Alexander Buchan

“ The frosts of winter are most severely felt in those localities where the slopes above them are destitute of vegetation, and consist only of bare rock and soil, or of snow. If, however, the slopes be covered with trees, the temperature is warmer at the base and up the sides of the mountain, the beneficial influence of forests consisting in the obstacle they offer to the descending currents of cold air and in distributing the cold produced by terrestrial radiation through a stratum of the atmosphere equalling in thickness the height of the trees. ”

“ Considering the weather of the year as made up of several distinct climates differing from each other according to temperature and moisture and their relations to each other, it may be divided into six distinct climates, characterised respectively by cold, cold with dry- ness, dryness with heat, heat, heat with moisture, and cold with moisture. Each of these six periods has a peculiar influence in increasing or diminishing the mortality, and each has its own group of diseases which rise to the maxi mum, or fall to the minimum mortality, or are subject to a rapid increase or a rapid decrease. ”

“ Though the space filled by the down-flowing current of cold air in the bottom of a valley is of greater extent than the bed of a river, it is yet only a difference of degree, the space being in all cases limited and well defined, so that in rising above it in ascending the slope the increased warmth is readily felt, and, as we have seen, in extreme frosts the destruction to trees and shrubs is seen rapidly to diminish. ”

“ Observation shows that when the quantity of vapour in the air is great, and also when the relative humidity is high, temperature falls little during the night, even though the sky be perfectly clear ; but when the quantity of vapour is small, or the relative humidity is low, temperature rapidly falls. On the other hand, during the day the temperature rises slowly, when the quantity of vapour is great, or relative humidity high, even though the sky be clear, but when the quantity of vapour is small, and humidity low, temperature rapidly rises. ”

“ Consequently, air under a pressure equal to that of two atmospheres will occupy only half the volume it occupied under the pressure of one atmosphere ; under the pressure of three atmospheres, one- third of that volume, &c. By doubling the pressure we double the elasticity. If, however, the temperature be increased, and the air occupy the same space, the pressure will be increased ; but if the pressure is to remain the same, the air must occupy a larger space. ”

“ But from the facts adduced, it is equally certain that, be the originating cause what it may, its effects are enormously modified by the distribution of land and water over the globe, by the wind, and by the absolute and relative humidity of the atmosphere. The smallness of the amount of the summer oscillation from the forenoon maximum to the afternoon minimum over the North Atlantic as far south as lat. 30, and its diminished amount, as far south at least as the equator, will no doubt play an important part in the unravelling of this difficulty. ”

“ This points to a chief acting force on which the great movements of the atmosphere depend, viz., simultaneous local irregularities in the distribution of temperature in the atmosphere. The local expansion of the atmosphere by heat during the day is greatest over land, where the air is clear, dry, and comparatively calm, and least over the ocean, where the sky is clouded, and the air loaded with moisture. ”

“ Atmospheric pressure is more uniformly distributed over the globe in April and October than in any of the other months. In May and November, being the months immediately following, occur the great annual rise and fall of temperature ; and since these rapid changes take place at very different rates, according to the relative distribution of land and water in each region, a comparison of the geographical distribution of May with that for the year brings out in strong relief the more prominent causes which influence climate, and some of the more striking results of these causes. ”

“ When it is borne in mind that in temperate climates, such as that of Great Britain, the majority of the deaths which occur in the winter months are occasioned or at least hastened by low temperatures, it will be recognized as of the most vital importance, especially to invalids, to know what are the local situations which afford the best protection against great cold. ”

“ The gradual narrowing of a valley tends to a more rapid lowering of the temperature for the obvious reason that the valley thereby resembles a basin almost closed, being thus a receptacle for the cold air-currents which descend from all sides. The bitterly cold furious gusts of wind which are often encountered in mountainous regions during night are simply the out-rush of cold air from such basins.
The two chief causes which tend to counteract these effects of terrestrial radiation are forests and sheets of water. ”

“ The investigation of this phase of climate, which can only be carried out by the examination of many thousands of daily weather charts, is as important as it is difficult, since till it be done the advantages and hazards offered by different sanataria cannot be compared and valued. It may in the meantime be enough to say that no place anywhere in Europe or even in Algeria offers an immunity from the risks arising from the occurrence of cold weather in winter at all comparable to that afforded by the climates of Egypt and Madeira. ”

“ As the air in contact with the declivities of hills and rising grounds becomes cooled by contact with the cooled surface, it acquires greater density, and consequently flows down the slopes and accumulates on the low-lying ground at their base. It follows, therefore, that places on rising ground are never exposed to the full intensity of frosts at night ; and the higher they are situated relatively to the immediately surrounding district the less are they exposed, since their relative elevation provides a ready escape downwards for the cold air almost as speedily as it is produced. ”

“ The infinitely diversified character which climate displays may be referred to the combined operation of different causes, which are chiefly reducible to these four distance from the equator, height above the sea, distance from the sea, and prevailing winds, which may thus be regarded as forming the great bases of the law of climate. ”

“ An excess prevails over North America to the north of the Lakes, over Arctic America, Greenland, the British Isles, and to the north of a line passing through the English Channel in a north easterly direction to the Arctic Sea. The excess in the southern hemisphere includes the southern half of south America and of Africa, the whole of Australia, and adjacent parts of the ocean. The influence of the land of the southern hemisphere, which in this month is colder than the surrounding seas, brings about an excess of pressure ”

