Page:Physical Geography of the Sea and its Meteorology.djvu/121

 Rh other evidence besides that of induction (§ 224) as to upward and downward movements amongst the particles of air. In violent winds especially are these upward and downward currents made obvious by the feathers, leaves, thistledown, dust, and trash that are blown about. It would be well if our wind gauges and vanes therefore were so constructed as to show the inclination as well as the azimuth of the wind. With such an improvement we might ascertain whether certain sudden changes in the weather be not owing quite as much to the inclination as to the direction of the wind.

246. The hot winds of the Andes.—We may seek in the vertical circulation of the atmosphere for an explanation in part, not only of hot and cold terms, but in a measure also of seasons of excessive drought, as well as of other phenomena with which all are familiar. Travellers in crossing the Andes tell of hot winds encountered there even on the mountain tops. Streaks of hot air are also frequently encountered in various parts of America, and I have no doubt in other countries also.

247. Certain "Hot Spells" explained.—To explain one of these sudden and severe "hot spells," let us suppose the neighbouring atmosphere to be well loaded with moisture at the temperature of 80° for example, and with the barometer at 30 in.; that from some cause this rain-laden air commences to ascend, and its vapour to be condensed. In this process the heat which was latent in the vapour becomes sensible in the all. Now the height to which this air rises may be such, were it dry air, as to reduce its temperature 80°, and bring it down to zero; but it is moist air, and the liberated heat may be sufficient to raise it to 20°, and so prevent the temperature from going below that reading. Thus this air is at least 20° above the normal temperature of the height to which it may have risen. Suppose that now, in the process of vertical circulation, it be brought down to the surface again, and submitted to the same barometric pressure as before: its temperature now will not be 80°, as before, but it will be 80° + 20°, or 100°. Thus by going up, precipitating its moisture; and coming down, it is made hot. 248. Reservoirs in the sky.—Whenever and wherever air in this condition descends to the surface, there will be a longer or