NOTES BY THE WAYSIDE.

Otago Witness, Issue 2426, 12 September 1900, Page 9

NOTES BY THE WAYSIDE.

By Geo. M. Thomsox, F.L.S.

Many visitors to and residents in Dunedin find fault with our climate as changeable and trying, and if one were here only to live an out-door life of sight-seeing and picnicking, the charge might be a true one. But for a fine working climate, in which one can be busy nearly all the year round with a minimum of inconvenience, there could be no better climate, and its arery -chanfefulness may have its charms. Those Avho have lived in a dry, tropical or sub-tropical region know lioav monotonous the very' sunshine becomes, when the sky — day after day and month after month • — is as burnished brass, and the eye aches for a patch of vivid green or the shadow of a great rock in a dry and thirsty land. They can appreciate the -feelings of the Indian civilian on his way to the home land, when the vessel in which he was voyaging up the English Channel ran into a, thick, Avetting mist, as he rubbed his hands together and exclaimed, " All ! this is the weather for me ; none of your blue skies ! " But, indeed, our- climate is not more changeable than some others with which it is often unfavourably compared. In all my experiences of Dunedin weather, I never experienced such sharp alterations of temperature as are common in Australia. What makes our changes here so noticeable is that the atmosphere is so frequently nearly saturated with water vapour that a fall in temperature of lOdeg causes a great amount of condensation Avilh formation ,of cloud. The occurrence of so much high and rugged ground in the immediate neighbourhood of Dunedin, Avhile it is the cause of most of the romantic beauty of its surroundings, is also responsible for much of the moisture whiclycharacterises the atmosphere here. Oiir climate is no doubt less sunny and more frequently wet than that of many another part of these favoured islands, but .where could one see more prevalent greenery (outside of the Emerald Isle), or finer cloud effects and more beautiful alternations of light and shade on the hills, than Aye have enjoyed this last summer and autumn. Words fail one in attempting to describe them. One afternoon recently the skies and surrounding scenery seemed to be made up of impossible tints, which changed with .the changing hours. The morning had ibeen close and bright, with the suspicion of a warm north-west -wind, — such a day as we often have when the rain is pouring doAvn on the West Coast, — but as the clay advanced leaden-coloured clouds began ■to gather over the f-ky. Out at .sea- especially, where there were no hills to arrest the movement of the south-Avesterly current which formed Oi m, they rolled up thick anr{ dink, c.niMug the ocean to asgUSGe a dee]) aud gloomy hue. The waters

of the bay seemed also of a dark leaden colour, deeper than that of the ocean, while the town and the hills of the Peninsula sJbood out beautifully clear and distinct against the sombre background. Though the wind was blowing gently from the west, the dark cloudr — which were at no great height — were moving due south, while above them was moving the cold foiitherly current, which was really forming them from the moisture of the warmer air below. This is a common phenomenon in this part, the cold current which brings down our south-westerly rains often blowing over the top of the warmer surface air and ultimately dispersing it. It seemed as if some great atmospheric disturbance was approaching, and the heavens appeared to 'be gathering up their forces for an outburst. But the threatened storm never came, the outlines gradually became softer and the sky le^s louring as the afternoon passed, and just shortly before the sun set behind the western hills his nearly level ■rays, breaking through an opening in the clouds, shot across the landscape, illuminating the hill 'tops and- pouring* their mellowing light over tie scene. It was a wonderful transformation, almost aAvful in its grandeur and superb beauty, but it was followed by a still more remarkable effect. When the sun had disappeared behind the high ground, the view to the north and east Avas simply marvellous. Pine Hill and Mount Cargill, North-East Valley and Signal Hill were bathed in dark and sombre greens and greys, tinted apparently with lilac, against a 'background of almost indigo colour, which changed to deep violet as the light waned. Such, cloud effects would be classed as "impossible on canvas, so startling were they- in their contrasts. But they are not uncommon in our skies, and for all who have eyes to see they constitute one of the redeeming and 'beautifying feature's of our climate.

