THE AIR; THE DISTRIBUTION OF WARMTH
Disregarding the very slight amount of heat radiated from the earth's interior to the surface, and also that received from other heavenly bodies, the sun must be regarded as the source of the heat received at the earth’s surface. The greatest intensity of heat is received in equatorial regions where the sun’s rays are practically vertical; the least intensity is in polar regions where the rays fall obliquely.
The inclination of the earth’s axis to the plane of the
Relative length of day and night.
Polar and tropical circles are the boundaries, not of climatic, but of light zones. The duration of daylight is of great importance; it governs, in no small degree, the maturing of crops, and therefore concerns practically all agricultural industries. In general, the regions of greatest productivity of staple food-stuffs are those in which the summer days are from 14 hours to 16 hours long. Both the navigator and the aviator must know whether he heads in the direction of increasing or of decreasing hours of daylight at any particular time of the year.
Climatic Zones.—Climatic zones correspond pretty closely to light zones, in position; but their boundaries are very irregular lines, called isothermal lines—that is, lines along which the annual mean temperature is the same. For all practical purposes, the climatic torrid zone is the zone where frost does not occur except at very high altitudes. Similarly, the southern-most line at which frost may occur is the southern boundary of the north temperate zone; and the line of mean temperature of 32° (0° C) may be considered its northern boundary. A more practical boundary is sometimes fixed at the northern limit at which barley will mature.
Climatology is chiefly concerned with the regions which will produce foodstuffs, and therefore sustain life. To a lesser degree it is concerned with the problems which affect transportation. In any case the problems are mainly those of temperature, pressure, moisture, wind and sunshine.
The Diffusion of Warmth.—The warmth of the various parts of the earth is modified chiefly by the movements of the air. Because of the vertical rays of the sun in equatorial regions, the air is not only warmed to a much higher temperature, but it is also warmed more quickly than in higher latitudes. Being expanded by the greater warmth, it becomes specifically lighter and is pushed upward by the denser cold air which flows in to take its place. The updraught of air flows poleward in upper currents, until it is chilled and descends to the surface again. A part of the descending current continues poleward but a considerable part flows back to tropical regions as a surface wind.[1] The actual movements of convection are much more complex. Calms alternate with eddying movements of great intensity. All general movements are deflected by the rotation of the earth on its axis—easterly in tropical latitudes, and westerly beyond the tropics. There are therefore three wind belts, one of easterly and two of westerly motion. Each of these has also a northerly and a southerly component; moreover, all three belts shift alternately north and south with the apparent movement of the sun. The belt of tropical easterly, or Trade Winds, extends a little further north than New Orleans in summer and its northern edge recedes as far south as Havana in winter. The position of the wind belts, month by month, is
The migration of the heat belt.
shown on the Coast Pilot charts of the United States Hydrographic Office.
The apparent motion of the sun, due to the inclination of the earth’s axis, carries the zone of greatest warmth far north in June and far south in December. Thereby the warmth of tropical regions is carried well into the temperate zones, and thereby the production of foodstuffs is extended to about the sixtieth parallel of latitude, north and south.
All this complexity of movement adds to the diffusion of warmth. The warm air of tropical regions is mixed with the cold air of circumpolar regions. Complex as they are, the general movements of diffusion may be classified as the horizontal movements which include the winds, and the vertical convectional movements with which are classed the cyclones and the anticyclones.
Temperature and Altitude.—The effects of altitude on temperature may be considered in two aspects—altitude along a sloping surface, such as that of a mountain range, or a high plateau, and altitude above the surface, directly into the air. Altitudes are measured usually from mean sea level.
The variations in temperature of the various plains, plateaus, and mountain ranges are very great. In general, the temperature decreases with altitude until, in tropical regions, the limit of perpetual snow is reached at a height of about 16,000 feet; it decreases with increase of latitude until, in circumpolar regions, the snow line is not much above sea level. The variations of temperature with height are governed by so many conditions that specific rules apply to specific localities only.
The study of the relations between temperature and vertical altitudes is a matter of great importance in meteorology, and it has been prosecuted diligently during the last quarter of a century in various parts of the United States, Canada, Europe, South America and Africa.
Many thousand flights have been made by kites, manned balloons, captive balloons, pilot balloons, sounding balloons, airplanes and dirigible airships. At Uccle, Belgium, a pilot balloon reached an altitude of 20.1 miles, or 32,430 meters. Up to an altitude of about 9 miles, temperature and pressure statistics of the air have been obtained for about every thousand feet of altitude; beyond that plane the measurements are incomplete.
