NORWESTERS.

Globe, Volume I, Issue 93, 17 September 1874, Page 3

NORWESTERS.

The following extract, from Dr Knight’s annual address, as president of the Wellington Philosophical Society, gives some explanation of the cause of the hot norwesters, which prevail in this province at certain seasons of the year ; “In connexion with the Meteorology of New Zealand, I wish to offer an explanation of the hot winds of the Canterbury Plains. The N.W. dry, hot winds of Australia, in their passage over the ocean, become surcharged with vapour at the expense of a noticeable part of their heat. In the philosophic language of the day, the heat of the air is made to do work by converting water into vapour, and by maintaining it in that state. The sensible heat thus absorbed by vapour is converted into latent heat—that is, the heat is not lost, but is engaged in the veiled work of maintaining water in an invisible state. The air in thus dissolving water becomes a carrier, or distributor of heat. It is not difficult to give an approximate estimate of the quantity of heat made sensible to the thermometer by the reconversion of vapour into water. The problem has been solved by experiments, and it is found that the amount of heat stored up in aqueous vapours from one pound of water is sufficient to heat lOOOlbs of water, one degree of Fahrenheit, or, as Tyndall puts it, to fuse olbs of cast iron.

Next as to the phenomena arising out of the fall of heavy rains on the flanks of the ranges on the West Coast. The quantity of heat liberated would be considerable did not other causes interfere with its escape. The rain itself carries down with it a small part of the sensible heat, but the greater portion of that set free is at once taken up by the atmosphere in rising to the altitude of the lofty ranges in its passage to the plains of the Middle Island. The sudden rarefaction is due to the loss of pressure of the column of air lying between the level of the sea and the average height of the ranges. This dilatation is accompanied by what used to be termed an increased capacity for heat, so that heat is absorbed by the air itself. This too, in the current phraseology of the day, is called work done at the expense of heat. So that altogether we have a diminution of sensible heat, but not an actual loss of heat, by the passage of the air over the ocean, and next by the ratification of air in its ascent to the higher regions of the ranges. When air expands by reason of the loss of pressure the undulations of the molecules of air, which we term heat, diminish also —or, in other words, the amplitude of the vibrations is diminished, and the distance between undulations increased. Motion imposed on the molecules of matter would go on for ever, just as any motion imposed on a mass would never cease, if there were no medium to which it could communicate its motion to other bodies, or in some other way exhaust itself. This inertia belongs to molecules as much as to masses. While the swing of the molecules is diminished, and the distance between the undulations is increased, the actual force, which we call heat, remains uudiminished. It is only necessary that the undulations should be again crowded together in order that the energy may be restored. So far I hope that I have made ray meaning clear, —that air in taking up moisture loses sensible heat. No one who has travelled on the West Coast of the Middle Island can have failed to notice when he leaves the forest road, and comes on the dreary sands of the coast, with a moderate sea breeze, how miserably cold the air is in the immediate vicinity of the breakers. This is owing to absorption of sensible heat by the solution of the spray in the current of air sweeping over the sea.

Next, in reference to the heat gained, if any,in the passage of the air over the extensive elevated region between the West Coast and the Canterbury plains. The air on the mountain ranges gains no addition of temperature from the direct radiant heat of the sun, for the rarified air is, to use a technical expression, almost transparent to the sun’s heat rays—it permits them to pass with very slight loss. The sun warms the ground but not the air. It is not so, however, with the heat radiating from the surface of the ground —this is almost entirely absorbed by the atmosphere, which thus gains sensible heat until at length it reaches the eastern slopes of the mountain ranges.

Here, where the column of air begins its descent to the plains, we ought to have a complete reversal of every phenomena that accompanied its passage from the level of the sea on the West Coast till it reaches the elevation of the mountain ranges. As the column of air sinks down, the increased extent of the vertical column gives increased pressure, and compresses more and more the lower stratum—the latent heat becomes sensible, and the thermometer rises. But it is necessary that I should give you here some proof that increased pressure adds to the energy of heat in aeriform bodies, or, in the old, expressive phraseology, converts the latent heat into heat recognised by our senses, and registered by the thermometer. Airy has explained that when the changes of volume and pressure are very rapid, the changes of temperature of the air are very great;—“ Upon suddenly condensing air it becomes very hot. We have verified the experiment, that if inflammable tinder is placed in the bottom of a cylinder in which a piston fits tightly and slides easily, when the piston is driven rapidly down so as to condense the air very much before it has time to impart the whole of its heat to the surrounding metal, the air will inflame the tinder.” And Airy remarked, “in the powerful air-pumps (driven by large steam engines) which were used to exhaust the air tubes upon the Atmospheric Kailway, that when the attenuated air in the tube, having acquired the temperature of the ground, was compressed by the operation of pumping so as to be able to open the last valve in opposition to the pressure of the atmospheric air. the emergent air was so hot as to be unbearable to the hand. If the heated air, without having lost caloric, be allowed to expand to its former dimensions it exhibits its former temperature ; that is, it cools by sudden expansion. And this is so well known that it has been proposed to supply apartments in hot climates with cool air, by compressing air in a close vessel, allowing the increased heat to escape by contact of the vessel with the external air or neighboring substances, and then permitting the condensed air (at the atmospheric temperature) to expand into the apartments, when it would have a much lower temperature.”

You see at once that the condition of the air in the tubes of the Atmospheric Railway bears the closest relationship to what takes place in the transit of the air from the West to the Bast Coast. First, the rarefaction of air in the tubes ; -this represents the expansion of air in its ascent to the mountain ranges. The subsequent compression of air, for raising the escape valve is analogous to to the descent of the column of air from the ranges ; —and the escape of the hot air at the valvular opening is a counterpart of the heated air sweeping over the Canterbury Plains.