“ One of the most important steps that could be taken would be an extensive series of observations from such countries as India, which offers such splendid contrasts of climate at all seasons, has a surface covered at one place with the richest vegetation, and at others with vast stretches of sandy deserts, and presents extensive plateaus and sharp ascending peaks all which conditions are indis pensable in collecting the data required for the solution of this vital problem of atmospheric physics. ”

“ The low areas of the American cyclones, as they proceed eastward along the north shores of the Gulf of Mexico, are often immediately followed to west and north-westward by areas of very high pressure, the necessary consequence of which is the setting in of a violent norther over the Southern States. Since similar barometric conditions do not occur in the region of Lower Egypt, its climate is free from these sudden changes which are so injurious to the health even of the robust. ”

“ Since the heat daily received by the ocean from the sun is diffused downwards through a very considerable depth, the surface of the ocean on which the atmosphere rests is much less heated during the day than is the surface of the land. ”

“ On the north side of the Himalayas it is about 4000 feet higher than on the south side, owing to the greater depth of snow falling on the south side and the greater dryness of the climate of Tibet, resulting in a more active" evaporation from the snows and stronger sun-heat on the north side, to which is to be added the comparative want of vegetation on the north side, thus favouring a more rapid melting of the snows. The snow-line is higher in the interior of continents than near their coasts, the rain fall there being less and the heat of summer greater ”

“ The forenoon maximum is conceived to be due to the rapidly increasing temperature, and the rapid evaporation owing to the great dryness of the air at this time of the day, and to the increased elasticity of the lowermost stratum of air which results therefrom, until a steady ascending current has set in. As the day advances, the vapour becomes more equally diffused upwards through the air, an ascending current, more or less strong and steady, is set in motion, a diminution of elasticity follows, and the pressure falls to the afternoon minimum. ”

“ Climate in its modern acceptation signifies that peculiar state of the atmosphere in regard to heat and moisture which prevails in any given place, together with its meteorological conditions generally in so far as they exert an influence on animal and vegetable life. ”

“ Observation shows, as might have been expected, that the rate at which the temperature falls with the height is a very variable quantity, varying with latitude, situation, the state of the air as regards moisture or dryness, and calm or windy weather, and particularly with the hour of the day and the season of the year. In reducing temperature observations for height, 1 for every 300 feet is generally adopted. ”

“ It follows from these relations of the wind to temperature and moisture that the S.W. wind in the British Isles is a very moist wind, being both an oceanic and equatorial current ; whereas the N.E. wind, on the other hand, is peculiarly dry and parching, because it is both a northerly and continental current. Owing to the circumstance of atmospheric pressure diminishing from the south of Europe northwards to Iceland, it follows that S.W. winds are the most prevalent in Great Britain ”

“ In truth, mere local situations may during periods of intense cold have the effect of maintaining a temperature many degrees above that which prevails close at hand a difference which must mitigate suffering and not unfrequently prolong life. ”

“ Of the periodical variations of atmospheric pressure, the most marked is the daily variation, which in tropical and sub-tropical regions is one of the most regular of recurring phenomena. In higher latitudes the diurnal oscillation is masked by the frequent fluctuations to which the pressure is subjected. If, however, hourly observations be regularly made for some time, the hourly oscillation will become apparent. ”

“ If a deep lake fills the basin, the cold air which is poured down on its surface having cooled the surface water, the cooled water sinks to a greater depth, and thus the air resting over the lakes is little if at all lowered in tem perature. Hence deep lakes may be regarded as sources of heat during winter, and places situated near their outlet are little exposed to cold gusts of wind, while places on their shores are free from the severe frosts which are peculiar to other low-lying situations. ”

“ Evaporation proceeds slowly from the damp soil usually found beneath trees, since it is more or less screened from the sun. Since, however, the air under the trees is little agitated or put in circulation by the wind, the vapour arising from the soil is mostly left to accumu late among the trees, and hence it is probable that forests diminish the evaporation, but increase the humidity, of climates within their influence. The humidity of forests is further increased by the circumstance that when rain falls less of it passes immediately along the surface into streams and rivers ”

“ The mortality from all causes and at all ages shows a large excess above the average from the middle of November to the middle of April, from which it falls to the minimum in the end of May ; it then slowly rises, and on the third week of July suddenly shoots up almost as high as the winter maximum of the year, at which it remains till the second week of August, falling thence as rapidly as it rose to a secondary minimum in October. ”

“ Here the highest pressure is in the centre of the system, and, as usually happens, the isobarics are less symmetrical than those near the centre of a cyclone. The winds, as usual in anticyclones, are light ; this, however, is the essential point of difference the winds do not flow inwards upon the centre, but outwards from the region of high pressure ; and it will be observed that in many cases they cut the isobarics at nearly right angles. ”

“ Oceanic climates are the most equable of all climates, showing for the same latitudes the least differences between the mean temperatures of the different hours of the day and the different months of the year, and being at all times the least subject to violent changes of temperature. So far as man is concerned, oceanic climates are only to be met with on board ship. ”