on Plate 148 show January as winter in the northern and summer in the southern hemisphere; and July as summer in the northern and winter in the southern hemisphere. The bounding lines of the red and blue tints are the isotherms (lines of equal temperature) of 70° F. and 30° F. respectively. It will be noted that the thermal equator-as the middle of the equatorial hot belt may be called-does not coincide with the geographical equator, but fluctuates in a way which is evidently influenced by the seasons, continents, oceans, and winds. When air is heated, it expands and becomes lighter; it therefore rises, and the ascending currents cause a decrease of pressure. Another important result follows in the formation of clouds and rain due to the presence of aqueous vapor in the ascending current. As the air rises it expands more and more, cooling at the same time through the absorption of heat due to this expansion, while it is still further chilled by the lower temperature of the upper regions of the atmosphere. As a consequence of the reduction in temperature, much of the aqueous vapor is condensed into clouds or precipitated in rain, giving off a large amount of heat, which tends to rarefy the air still further and so to intensify the upward draft. We have here, briefly outlined, an explanation of the weather characteristics, which in some degree mark all regions of low barometer and cyclonic circulation, whether these extend over thousands of miles of ocean and continent as in the cases shown on Plate 148, or are localized in the narrower and more intense form of tropical hurricanes. The weather of all such regions is characterized by more or less cloudiness, frequently accompanied by rain, by low barometer, and by winds which, while not necessarily violent, are usually stronger than in an anti-cyclone, since the determining cause of these winds—an upward rush of warm air with the attendant features that have been described-naturally takes on a somewhat more violent form than does the downward current of cool and heavy air which is the determining cause of anti-cyclonic circulation. The velocity of the wind, as well as its direction, depends upon relative pressures in adjoining areas; being determined by the steepness of the "barometric gradient," or the amount of difference in the reading of the barometer for a given distance. This is well illustrated in the cyclone and anti-cyclone of Plate 149, in which the distance between the successive iso bars indicates the steepness of the barometric gradient. Where the isobars are crowded closely together, the change in pressure is very great for a small change in position; or in other words the gradient is very steep. At such places the velocity of the wind is high. Where the isobars are separated more widely, they indicate a gradual change in pressure or a gradual slope to the gradient, and here the winds are more moderate. To compare barometric gradients, they are commonly reduced to hundredths-of-an-inch per 15 sea miles. On this scale the steepest gradient ever observed was in the cyclone that passed over False Point, India, in September, 1885. This gradient was 238; which means that in a distance of 15 miles, there existed a difference in barometric reading of 21% inches. It will be observed in Plate 149, that the wind blows partly across and partly along the isobars, forming the spiral whirl which has been elsewhere described. The following description and explanation of a sudden shift of wind over the North Atlantic Ocean is from the Pilot Chart of the U. S. Hydrographic Office for July, 1900 (Plate 150). It illustrates in a very striking way the relation between the direction of the wind and the barometric pressure. During the course of March 17, 18 and 19 last, the steamers along that portion of the transatlantic routes lying to the eastward of the meridian of 60° experienced a sudden shift of the wind from S.E. to N.W., accompanied by a marked increase in force, as indicated by the following table. The shift progressed steadily eastward, the date and hour at which it was noted aboard the several steamers distributed along the lane depending upon the position of the various vessels at the time, becoming successively later as the longitude diminished. Vessel. Date. Long., REMARKS. Strong to fresh breeze (from the South); at 8 a. m. wind shifted suddenly to NW. 42 Southerly wind until midnight, when wind veered to Southerly wind shifting at midnight to NW.; moderate gale. At 3 p. m. wind shifted suddenly from South to N NW. and blew a moderate gale, 42 53 47 Anchoria... Mar. 18 50 At 2 a. m. wind shifted from S SE. to N NW. Wind S SE., 2, until 5 a. m., when it shifted through SW., Symra At 8 a. m. wind shifted from SE. to NW. Wind SE.; at 5 p. m. wind hauled to W SW., later to NW. At 11 p. m. wind shifted (from SSE.) to NW., moderate gale. |