be light, there may occur an increase in temperature up to a height of two hundred to four hundred feet, and then a fall, reaching the surface temperature at about two thousand feet elevation, unless the ground be wet, or the location be adjacent to a considerable body of water.
A Cloud Covering Cools by Day and Warms by Night. One of the principal functions of clouds is to conserve the heat of the sun. A covering of cloud, fog, or dense haze may not only screen off the heat of day, but greatly retard the lowering of temperature at night by reflecting and radiating back to the ground much of the heat that it has lost.
The Temperature of Oceans, Lakes, and Rivers. The same quantity of heat falling upon different kinds of matter produces different temperatures, depending on the capacity (specific heat) of each kind of matter to absorb or hold heat; this is notably apparent when the matter is land, water, or air; for the same quantity of heat will raise the temperature of a water surface only about one fourth as much as it will a land surface. Water rejects by reflection a considerable amount of the solar rays that fall upon it, while land reflects but a small part; and of that which is received upon the top layer of water much is rendered latent in the process of evaporation and does not impart warmth to the water. Solar rays also penetrate water to a considerable depth and are quite uniformly absorbed by the whole stratum penetrated. These conditions cause large water surfaces and the air immediately over them to have a much lower temperature during the day and a much higher temperature during the night; and also lower temperatures during summer and higher temperatures during winter, than occur over a land surface of the same latitude.
Fresh Water and Salt Water Have Different Freezing Temperatures. In the ratio of 93.5 to 100 the specific heat of sea water is less than that of fresh water. Sea water is a better conductor of heat, so that it penetrates to a greater depth in salt water in the same period of time than it does in fresh water. Sea water regularly contracts with falling temperature until its greatest density occurs at four degrees below freezing, when it becomes solid ice and expands in the process of freezing; otherwise it would not float.
A Wonderful Phenomenon. In this respect a most wonderful and unexplainable phenomenon occurs with regard to fresh water. Not only sea water but practically all other forms of matter—liquid, solid, and gaseous—expand with increasing heat and contract with decreasing heat, except fresh water between 39° and 32°, which actually expands with falling temperature. It seems as though the Creator had gone over His work and made revisions and corrections here and there, for unless the law with regard to the contraction of liquids with falling temperatures had been reversed for fresh water between 39° and 32° our rivulets, streams, lakes, and rivers would freeze from the bottom upward and the life of inland water be wholly or partly destroyed.
Even more calamitous would be the floods of springtime, for melting snows and falling rains would spread over and erode the cultivated fields of the husbandman instead of being carried away by the open channels of streams, as is largely done now.
The Freezing of Fresh and of Salt Bodies of Water. The freezing of water does not take place upon the surface of water only, as many suppose. Congelation takes place about millions of minute atoms of matter carried by the water in suspension. Water expands in the process of freezing and each particle of ice, no matter in what part of the body of water it is formed, immediately rises to the surface because of the gain in its buoyance as it changes from the liquid to the solid form.
When the surface of water cools by radiation to a cooler air it gains in specific gravity and sinks and warmer water comes up to take its place and in turn be cooled and sink; thus a circulation is established which continues in fresh water until every part of the body of water has fallen to 39° and in salt water to 28°. At these temperatures the two waters reach their maximum density. With the further cooling of salt water particles of ice form and rise to the top, as already described. With the cooling of fresh water below 39° the law that holds good for all higher temperatures is reversed and expansion of volume begins, which continues until 32° is reached. Therefore, fresh water of any temperature between 39° and 32° may float upon water that is considerably warmer; in fact, it has less specific gravity at 32° than at 46°. At 32° that which was a liquid becomes a solid and still further suddenly expands its volume.
The Cold of Ocean Bottoms. Few have any idea of the enormous volume of cold water that lies upon the surface of the earth, three fourths of which is covered with oceans whose depths average two miles and in many places are five miles. Below one mile in depth these oceans are always at about the freezing point of salt water, which is 28°, except in the tropics, where it is but little warmer, varying between 34° and 36°.
How Temperatures of Inclosed Seas Differ from Those of Oceans. We will take the Red Sea as an example. It is 180 miles wide and extends in a nearly north and south direction for 1450 miles, about one half of it lying within the tropics. Evaporation takes place at a rapid rate, but only the surface water of the Indian Ocean on the south is able to enter to take the place of that which is lost, for a bar or sill at the entrance, extending from the bottom to within twelve hundred feet of the surface, separates the deep water of the sea from that of the outside ocean. Its surface temperatures vary about as the Indian Ocean, being 85° in summer and 70° in winter. Both bodies of water decrease in temperature at about the same rate down to the level of the sill, where the temperature remains constant the year through at 70°. Here a marked difference occurs, for the sea, which has a depth of 7200 feet, maintains the same temperature of 70° all the way down to the bottom; while the ocean continues to decrease in temperature down to a depth of about six thousand feet, where a temperature of 34° to 36° prevails throughout the year. A similar condition exists with relation to the Mediterranean and the Atlantic Ocean. At the top of the sill, which is 1140 feet below the surface, the temperature of both bodies is 55°, and this degree of heat is maintained all the way down to the bottom of the Mediterranean, while in the Atlantic Ocean, at the same depth as the bottom of the Mediterranean, the temperature is only 35°.