The fall in temperature with the increase in altitude has been in the traditional ratio of 1° F for every 300 feet[2]—the conventional temperature gradient. This has been a convenient ratio for general purposes, but it cannot be used in specific cases. Within the first 2 miles the temperature gradient is very irregular; at times there is even a rise in temperature with increased altitude; that is, the temperature gradient becomes negative. The rise in temperature with increasing altitude is technically known as inversion.
Inversion may occur in winter, when comparatively still air settles on a level surface or in a basin. It is pretty apt to be noticeable when a cloud layer separates two layers of air; the 
Temperature records made by a sounding balloon at Avalon, California, July, 1913. Note that an inversion of temperature occurs at the altitude of about 12 miles, and at 20 miles the temperature is about 20 degrees higher than at 12 miles. upper layer may be the warmer; indeed, the airman is quite apt to find a higher temperature above than below. Above a height of 2 miles, when the air is moderately still, the fall in temperature is apt to be fairly uniform. At a height varying from nearly 7 to 10 miles the fall in temperature ceases. Above this plane it remains stationary, or perhaps it rises. In one instance, a steady rise of temperature was observed between the altitudes of 8 miles and 20 miles.
The plane which separates the stratum of falling temperature from that of stationary temperature is sometimes, but rather loosely, called the isothermal layer. It varies in height, being highest at the equator; it is likewise higher in summer than in winter. It separates the shell of the atmosphere into two distinct layers—the stratosphere, and the troposphere.
The air of the stratosphere is remarkable chiefly for its apparent inertness. At its lower part the temperature does not vary much from −67° F (−55° C). If, as seems probable, there is a rise of temperature with increase of altitude, the rise is normal rather than abnormal. It seems to be due to the fact that the base of the stratosphere is chilled by masses of extremely cold air that constantly are thrown upward against it.
The humidity of the air of the stratosphere is very low—so low that visible clouds do not form. Therefore, if the dew-point is ever reached, the condensation is confined to ice spicules so few in number that they do not affect the visibility of the air. There is no vertical convection; therefore they sink slowly; and if they are greater in size than are molecules of water vapor they sink more rapidly than the water vapor diffuses itself.
It seems certain that the air of the stratosphere contains dust a-plenty—both cosmic dust and dust that is hurled into it by volcanic eruptions. If dust is absent, the air of the stratosphere differs from that below it and from space above it. One thing is certain, the radio-activity within the stratosphere indicates the presence of dust particles highly electrified.
The depth of the troposphere is inconsiderable compared with that of the stratosphere; aviation has probably scaled its height probably within pistol shot distance of the isothermal layer. The troposphere is the region of convection. Its height is practically the height of cirrus clouds, and all the great movements of the air—wind, cloud, storm, and precipitation—take place within its limits.
Experience has taught the meteorologist that conditions in mid-air of the troposphere, in many instances, are the key to conditions at the surface. They are far more important in air flight; for the airman encounters bumps and holes, both of which are due to sudden inequalities in temperature. The airman and the navigating officer of the airship are likely to encounter cross-winds, the updraught of thunder-storms, and the vagaries of cloud-formation; these, too, are due to irregular conditions of temperature, all of which must be understood and reckoned with in flight.
Air Altitudes and Terrain Altitudes.—The laws and values which apply to vertical altitudes in free air are not applicable to altitudes on the earth’s surface. In general, temperature decreases with altitude, but this is not always true. At various times the temperature of mountain valley floors is lower than that of the foot-hill slopes several hundred feet higher. On very cold, still nights, low spots, such as stream valleys, are almost always colder than higher ground. In regions where late frosts prevail, fruit growers have learned to take advantage of this fact. The difference between the temperature of a low spot and higher ground a few rods away is at times the difference between freezing and non-freezing temperature.
In tropical regions where mountains lie against the coast the difference between sea level temperature and that of the foot-hills a thousand feet higher is very marked. Thus, the temperature of the business district of Victoria, Hongkong, is almost intolerable to Europeans; on the Peak, a few hundred feet higher, the climate is pleasant. The same difference is noticeable between Rio Janeiro and its suburb, the Corcovado; it is even more noticeable in comparing the climate of Vera Cruz with that of Puebla or Orizaba.
