Distribution of Heat and Light.—The distribution over the earth's surface of the heat and light received from the sun is not only of fundamental importance as respects climate, but furnishes a part of the essential conditions on which depend the presence and distribution of living organisms. The heat and light, or more accurately, the radiant energy of the sun, the full significance of which is probably not thoroughly understood, we term, for convenience, insolation. The intensity and seasonal distribution of insolation are prime factors on which many important results hinge.
Owing to the inclination of the axis about which the earth rotates (23° 27') to the plane in which the earth travels about the sun, or the plane of the ecliptic, the northern end of the axis is turned towards the sun in summer and away from it in winter—that is, the axis of rotation of the earth at all times is parallel to the same imaginary straight line. As a result, the sun appears to migrate northward in the heavens during the spring-time of each year, being vertical over the equator on March 21st, and to an observer in north latitude 23° 27' rises higher and higher each moon, until on June 21st it is vertically overhead; and then returns southward. The latitude in which the sun is in the zenith at the time of its greatest northward migration determines the position of an imaginary line on the earth's surface, named the Tropic of Cancer. This line, as shown on the accompanying maps, crosses the Bahama Islands, passes about 40 miles to the northward of Havana, divides Mexico into two approximately equal parts, and cuts the peninsula of Lower California near its southern end. The portion of the continent to the south of the Tropic of Cancer lies within the torrid zone.
When the sun is vertical over the equator, as it is about March 21st and September 23d each year, its rays, not allowing for refraction, are tangent to the earth's surface at the poles, and the hours of light and darkness are equal the world over. During the winter season the sun appears to migrate southward of the equator until December 21st,
when it is vertical at noon at all points situated in south latitude 23° 27', which is termed the Tropic of Capricorn. Its rays are then tangent to the earth's surface in the northern hemisphere in latitude 66° 33', which defines the position of the arctic circle. This imaginary line on the earth's surface, as is indicated on the accompanying maps, crosses Canada to the north of Hudson Bay, and passes through Alaska near where the Porcupine River joins the Yukon. To the north of the arctic circle lies the frigid zone. Between the torrid and frigid zones is situated the temperate zone, within which is included about seven-eighths of North America, exclusive of Greenland. The relation of the continent to the three great zones of climate into which the northern hemisphere is divided is thus most fortunate so far as man's activities are concerned.
The climatic zones just referred to, while based on precise astronomical data and representing important facts, are not separated one from another by tangible lines, and might easily pass undiscovered by one who studied only the surface characteristics of the earth. Each summer a wave of heat and light sweeps northward over the continent and reaches beyond the pole; and each winter a counteracting wave of cold and darkness moves southward, the influence of which is marked even well within the torrid zone. A comparison of the isothermal lines drawn on the map forming Plate II with the parallels of latitude shows at a glance that there is only a general relationship between the two. In order to understand this discrepancy between what might be expected from astronomical considerations in reference to the distribution of solar energy and the actual conditions as learned by observation, it is necessary to take a more critical view of the manner in which insolation is received by the continent, and also to consider secondary conditions which exert far-reaching influences on its distribution.
The amount of heat, or to avoid objections, the distribution of insolation over North America, depends on three primary conditions: First, the angle at which the sun's rays strike the earth, the range being from zero to 90°; second,
the length of time a particular locality is exposed to sunlight; and third, variations in the distance of the earth from the sun. Each of these conditions varies from day to day for every locality throughout the continent. The sun is highest in the heavens in the torrid zone, being twice vertically overhead each year at every locality, and the hours of light and darkness each day are approximately equal throughout the year. North of the torrid zone, however, the rays of the sun become more and more oblique to the earth's surface, and hence insolation becomes weaker and weaker for a given period of sunshine as one travels from south to north. But the hours of sunlight each day undergo marked variations, lengthening from December 21st to June 21st, and shortening as the sun makes its southward migration. At the north pole, as all know, there are six months of light and six months of darkness each year. The amount of insolation reaching the northern portion of the continent each day increases with the lengthening of the hours of light, and during midsummer is greater for a given area in a single day (twenty-four hours) than the amount received by a similar area in the torrid zone. The almost magical springing into life and bloom of the vegetation over the northern portion of the continent with the lengthening of the hours of sunshine each summer is thus explained. In the portion of the continent within the temperate zone, more especially within the continental basin, the large number of hours of sunshine during a summer's day is frequently accompanied by a temperature as great as is usually experienced in the torrid zone. It is the high summer temperature of this region, together with the lengthened duration of sunshine in the growing season, that makes the Mississippi basin and the adjacent region on the east and north so favourable for agriculture when the requisite amount of moisture is present.
The distribution of heat over the earth's surface depends not only on the direct influence of insolation, but on its transfer from one locality to another through the agency of the winds and ocean currents. The movements of the waters of the ocean, it will be remembered, are largely
under the control of the winds, so that the essential factor in the transfer of heat from place to place is atmospheric circulation. The primary causes of movements in the air, as is a matter of current knowledge, are the differences that arise in temperature at various localities. In regions where the air becomes more highly heated than over adjacent areas it expands, and in consequence becomes lighter, volume for volume, than the air over neighbouring areas, and is forced upward and overflows aloft. The overflow or dispersion of the warmer and lighter air above gives origin to a reduction in barometric pressure, the column of mercury in a barometer being counterbalanced by the pressure of the air above it. Briefly stated, the air near the earth's surface flows towards regions of low, and away from regions of high barometric pressure, and winds are established. The directions taken by the winds are influenced or controlled in various ways.
The Planetary Winds.—The great movements in the atmosphere originate from differences in temperature between the warm equatorial and cold polar regions. This alone would cause the cold air from either pole to flow towards the equator as surface winds, and the warm air in the equatorial belt to ascend and overflow aloft towards either pole. The earth's rotation, however, influences the direction of these winds and causes them to be deflected from the lines of longitude which they would otherwise follow. In the northern hemisphere the air-currents are deflected to the right and in the southern hemisphere to the left of their initial directions. The best known examples of these planetary winds, as they are termed, are the trade-winds, which blow from the northeast in the northern and from the southeast in the southern hemisphere. Between these two belts of converging winds lies the equatorial belt of calm, some 300 miles wide, which also encircles the earth and is termed the doldrums.