How the Temperature of Water Changes with Latitude, Season, and Depth. It is impossible to name a given temperature as prevailing over bodies of water at all places on the same parallel of latitude, because ocean currents soon move water heated in one latitude to a higher or a lower position. At the equator the surface temperature is between 82° and 84°; it changes less than one degree between day and night, and not over five degrees between winter and summer; and below twenty-four hundred feet there is no difference between the seasons, the daily variation ceasing at less than a hundred feet. Below six thousand feet the temperature is always near the freezing point of fresh water.
In the middle latitudes the surface variation is from 50° in winter to 68° in summer.
At latitude 70° N. the surface temperature has but a small daily variation, and a yearly range of from 35° for winter to 45° for summer; at a depth of twenty-four hundred feet it remains steady at 32°.
From this level there is a gradual decrease to a depth of six thousand feet, where a constant temperature of 28° exists, and below this there is no change. The temperature of Lake Superior decreases down to a depth of two hundred forty feet, where a temperature of 39° continues throughout the year, as it does downward for the remainder of the distance to the bottom, which has an average depth of nine hundred feet.
Direction of Wind Affects Shore Temperature of Water. Onshore winds skim off the warm surface water and drive it shoreward, where it banks up, and, pressing downward, causes the colder water beneath to flow back seaward. In like manner, offshore winds blow off the top water near the shore and send it out to sea, and colder water rises to take its place.
Great Heat of the Earth’s Interior. We are ignorant of the conditions of matter under the heating effect of the enormous pressure that exists near the center of the earth, but it is probable that pressure prevents it from changing from a solid to a liquid or a gaseous form. The surface of the solid earth rises to a much higher temperature as the solar rays fall upon it than does a water surface, or the air immediately above, because it is a poor reflector, a poor conductor, and a poor radiator, and when dry does not get any cooling effect from evaporation. Solar heat ceases to be apparent at a depth that varies with the latitude and the conditions of the soil with regard to moisture and specific heat, but everywhere at less than fifty feet.
At the poles and for some distance away the earth is covered with ice or snow the entire year and is frozen to a considerable depth. In the interior of Siberia and some parts of Alaska only a thin stratum of soil thaws out under the heat of summer. Beginning at about fifty feet, there is an increase of temperature downward, but it is not the same for all places, varying from a degree for forty feet to a degree for one hundred feet. Taking the average of the increase with depth, water would boil at ninety-five hundred feet and the hardest rock be molten at thirty miles. At a depth of 3490 feet near Berlin, the temperature was found to be 116°, while it was only 108° at the same depth at Wheeling, West Virginia, and in both places there is no change from day to night or from winter to summer.
Soil Usually Warmer Than Air Next Above. In summer, June to August, the bare, dry, top soil is warmer than the air ten feet above during all hours of the day and night, at times the difference being as much as forty degrees at midday. During winter, December to February, it is slightly cooler, except between 9 A.M. and 3 P.M. when the excess is seldom more than ten degrees. Evaporation from a wet soil lowers its temperature below that of the air immediately above through the rendering latent of a large quantity of heat. A melting snow surface also is below the temperature of the air because of the heat employed in changing the snow to the liquid form.
Let Mother Earth Cool and Refresh You During the Heat of Summer. How little the average man realizes the possibilities for improving his condition that lie close at hand. He does not know, or he is indifferent to the fact, that only three feet from the surface of the ground it is as cool at midday as at midnight, and that there is no diurnal variation in temperature below that depth, and no annual variation below a depth of from thirty to forty feet. If one were to set down the temperature of each day, add the numbers at the end of the year, and divide the sum by 365 the quotient would equal the temperature always found at that place at a depth of about thirty feet. The temperature of a deep-flowing spring is always about the mean annual air temperature of the place. Here is health-giving coolness for summer and warmth for winter of which one takes little heed and derives practically no profit.
Remarkable, is it not? And these beneficent conditions are universal and available for all, except to those crowded into congested centers of population. The temperature is 54° in the Mammoth Cave in Kentucky and shows no change from day to day and from winter to summer.