The question is often asked, but seldom satisfactorily answered — How far does the cutting down and clearing away of bush affect the climate of a district? The popular idea is that it tends to make it drier. As far as- any alteration in the rainfall is concerned this view is probably incorrect. The -presence of large masses of high land in the neighbourhood of the &ea probably modifies the meteorological conditions profouridly, but the removal of a few thousand acres of the surface vegetation can have very little effect on the condensation and precipitation of moisture in the district. But although the clearing of the bush may not affect the climate in general, it has remarkable and very serious local effects. Thus, in a sense, the corporation has shown extraordinary carelessness and neglect of prudential considerations in alloAving any of the Jan d in the catchment area of Ross's Creek to be alienated and the bush destroyed. Land covered with uncleared timber retains a gmit pioporlion of the anoisture which falls upon itj and only parts with it slowly. Il'is not a difference of evaporation, for tho probability almost certainly is thai bush lend evaporates water at a far greater rate per acre thaii ie

the case with cleared ground. It is more probable that the cause is due to the more spongy nature of the surface materials and the mechanical entanglements of roots and fallen vegetable debris. On cleared land the rain may, and probably does, fall as frequently and as heavily as* before, but instead of being retained by the surface vegetation and slowly yielded up by the never-failing streams and rills, it now rushes ofi the sloping ground, carrying away "the best of the soil into the valleys, and does damage by its unrestrained vehemence. Hundreds of acres of land formerly covered Avith noble bush, the soakage from which formerly,, fed the northern reservoir, have been cleared during the last two or three decades, to make way for grass fields which cannot retain the moisture which the forest formerly held. And, to make matters worse, houses have been erected in this catchment area, and the drainage from these must find its way sooner or later into the toAvn water supply. It is wonderful how complacently we accept all these conditions without so much as raising a finger to alter them. Give us a plague scare and we begin to 'bustle round and find out the objectionable features of our neighbours' Back yards, but as to supplying the citizens with clean water — why, that is a totally different matter.

There is one kind of weathei* of a A r ery trying and objectionable character which is experienced in all the open country on Ihe east side of this island, from Southland to Marlborough, but which we get in a A-ery mild form in Dunedin, thanks to the hilly environment. I refer to the hot nor'wester, which in the Canterbury Plains are a perfect abomination, sAveeping the very soil off the ploughed fields, and parching the vegetation. The explanation of these hot winds is very simple, and is familiar to every student of physical science, but is probably not popularly known. i When a north-westerly Avind which has traversed the Tasman Sea reaches the West Coast of this island, saturated Avith moisture, it meets with a mountain range 6000 or more feet in height, against' which it is forced, and which it has to surmount. In accordance with a well-knoAvn physical laAv, the energy required to raise the mass of air over these mountains causes a very considerable expenditure of heat, or, as avc would otherwise express it, a fall in temperature, and this results in a great condensation of moisture. But when water vapour condenses, a large amount of heat is liberated. Tho result is that the wind, in being forced up the mountain fides, pours out an immense quantity of rain on tho West Coast, but reaches the summit of the ranges as warm as it was at the foot, this being due to the latent heat Avhieh has been parted with. Coming down the eastern slopes of the mountains it regains as much heat us it io^fc in consequence of being forced up the other side, and so pours cloaui on Die open country as a hot wind. This rise of temperature causes it to expand very considerably, so that what

is a qtiiet, mild, rain-bearing wind at Hokitika appears on the Canterbury Plains as a strong, hot, and parching blast, which shrivels the A T egetation and makes life generally rather disagreeable.

The phenomenon is common to many countries of similar conformation. Thus southerly winds from the Mediterranean, which cause heavy precipitation on the Italian slopes of the Alps, bloAv over Southern Germany as hot, dry, scorching winds, exactly as our nor 1 -westers do. This wind is called the " fohn " in Germany, and is very much disliked. Though these winds "feel" heavy,— in Southland Aye- used to notice that they seemed to blow doAvn on us at a considerable angle, — they are really so expanded by heat as to lower the barometric pressure very considerably over a wide area. Therefore, after they have blown for a feAV hours, they are almost invariably replaced by a colder, heavier wind from the south-wesL. This inrush of cold wind usually lowers the temperature of the warmer air to below its saturation poinjt, and brings doAvn a more or less prolonged rainfall. We had a very remarkable example of this only as recently as yesterday, when a warm northwest breeze, — hardly felt in Dunedin as a wind at all, — dropped the barometer very sharply, and into the area of loav pressure so produced there came an inrush of cold southerly wind, Avith a very heavy downpour of rain. Of course, such an explanation is only a very partial one, for we know little of the gigantic processes which are constantly at work in the atmosphere ; but it gives a general idea of one. at least, of the potent causes of weather change.

If we would only look at our changes of weather from a point of view, then even the bad ones might become bearable ; but one must truly have a very philosophic temperament to be able to 30 out in the morning Avithout top-coat and umbrella, and get caught in a coi'd southwester at noon, without betraying any tendency to use unparliamentary expressions. I am reminded in these circumstances of a very excellent article on " Boils "' in the " Family Physician," in Avhieh it is stated that " scientific men call them fimmculi, but even then they are rather painful.'' Dunedin, May 3, 1900.