On the other hand, extremely hot days in the foot-hills of the Sierra Nevada Mountains are apt to be cool days along the coast. The explanation is not hard to find: the ascending hot air of the foot-hills is replaced by cold air blowing in from the ocean.
In many instances the difference between low valley and hill stations is quite as much hygienic as climatic. It is the difference between moist, dusty and miasmatic air on the one hand; and clear, dry air on the other.
Temperature and Latitude.—In general, the mean temperature of the air decreases as latitude increases. In the southern hemisphere, which has chiefly an ocean surface, the decrease is quite regular and the direction of the isotherms does not vary much from that of the parallels. In the northern hemisphere the decrease is by no means regular, and the isotherms wander greatly from the parallels.
The following illustrates the decrease in the United States, as affected by latitude. In column I, the stations from south to north are approximately along the ninety-sixth meridian; in column II they are situated along the Atlantic Coast.
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I
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II
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Mean temperature, January.
The same results are seen in the temperature range in Europe, from Athens to Petrograd, or in South America, from Guayaquil to Punta Arenas. Within the tropics and also in polar regions, temperature changes due to latitude are not regular, nor are they great. In the main they are due to causes and conditions more or less local in character.
Mean Temperatures.—The daily, the monthly and the yearly means are required in weather service. The most accurate daily means would require the average of the hourly observations for the day; but the results would not be commensurate with the labor involved. The investigations of General A. W. Greely, while at the head of the U. S. Weather Bureau, showed that the average deduced from readings made at 7.00 a.m., 2.00 p.m., and 9.00 p.m., taking the last named twice and dividing by 4 gave a result very closely approaching the average of hourly means. This method has much to recommend it.
In the various Weather Bureau stations, where temperatures are recorded by regular observers, and in the various military field stations, the daily mean is found by taking half the sum of the daily maximum and the daily minimum. This mean is slightly in excess of the mean deduced by the preceding methods, but the error is so small that it may be disregarded.
The cooperative observer’s day begins and ends with the time that the maximum thermometer is set—usually from sunset to the following sunset. The daily mean thus established, however, does not differ materially from the true mean. The monthly and the yearly means are sufficiently accurate for practical purposes.
The yearly mean is deduced by dividing the sum of the monthly averages by 12. A closer average may be found by adding the monthly sums and dividing by the number of days in the year on which observations are made.
The mean annual temperature of a region is not a key to its temperature conditions or to its habitability. Thus, New York City and San Francisco, both seaports, situated not far apart in latitude, have about the same mean yearly temperature. But while the difference between the winter and the summer means in San Francisco is not more than 8 degrees, in New York it is about 32 degrees, and while the difference
Mean temperature, July
between the warmest and the coldest month in San Francisco is 10 degrees, in New York it is 44 degrees.
In studying the temperature of a locality, therefore, in addition to the question of mean annual temperature, various other elements must be taken into consideration. In the main, these are the daily range, the range of monthly means, and the seasonal range. These are affected in turn by latitude, by altitude above sea level, by the direction of prevailing winds, and by distance from the sea. In a minor way they are also affected by the moisture and the smoke content of the air.
The mean annual temperature of a place varies but little from year to year. In New York City, the range of yearly means has varied about 6 degrees in ninety-seven years. The average of each ten-year period for that time varies but a trifle from the normal of 52°. The records of Cooperstown, New York, have been kept continuously since 1854. The averages of ten-year periods show neither apparent gain nor loss in temperature.[4]
Temperature Ranges.—The daily, monthly, yearly and extreme ranges all have an important bearing on the climate of a region. The daily range is a part of the records of every Weather Bureau station; and the greatest daily range in each month is an item of separate record.
The various ranges are usually, though not at all stations, least in tropical regions and greatest in inland regions where the humidity is low. In temperate latitudes they are lower on the coasts than in the interior. In the United States the average daily range is somewhat less along the Pacific Coast than along the Atlantic Coast; and the daily ranges of inland stations are greater than those of coast stations. The reason therefor is that the drier air of inland stations permits greater radiation of heat at night and greater absorption during the day. For a similar reason the daily ranges at stations of considerable altitude are apt to be greater than those at or near sea level.