During the extreme heat of summer and the cold of winter many could profitably, healthfully, and pleasantly live below ground. During such periods the cellar of the house, which should be deep and spacious, even extending beyond the dimensions of the edifice above, if a continuous supply of pure air could be forced through it, or natural ventilation accomplished by the plan outlined below, should be the lounging, resting, and sleeping place of the occupants of the household. It is not impossible or extremely difficult to change the stagnant, moist, germ-laden, ill-smelling air of the average cellar, in which it is positively dangerous to spend much time, into active, pure, and delightfully healthful air,—air in which the worn and weary worker from the heat of the farmer’s field, or the artisan and the clerk from the debilitating temperatures of the factory and the office could recuperate from the toil of the day, and from which they would go forth each morning invigorated for another day’s efficient service, instead of dragging weary limbs from hot, sleepless beds, each morning less in energy than the day before. As is shown in other parts of this book, the researches of Huntington have proven conclusively that man is at his lowest physical and mental points of efficiency, and more subject to the contraction of disease through weakness, in midsummer and midwinter, and that the hotter the summer and the colder the winter the less is his energy and the lower is his power of resistance.
The whole problem is one of ventilation. While this is simple, it must be scientifically done. The ideal location for a living cellar is a hillside. It is easy to install ventilators in the roof of a cellar no matter where located, but these are of no avail whatever if there is not adequate air drainage at the bottom of the cellar. From the cellar in a hillside a conduit can lead from the bottom of the inclosure and have its opening at a lower level, thereby accomplishing drainage and circulation, which are all-important in the creating of a sanitary condition of air under the cool earth. For each thousand cubic feet of cellar space there should project from the roof, to a height of at least six feet above ground, a separate ventilator shaft of at least one square foot cross-section dimension. A like ventilating capacity should be provided from the bottom outward to a lower level, but here two or more shafts may be combined in one, so the proper capacity is secured. During the day the draft will be upward through this system. But at night, except when the wind is brisk, the direction of movement of the air is reversed, and the cool air of the minimum temperature of night or early morning, because of its greater density, drops down into the cellar. The drainage shaft should be provided with a damper, which should be closed in the early morning, about daybreak, entrapping the cold air of night. The lower opening should be covered with wire netting, to exclude small animals, and the whole construction be of concrete, rendering it imperishable and rat-proof.
Inexpensive but Efficient Cold Storage. Such a sanitary cellar as described above provides an excellent storage for fruits and vegetables, comparing favorably with the much more expensive artificial refrigeration. By an intelligent manipulation of the damper in the lower shaft, cool storage may be provided for fruit and other produce in the early fall, and protection secured against the extreme cold of winter.
Why Does Air Cool with Ascent and Heat with Descent? If a mass of air be elevated 183 feet it will be found to have lost one degree in temperature, because there is less air above to exert pressure upon it and it therefore expands to greater volume, and in the process of expansion work is performed which employs heat and renders it latent. One minute, one hour, or a thousand years thereafter, if this same air be lowered to its former elevation, it will be compressed into its previous dimensions and the heat energy that formerly was employed to expand it will be restored to the sensible condition. This ratio of 183 feet to one degree does not hold for any extended movement, because, as soon as the dew point of the air is reached, condensation in the form of cloud or rain occurs and the heat of condensation is released; that is to say, the same quantity of heat employed to create the water vapor at some previous time and thereby rendered latent is now become sensible and partly makes up for the loss by expansion as the air ascends. The average is therefore about three hundred feet for one degree.
Height of Freezing Cold in the Free Air. The frost level remains constant, winter and summer, over the equator at about eighteen thousand feet. Elsewhere this level rises and falls with the seasons, the amplitude of the movement increasing with latitude and being greater over land than over water on the same parallel.
Daily Range of Temperature in the Free Air. The difference between the temperature of day and that of night decreases with altitude in the free air and ceases at about eight thousand feet. It is greatest during clear weather and least in cloudy weather. Narrow valleys may show a greater daily range than hilltops. When the sky is clear, radiation from the hillsides may heat the air in a valley to almost furnace heat at midday, while at night the air, coming in contact with cool vegetation higher up, chills and, gaining in weight by contraction, flows down and collects in the valley, making the bottom of the valley warmer during day and colder during night than the air above. Often moisture-laden winds precipitate much of their water vapor as the air cools by expansion in passing over a mountain range. These winds carry a comparatively dry air over to the leeward side of the mountain, where the daily range of temperature will be much greater than on the windward side at the same elevation. San Francisco, where the prevailing winds come from the ocean, has a less range than New York, where the predominating winds are from the land; but New York is influenced by its proximity to the ocean, for its range is much less than at Denver, in the interior of the continent. The range is less on the east side of Lake Michigan than on the west side, as it is with relation to all similar bodies of water.
Man Soon Adjusts Himself to Changes in Altitude. In Colonial days it was noted that horses coming down from the mountains in North Carolina ran swifter in the races the first day or two after changing to a lower level. In going to a higher altitude an increase in the number of red corpuscles in the blood enables it to absorb oxygen more readily, and thus compensate for the loss in the density of the air. Because of this gain in the chemical activities of the life current, one feels a marked increase in strength on coming to a lower level, but the gain lasts for only a short time before there is a readjustment to former conditions. Persons with weak hearts may not be able to live at an altitude of four thousand feet, and most people experience inconvenience, at least, on first reaching ten thousand feet; but nature is accommodating, and a number of large cities prosper at altitudes of from one to two miles.