Page:The American Cyclopædia (1879) Volume XI.djvu/451

 METEOROLOGY 439 in 1871. In this figure the arrows show the direction toward which the wind moves ; the respective monthly lines show by the changes DIAGRAM VI. Diurnal Variations in Force of Wind at Philadelphia. in their directions, at every point from feather to barb, the changes of the wind from hour to hour. It will be seen, in fact, that in May the early morning winds are northerly (land breezes), while the evening winds are souther- ly_(sea breezes). A daily period in the force with which the wind blows is as conspicuous as the periodicity in its direction. This is well exemplified in diagram VI., from Loomis, showing the average pressure of the wind in pounds per square foot for each hour, derived from the observations of Bache at Philadel- phia. The annual variations of the wind may be traced back to the same ultimate cause, namely, the varying influence of the unequally heated portions of the continents and oceans. The most remarkable annual variations in the winds are those known as monsoons, in the Indian ocean. The most remarkable perma- nent or nearly permanent winds are the trade Shaded circles indicate regions of numerous calms. DIAGRAM VII. Isobars and Prevailing Winds for January. February, and March Shaded circles indicate regions of numerous calms. DIAGEAM VIII. Isobars and Prevailing Winds for July, August, and September.