THE POPULAR EDUCATOR. 



METEOROLOGY. I. 



SCOPE OF THE SCIENCE AERIAL PHENOMENA WINDS 



ISOBAROMETRIC LINES STORMS MEAN TEMPERATURE 



ISOTHERMAL LINES EFFECTS OF LOCAL CAUSES ON 

 CLIMATE. 



THE earth, we inhabit is, as is well known, completely sur- 

 rounded by an envelope of gaseous matter called the atmosphere, 

 which is kept close to its surface by the action of gravitation. 

 This aerial layer is the seat of many very important and inte- 

 resting phenomena, which it is the province of the science of 

 Meteorology to inquire into and explain. The science is, how- 

 ever, now usually restricted more particularly to those atmo- 

 spheric phenomena which influence weather and climate. 



Owing to the very many causes that are at work, there is 

 great difficulty in tracing out all the laws by which these 

 phenomena are governed ; and though many weather prognos- 

 tics have long passed into current proverbs, it is only recently 

 that true progress has been made in the study of the science. 



In early times, when the occupations of most men kept them 

 much in the open air, and consequently exposed to all the 

 vicissitudes of the weather, they would naturally make various 

 observations, which would enable them to foretell to a limited 

 extent the probable changes. These observations were handed 

 down from generation to generation ; but were far too vague and 

 general to serve in any way as the foundation of a science. 



In modern times, however, various instruments have been 

 devised for ascertaining accurately the pressure, the tempera- 

 ture, the humidity, and other important matters in connection 

 with atmospheric changes ; and now, by means of the baro- 

 meter, thermometer, and similar instruments, constant records 

 are being kept, in different places, of all these changes. 



In a science like Meteorology, it is only by the careful exami- 

 nation of long-continued records of this kind that true progress 

 can be made. The first thing required is to observe the pheno- 

 mena accurately, and we must endeavour to explain and account 

 for them. To start hypotheses first, and then endeavour to 

 reconcile observations with them, is a mistake. 



So numerous and varied are the phenomena with which we 

 have to deal, that it is somewhat difficult to classify them. For 

 the sake of convenience we may divide them into aerial, aqueous, 

 and optical phenomena. It must not, however, be supposed 

 that these classes are indicative of fundamental differences. 



The chief physical properties of the air have already been 

 referred to in our lessons on Pneumatics and Chemistry, to 

 which, accordingly, we must refer the student. In this aerial 

 ocean we meet with regular tides, but currents produced by 

 other .causes are far more common and important. 



These currents in the air are known as the winds, and they 

 usually arise either from a change in temperature, or in the 

 amount of watery vapour it holds in solution. The direction of 

 the wind is always indicated by the point of the compass, from 

 which it appears to come ; and as the weather and climate of 

 any place are greatly affected by the prevailing direction, con- 

 siderable attention has been directed to the subject, and records 

 are kept in many places showing the direction and intensity of 

 the wind at stated times every day. From such registers it 

 appears that in this country S.W. winds are the most prevalent, 

 since they blow on the average 225 days out of every 1,000. We 

 find, too, that westerly winds are more common than easterly, in 

 the proportion of about 220 to 145. 



The prevailing character of any wind depends chiefly upon the 

 countries over which it passes. With us, a west wind is usually 

 moist and mild, having swept over the Atlantic Ocean, and thus 

 become partly charged with vapour and warmed. So, too, in 

 the south of Europe, especially in places where the Mediter- 

 ranean is narrow, a south wind is very hot and dry, from having 

 swept over the arid deserts of Africa. In Italy this is especially 

 noticeable, and this wind is there distinguished as the "Sirocco." 

 A similar wind, but more injurious in its effects, is very preva- 

 lent in the desert regions of Arabia and Syria, and is known as 

 the " Simoom." This is frequently spoken of as poisonous. It 

 appears, however, that it is merely a very hot, dry wind, com- 

 pletely charged with fine dust. Everything exposed to its 

 influence becomes rapidly dried up; the skin is parched and 

 dry ; a general languor comes over everything ; and if the 

 traveller is unable to find a place of shelter, he not unfrequently 

 perisl es. 