Along the Atlantic Coast the greatest daily range within a month does not often exceed 30 degrees; and the average monthly range is not far from 20 degrees. Away from the coast belt daily ranges above 40 degrees are common. In the Plateau Region of the western highlands the average of daily ranges in June varies from 40 degrees to 50 degrees. At Pacific Coast stations the average daily range is not far from 15 degrees. In Arizona, a part of the Plateau Region, owing to dry air and altitude, the daily ranges have usually been greater than in most other parts of the United States. At Florence, Arizona, a daily range of 63 degrees has been recorded, and ranges above 45 degrees are noted occasionally.
Undoubtedly the greatest daily ranges occur in the high desert plateaus of Asia. The temperature records for this region are few in number, and not always trustworthy. One fact, however, has been established beyond reasonable doubt: excessively hot days are sometimes followed by freezing temperature at night.
Excessive extremes are characteristic of inland regions; and in Siberia, where inland distances are greater than in the American continent, the extremes of temperature are also greater. At Verkoyansk a minimum of −96° F has been noted; and at Wargla, a caravan station in the Sahara, a maximum of 127° F has been reported by a trained observer.[5] In the northern part of the United States, a temperature of −30° accompanying a cold wave, is not uncommon.
In the United States, the highest official temperature record, 134°, is reported at Greenland Ranch, California; the lowest, −67°, at Poplar River, Montana. It seems certain that desert regions in low latitudes are the hottest places in the world.
Temperature Normals.—The daily normals of a station are the averages of each day of the year for a period of not less than ten years. The monthly normal is the average for the particular month for not less than the same length of time; the yearly normal is computed from an average of the monthly normals. It is the custom of Weather Bureau stations to extend the computation of the means to the end of successive years[6] but normals once established seldom change materially.
It is the custom of many observers to note, as a part of the daily record, the number of degrees above or below the daily normal; this is the “departure from the normal.” If below the normal the number is prefixed by a minus sign. It is an excellent plan to carry the algebraic sum of the daily departures to the end of the year. Many of the daily neswpapers desire these figures as a matter of public interest.
The monthly normals, by comparison, furnish the most instructive data concerning the temperature conditions of a given locality. Thus the January mean at Devils Lake, North Dakota, is 0°; at New Orleans it is 53°. The one is an inland station in comparatively high latitude; the other is practically a coast station in much lower latitude. The January mean of San Francisco is 50°; that of New York is 30°. Both are coast stations, but San Francisco is warmed by ocean winds. For the eastern part of the United States, the coast stations excepted, January normals are not far from 30°, and July normals range from 70° to 75°. At Moorhead, Minnesota, the summer mean is 67°; at San Francisco, 58°; at Seattle, 63°; at New Orleans, 81°; at Key West, 83°; at Yuma, 90°. A few stations excepted, January is the coldest and July the warmest month.
Temperature and Prevailing Winds.—Land winds are marked by great ranges in temperature. In regions far from the sea, changing winds are far more frequent than in maritime regions. Some of these winds, like the anticyclones which bring cold waves, are widespread in prevalence; others, like the simoon, an intensely hot and dry wind, are confined mainly to desert regions.
Throughout the greater part of Europe and the United States, westerly winds prevail; in summer they are frequently from the southwest, and in winter mainly from the northwest. The Pacific Coast of the United States receives ocean winds, and the winters are mild; west of the high mountain ranges zero temperatures rarely if ever occur. Along the coast, summer temperatures are never high; towards the foot-hills they occasionally exceed 100°.
East of the Rocky Mountains moist, southerly winds are common during the summer months, and occasionally these extend to the northern border. In the southern half of the United States the prevailing winds are persistent, moist, and hot. In the northern part they are not so moist, but very warm. Rhode Island and Delaware possibly excepted, summer temperatures of 100° and over occur in every state, when hot westerly winds prevail for a few days.
It is obvious that sea winds are more equable in temperature than land winds. Thus, summer days in San Francisco do not often reach 90°, and freezing weather occurs perhaps two or three times in a decade. When such temperatures occur they come almost always with land winds. The normal wind at this station is from the Pacific Ocean. In New York City, on the other hand, prevailing winds are land winds; and within a period of eight months a range of 115 degrees, −13° and 102°, has occurred.
Temperature and Radiation.—Very dry, clear air permits the sun’s rays to pass readily to the earth with but little perceptible loss—that is, dry, clean air is diathermous to the heat rays that impart the feeling of warmth to living bodies. [7] The heat is in turn absorbed by the earth. Earth temperature at the surface, or to a depth of an inch or two, may be many degrees higher than that of the air. Thus, in desert regions, pieces of metal lying on the ground in the sun become so hot they cannot be held in the hand. All this is due to the absorption of rays to which the air is diathermous.
