Page:Physical Geography of the Sea and its Meteorology.djvu/167

 Rh from the pure snows of the Andes. Between this range and the coast, instead of marshes and a jungle, there is a desert—a rainless country, upon which the rays of the sun play with sufficient force not only to counteract the trade-wind power and produce a calm, but to turn the scale, and draw the air back from the sea, and so cause the sea breeze to blow regularly.

318. Influences which regulate their strength.—On the coast of Africa, on the contrary, a rank vegetable growth screens the soil from the scorching rays of the sun, and the rarefaction is not every day sufficient to do more than counteract the trade-wind force and produce a calm. The same intensity of ray, however, playing upon the intertropical vegetation of a lee-shore, is so much force added to the sea breeze; and hence, in Brazil, the sea breeze is fresh, and strong, and healthful; the land breeze feeble, and therefore not so sickly. Thus we perceive that the strength as well as regularity of the land and sea breezes not only depend upon the topography of a place, but also upon its situation with regard to the prevailing winds; and also that a given difference of temperature between land and water, though it may be sufficient to produce the phenomena of land and sea breezes at one place, will not be adequate to the same effect at another; and the reason is perfectly philosophical. 319. Land breezes from the west coast of Africa scorching hot.—It is easier to obstruct and turn back the current in a sluggish than in a rapid stream. So, also, in turning a current of air first upon the land, then upon the sea—very slight alternations of temperature would suffice for this on those coasts where calms would prevail were it not for the land and sea breezes, as for instance, in and about the region of equatorial calms; there the air is in a state of rest, and will obey the slightest call in any direction; not so in regions where the trades blow over the land, and are strong. It requires, under such circumstances, a considerable degree of rarefaction to check them and produce a calm, and a still farther rarefaction to turn them back, and convert them into a regular sea breeze. Hence the scorching land breeze (§ 317) on the west coast of Africa: the heat there may not have been intense enough to produce the degree of rarefaction required to check and turn back the south-east trades. In that part of the world, their natural course is from the land to the sea, and therefore, if this view be correct, the sea breeze should be more feeble than the land breeze, neither should it last so long.