The trades, monsoons, and other periodical winds, have 

 already been explained in treating of Pneumatics. Near the 

 equator, and running almost parallel to it, is a belt some 4 or 

 5 in breadth, known as the Region of Calms. The position of 

 this varies with that of the sun, and the whole region is charac- 

 terised by very heavy rains and frequent thunderstorms. 



As the winds are mainly produced by variations in the baro- 

 metric pressure, great light is often thrown on their phenomena 

 by observing the different pressure at neighbouring places. 

 Maps are now drawn with isoparametric lines marked on them ; 

 that is, lines passing through those places where the mean 

 pressure is the same ; and when these are carefully constructed, 

 much may be learnt from them. 



The climate of most places is greatly influenced by local 

 peculiarities, as will be more fully explained, and hence these 

 must be taken into account. In tropical regions, however, 

 these local influences are almost overpowered by the great and 

 regular currents which prevail, and therefore they interfere far 

 less than they do in more temperate climes. 



Tha investigation of the laws governing storms and their 

 movements is one of the most important but most difficult 

 questions in Meteorology. The great destructive power which 

 they possess, and the extent to which they may be guarded 

 against when expected, show the importance of this inquiry. 

 What is most needed for this purpose is a complete set of simul- 

 taneous observations made during the prevalence of any storm ; 

 and, as observers are now stationed in different parts of Europe, 

 these observations have frequently been obtained. 



The best method of examining them is to lay down on a map 

 isobarometric lines showing the pressure during the storm in 

 different- places, since this appears to be the most important 

 item in the inquiry. By charts of this kind it is found that the 

 centre of the storm is marked by a region of unusually low 

 pressure, the barometer often standing considerably below 29- 

 inches. The barometric lines, too, are usually circular or ellip- 

 tical. Occasionally, these curves are very irregular, but this 

 often arises from two or three storms which have parted from 

 the original one, and sometimes re-ixnite with it. 



As a storm dies out, the central depression becomes much 

 less, and occupies a more limited area. The direction in which 

 storms travel in Europe is usually from S.W. or W.S.W., and 

 is more or less circular. The rate at which they move varies 

 considerably, but is usually about eighteen or twenty miles an 

 hour, though it has been known to be twice as great. 



Since the west of Ireland is some 450 miles from the east 

 coast of England, a storm appearing in the former locality 

 may at once be announced to eastern seaports, and thus nearly 

 twenty-four hours' notice of its approach may be gained. In 

 order, however, to be of much service, these notices must be 

 transmitted at frequent intervals. If only daily signals are 

 sent, the storm may outstrip them. In calm weather, one 

 report would, of course, suffice, but in unsettled weather they 

 should be much more frequent. 



The direction of the wind during a storm is not directly 

 towards the point of least pressure, but usually to some point a 

 little to the right of it, so that the wind flows in a spiral 

 direction round the area of low pressure. The force at any 

 place is usually proportional to the difference in the pressures 

 at the places between which it is situated. 



In tropical regions storms are much more frequent, and ar& 

 there preceded by a very sudden and remarkable fall in the 

 barometer. During a storm which raged at Guadaloupe on the 

 6th of September, 1865, the mercury fell T693 in. in one hour 

 and ten minutes. This sudden fall relieves the ocean of a large 

 portion of its pressure, and hence the water is often raised to a- 

 considerable height by the greater pressure all around. To this 

 cause must be attributed those storm-waves which frequently 

 accompany tropical hurricanes, and often cause a great loss of life 

 and property. The rotating character of these storms is very 

 clearly seen, and when the centre is over any place, the clouds 

 in the zenith are frequently seen to be revolving rapidly. 



In the tropics the movements of the barometer are very 

 regular indeed, the daily variation being most distinctly observed. 

 Any deviation from this at once indicates a disturbance of the 

 atmosphere in the locality, and affords a sure method of fore- 

 telling a storm. An isolated observer may, in fact, easily ascer- 

 tain their approach, and give warning of it. 



In our latitudes the irregular variations of the barometer are 



