« AnteriorContinuar »
and influence, that the atmospheric pressure, the hygrometrical, thermometrical, and diaphanic state of the ambient air, the calorific effects of solar rays, and, lastly, the animal and vegetable life, are all affected by their presence and action.
Both currents and winds disclose, through the courses of clouds, that the number of superposed belts or strata of circulation, their respective direction and velocity, are subject to infinite variation*, but that their movement with respect to our planet is constant, being, so far as the evidence has gone, either parallel to the earth's surface, or at low angles of inclination to it. To this may be added, that a calm in one region of the atmosphere, and a strong agitation and circulation in another, whether below or above it, is of common occurrence, as is also a diametrically opposed movement between that circulation and the surface winds.
The register of currents and surface -winds kept during four years, and condensed in the following table (p. 163.), leads to the belief that, as regards the difference between their respective directions, the ratio of that difference follows the increase and decrease of the sun's declination.
The observations collected in ascending high mountains tend to establish the general fact, that the thermometrical condition of these currents is as variable as their direction; and that, contrary to the
* "It is obvious, from the courses of clouds and other light bodies which sometimes float in the atmosphere, that the movements of the latter are mainly horizontal, or parallel to the earth's surface. Notwithstanding this, the common theory of winds supposes a constant rising of the atmosphere in the equatorial regions, connected with a flow in the higher atmosphere towards the polar regions, and a counter flow at the surface towards the equator, to supply the ascending current. This movement, however, has never yet been discovered; and it is easy to perceive that, if it existed in the manner supposed, its magnitude and velocity must be altogether too great to have eluded observation."— Meteorological Skelchet.
Table I.—Showing the Monthly Number of Currents contrary in Direction to Surface Winds.
law of hydrostatics, the colder current moves between two warmer, entirely by virtue of its volume. Thus, on ascending Mouna Roa (Sandwich Islands), I noted, ■within the elevation even of 6000 feet, three currents, of different directions, intensity, and therraometrical condition. That of Hilo (Byron's Bay) was a very light S.E. current, with a temperature of 86°; that at an elevation of 6000 feet was a brisk N.W. wind temperature 67°; while, as an intermediate between these, at an elevation of 4000 feet, a strong westerly wind moved in the temperature of 55°. On Mount Kosciuszko, New South Wales, the stratum of air at 3000 feet was considerably colder, during the daytime, than at the elevation of 6500; and in ascending Ben Lomond (Van Diemen's Land) a similar fact was observed. Moreover, on the last-named mountain, 5002 feet above the level of the sea, I encountered, on the 28th of November, 1841, before noon, a thunder storm, coming from the equatorial region, and attended with copious rain, and a temperature of 56°. On the same day, about four o'clock in the afternoon, it became calm and clear above, misty around, and densely clouded below; while at Avoca Vale, 4200 feet lower down, to the leeward of
Ben Lomond, there fell at the same hour (four o'clock) a hail storm, which thus must have originated in a stratum of air far below the point of congelation, and moving between 5002 feet and 800 feet of elevation, and between the 56° temperature of Ben Lomond and the 80° which was the temperature of Avoca Vale before the outbreak of the storm. This storm was in both places succeeded by a polar wind.
To these facts may be added that of rain being often observed to fall in Van Diemen's Land, on a winter's morning, when the temperature is below the freezing point; and that also of the melting snows which I witnessed on the crest of the Cordillieras in Chili, at an elevation of 15,000 feet, while the snow lower down, at the elevation of 10,000 feet, was found unaltered.
Independently of these thermometrical phenomena, the currents are attended by some extraordinary ones, as exemplified in their respective oscillations from one region to another. Thus, an upper current of a lower temperature than the surface wind has been observed to dislodge that wind, and to take its place, as was most probably the case in the above reported phenomenon of hail, in which the equatorial wind that accompanied the storm on Ben Lomond, was succeeded by the polar wind, after the latter had discharged its elements of hail on Avoca Vale.
In the action of the one current upon the other, a gradual commingling of the two currents not unfrequently takes place; sometimes that action is abrupt; and, in that case, the deflection of the lower current is immediately followed by an increase of atmospheric pressure.
This is well exemplified in both New South Wales and Van Diemen's Land, where the sudden southerly squall rapidly succeeds a N. W. or W. wind, and produces a rise in the barometer.
The only laws, however, that can be detected as governing the circulation of these currents, so little