Page:The American Cyclopædia (1879) Volume XI.djvu/452

 440 METEOROLOGY winds, which flow from the northeast and southeast, in the northern and southern hemi- spheres respectively, toward the equator, and include between them a narrow belt of equa- torial calms. The latter has a breadth of a few degrees only, and varies its position with the seasons, lying somewhat further to the north of its mean position in July, and to the south in January. On account of the intimate relation between the direction of the wind and the barometric pressure, it is best to represent these two meteorological elements on one map ; and accordingly on the accompanying charts, Nos. VII. and VIII. (compiled from those published by the British admiralty in 1872), are shown by arrows flying with the wind the prevailing winds for the months of July and January, over the entire globe; regions of prevailing calms are designated by shaded circles. The comparison of these charts will show more clearly than words the great changes which take place in the movements of the air during the year. The principal regions of calms and light variable winds, besides the equatorial belt already mentioned, are those belts about 10 broad, lying K and S. of the trade wind regions, in the respective hemi- spheres. Coffin (1853) has moreover shown that N. of the belt of calms in the Atlantic the winds both of Europe and America show com- paratively slight but decided indications of monsoon features, due to the over-heating of the land in summer and its cooling in winter. Similar phenomena are observed in reference to the Sahara of northern Africa, and to the continent of Australia. Of the non-periodic variations of wind, the most important are thosa attending storms. Dove has shown that, at any station in the northern temperate zone, the wind veers more frequently than it backs ; that is to say, the successive wind changes are more frequently in the order S., W., N., &c., than they are in the reverse order, S., E., K, &c. But this interesting law may be considered as a simple consequence of the position of the stations in relation to the paths generally pursued by storm centres. Red- field (1821-'57) was the first to prove that in all extensive severe storms there is always present a system of surface winds revolving about and blowing in toward a storm centre, which latter has a progressive motion along the surface of the earth. At a short distance above the earth's surface is a system of outward-moving spiral currents immediately above the lower inward- moving winds. The independent investigations of Espy (1837), Reid, Piddington, Ley, and others, have abundantly confirmed the correct- ness of Redfield's conclusions; and the studies of later ^yriter8 have shown that such a sys- tem of winds is characteristic of severe storms throughout the world. (See HURRICANE, and STORMS.) The relation between the wind and temperature is graphically represented by the thermal wind rose, which gives for each direc- tion of the wind the average temperature re- WIND. NEW HAVEN. VIENNA. LONDON. BT. PETEB8- BUEG. Year. Jan. July. Jan. Jnly. Jan. July. N 46-3 26 6 66-8' 82-4' 61 -0 13 8" 58.6 N.E. . 48-4 26-4 70-2 80-3 61-6 7-1 62-7 E... 49-5 25-5 74-8 81-6 65-0 18-8 64-7 8.E. 50-2 29-3 71-8 85-6 66-8 10-7 65-5 8.... 52-2 80-7 72-5 40-4 65-4 13-6 65-4 8. W 53-0 85-4 72-5 89-8 63-7 19-7 64-9 W... 47-9 86-0 65-9 86-8 63-6 19-0 61-6 N. W 44-5 32-0 66-0 83-3 62-0 18-1 60-2 N... 46-8 26-0 66-8 82-4 61-0 13-8 58-6 maining after eliminating the effects of the daily and annual changes and the lesser local peculiarities. The following table presents a few of the stations for which sufficiently long periods of observation are available to render the average results trustworthy : TJiermal Wind Rose. The connection between the direction of the wind and the position of the moon in its orbit has been investigated by Schubler and Eisen- lohr for stations in Germany, and the results, as shown in the following table by Schubler, would indicate that K and E. winds are more frequent at the time of the last quarter of the moon, and that S. and W. winds prevail at the second octant : Table showing the Prevalence of eacti Wind on the Days of the respective Phases of the Moon. LUNAR PHASES. & 6 8 7 11 12 H W / OD >p
 * L-

A Mean direction. 18 1-.' 10 ir, 29 44 40
 * w

H5 41 13 IT Is Hi 111 172845 1724'55 12 28 j 62 12 25 59 13 35 88 288.S.W. 24 S. W. byW. 22 !W. by S. 23W. 18 W. First quarter Second octant Full moon Last quarter 4. The Aqueous Vapor, whose presence in the atmosphere is of the highest importance in a meteorological point of view, as well as in reference to animal and vegetable life, may here be considered as having its ultimate ori- gin in the ocean, whence it is drawn by surface evaporation under the influence of solar heat. Its subsequent diffusion throughout the atmos- phere is in a slight degree due to Dalton's prin- ciple of the diffusion of gases, but principally to its convection by the winds ; its deposition, whether in the form of clouds or rain, is the result of the subsequent cooling of the moist air. Of the rain which falls upon the land, a large portion is again evaporated, and thus returned to the atmosphere, and ultimately to the ocean. The quantity of moisture in the air is determined for meteorological purposes by means of observations with either the hair hygrometer of De Saussure, the wet and dry bulb thermometers of August, or the dew-point instruments of Daniell, Regnault, Bache, and others. (See HTGROMETRY.) Measurements of evaporation have such a peculiarly local sig- nificance that it has been difficult to ascertain any practicable means of making these com- parable with each other; but it may be said