But if dry, clean air permits excessive absorption, it also permits rapid radiation. The nights, in regions of very dry air, may be bitterly cold although mid-afternoon has been intolerably hot; indeed, at considerable altitudes, freezing temperatures during summer nights are not unknown in desert regions.
Both the moisture and the dust and smoke content of the air modify the absorption of the sun’s heat and its radiation by the earth. The moisture and, to a less extent, the dust and smoke content of the air absorb a considerable and likewise a measureable part of the heat that passes readily through dry clean air. And if they intercept and retard the passage of heat coming to the earth, they also retard radiation from the earth at night. In other words, the amount of insolation—that is, of solar heat—received at the earth’s surface is quite as variable as is the daily range; indeed, it is the highest expression of the daily range. At sea level dry air does not always indicate warm days and cool nights; but at levels of 5000 feet or more this is the rule rather than the exception. At any level, changes in temperature are more rapid in dry than in moist air, and the reason therefor is obvious.
Over areas of moist air a considerable part of the heat of insolation is absorbed in another way. Almost always in such areas there is a considerable water in the form of mist—that is, minute droplets of water. When the sun’s warmth converts these to vapor the absorbed heat becomes latent heat, and no longer appears as sensible heat. This fact furnishes another reason why the air over desert regions, as well as the ground surface itself, becomes heated to a higher degree.[8]
Conditions of temperature exercise a great control, not only over civilization, but over the distribution of life itself. Humanity may overcome its environment so far as temperature is concerned—man can command fire, food, and fuel to be brought to him; but other forms of life cannot rise superior to conditions of temperature. The line beyond which grass will not grow is determined in part by temperature; it marks the limit beyond which grazing animals cannot thrive, and, with a few exceptions, cannot survive. Conditions of temperature, such as obtain in the temperate zones, stimulate both the bodily and the mental faculties of humanity.[9] Therefore they have resulted in a civilization fundamentally different from that of tropical regions.
- ↑ This explanation is not accepted by all meteorologists, but it is supported by evidence that cannot be disregarded.
- ↑ This does not refer to the adiabatic cooling of air by expansion—about 1° F per 183 feet, or 1° C per 100 meters.
- ↑ The temperatures noted in the rest of this chapter are Fahrenheit, this scale being used for Weather Bureau reports.
- ↑ The history of the cultivation of the grape in Europe shows even more conclusively that no material changes have occurred in two thousand years. The grape of southern and western Europe is semi-hardy; it likewise is sensitive to temperature changes. But in twenty centuries its limits of latitude have not changed.
- ↑ If the figures are authentic, the absolute range for the earth, so far as is known, is 217 degrees.
- ↑ Bulletin R, U. S. Weather Bureau, the first edition in 1908, contains the normals of nearly two hundred stations computed by Professor Frank H. Bigelow. The changes since that time are very slight.
- ↑ Not all the heat rays impart the feeling of warmth; in many instances, they blister and burn the skin without imparting this sensation. It is thought that heat of this character consists of wave-lengths which, though they may destroy living tissue of certain kinds, do not stimulate the nerves to which the temperature sense responds. Thus, in popular tradition, there is “sensible” and also “insensible” heat. These terms, though inexact, are not without meaning. On dry, winter days, the flagstones of a sidewalk frequently absorb enough heat to melt ice and snow, even though the temperature of the air is as low as 20°; and occasionally side walks and hard-paved streets are slushy with the thermometer scarcely above 25°.
- ↑ A moist surface, and very moist air as well, does not have a temperture materially higher than the wet-bulb thermometer; a dry surface, or very dry air, acquires a temperature approximating that registered by the black-bulb thermometer.
- ↑ “Every species of plant and animal has an optimum temperature at which it thrives most vigorously, and man is no exception. The optimum may vary a little from individual to individual, but not much. It is more likely to vary from one type of activity to another. For physical health among the white race as a whole, the best temperature is an average of 64° F for day and night together. In other words, people’s health and strength are greatest when the temperature drops to about 56° to 6o° at night, and rises to somewhere between 68° and 72° during the middle of the day. For mental activity the temperature is much lower than for physical, being an average of approximately 40°. In other words, people’s minds are most alert and inventive when the temperature falls to about freezing at night and rises to 45° or 50° by day.”—Huntington and Cushing’s Human Geography.