The effect of latitude will be understood by reference to the following figure: As the latitude increases, the rays of the sun will fall with increasing obliquity, and they lose in power by being spread over a larger surface, and by traversing a greater depth of air, which absorbs more of their heat.
The same beam A, '
Solar energy is about 7 per cent greater at perihelion (the point in the earth's orbit near est the sun) than at aphelion (the point far thest away). As perihelion occurs in Decem ber, or the summer time of the southern hemi spere, and aphelion in its winter, that region has a greater annual range in the intensity of solar insolation than the northern hemisphere. If the land surfaces of the two hemispheres were equal in area, the southern would have colder winters and warmer summers than the northern and this is the case in portions of the southern hemisphere where the land area is large. But the great capacity of water for heat and the slowness with which it radiates the same, modifies seasonal extremes that oth erwise would be much greater.
Variations in Climates.— If the earth were all water or all land and if the land were every where of the same elevation, most of the fac tors that cause variations in climates — often considerable for regions closely contiguous would be eliminated from the equation. Every point on the same parallel of latitude would have the same mean annual temperature and the same average heat in summer and the same average cold in winter. New York and Lon don, separated by 11 degrees of latitude, would not, as now, have about the same mean annual temperature. If it were all water, there would be no such extremes of heat and cold as we now know. It is probable that a thermometer exposed in shade four feet from the surface of the earth would not anywhere — even at the equator — ever register above 90° F.; there would be no frost within 35° or 40° of the equa tor, and zero temperatures would be recorded only in regions within 30° of the poles. If it were all land the heat would be much more in tense than now in the tropics and in the temper ate and frigid zones the heat of summer and the cold of winter would reach extremes un known at this time.
All the anomalies of climates are caused by the different specific heat capacities of land and water; their different powers of conduction and radiation; the irregular distribution of these two surfaces; the widely-varying eleva tions of the land; the trend of mountain ranges; the prevailing direction of the winds, and the carrying of large quantities of heat by ocean currents from the equator toward the poles and the relative quantities of cloud and rain or snow. It is germane to a proper under
standing of climate to know something in detail of the manner in which the air is heated. At 100 or 200 miles above the earth's surface there is only the hypothetical ether, which, while too tenuous to be detected or measured by any methods or appliances so far known, is sup posed to be the medium that transmits solar energy to the earth and diffuses it through space. This energy, coming in many different wave-lengths and with widely varying inten sities of vibration, produces several different phenomena as it is absorbed by or passes through the air, or as it impinges on the sur face of the earth. The waves differ in their effects on different objects, depending on the length and the absorptive response of the sub stances upon which they fall. The waves have heating, lighting and chemical effects simul taneously in themselves and it is only the na ture of the objects upon which they fall that tends to differentiate them. The atmosphere, even at the surface of the earth, absorbs but a small part of the heat-waves. They there fore reach the earth and warm its surface; and the earth in turn, by radiation, convection and conduction sends back into the air long heat-waves, which, unlike the shorter solar waves, are readily absorbed by the atmosphere. The atmosphere is thus mainly warmed from the bottom upward. This accounts for the per petual freezing temperatures of very high moun tain peaks, although they are nearer the sun than are the bases from which they rise. At the height of one mile in free air the temperature is about the same at midday as at midnight. Only during recent years have we begun to realize how extremely thin is the stratum of air next the earth that has sufficient heat for the inception, growth and maturity of both an imal and vegetable life. The raising of the ther i..ometer shelter at the New York city observa tory from an elevation of 150 feet above the street to an elevation of 300 feet has caused an apparent lowering of the mean annual tempera ture of 2.5° F. On the hottest day in summer, if one could be lifted up to a height of only 1,000 feet in free air, he would find a marked change in temperature. The United States Weather Bureau at 16 stations made a total of over 1,200 kite observations in the United States in 1897. They showed an average de crease of 7.4° F. for the first 1,000 feet of ascent during the warm months and when the observations were taken near the hour of daily maximum heat the decrease was fre quently as much as 15°. At the height of six miles the cirrus clouds common to this level are, on account of the low temperature, always composed of minute ice spiculx, never of wa tery droplets like the lower cumulus clouds. In the middle latitudes of both hemispheres the air at this height is ceaselessly flowing toward the east, passing uninterruptedly over the cy clonic and anti-cyclonic systems that cause our storms and cold waves at the surface of the earth. Glaisher and an assistant ascended to a height of about 30,000 feet. They suffered greatly from the cold, which measured many degrees below zero although the time of year was 5 September. At the height of six miles the average temperature, determined by many balloon ascensions, is about — 50° F.