METEOROLOGY. 



temperature of January, and the solid lines that 

 for July. 



The part of the globe having the highest mean 

 annual temperature forms an irregularly shaped 

 belt, lying along the equator, and comprised 

 between the north and the south isothermals of 

 80. On either side of this warm belt the tem- 

 perature diminishes towards the poles ; and the 

 lines shewing successively this diminution are, 

 speaking in a very loose sense, arranged parallel 

 to the equator, thus shewing the all-predominating 

 influence of the sun as the source of terrestrial 

 heat. The coldest portion of the earth's surface 

 is a small oval-shaped patch near to but not 

 surrounding the north pole, its mean temperature 

 being 4. Its narrowest diameter lies north and 

 south, nearly touching the pole on the one side, 

 and extending on the other as far south as 72 0< 3o 

 N. lat. in 130 W. long. Part of it is seen in the 

 diagram. 



While the decrease of temperature in advancing 

 towards the poles corresponds in a general way to 

 what may be called the solar climate, there are 

 deviations brought about by disturbing causes too 

 important to be overlooked. The chief of these 

 disturbing causes are (i) the currents of the sea ; 

 and (2) the prevailing winds. 



Of ocean-currents affecting temperature, the 

 most marked and important is the Gulf-stream in 

 the North Atlantic, which, by conveying warm 

 water to the arctic regions, pushes the isothermals 

 many degrees to the northward. There is a similar, 

 though much feebler, current passing from the 

 North Pacific to the Arctic Sea through Behring's 

 Strait, and there, accordingly, the isothermals are 

 pushed a little to the northward. An opposite 

 effect is produced by two cold currents from the 

 Antarctic Ocean, flowing, the one along the coast 

 of Peru, the other along the west coast of Africa. 



Since winds bring with them the temperature 

 of the regions they have crossed, the equatorial 

 current is a warm wind, and the polar a cold wind ; 

 also winds arriving from the ocean are not subject 

 to such variation of temperature during the year 

 as winds from a continent. As an atmosphere 

 loaded with vapour obstructs both solar and 

 nocturnal radiation, it follows that moist winds 

 are accompanied with a warm temperature in 

 winter, and a cool temperature in summer ; and 

 dry winds with cold winters and hot summers. 

 The direction of mountain-ranges is also an 

 important element to be taken into account in 

 estimating the influence of winds on temperature. 

 These considerations explain the position of the 

 isothermals in the north temperate zone, where the 

 prevailing wind is the south-west or anti-trade. 

 In January, the western parts of each conti- 

 nent enjoy a comparatively high temperature, 

 from their proximity to the ocean, whose high 

 temperature the winds waft thither ; and they are 

 further protected from extreme cold by their moist 

 atmosphere and clouded skies. But in the interior 

 of the continents it is otherwise ; for the winds 

 getting colder as they advance, and being deprived 

 of their moisture as they cross the mountains in 

 the west, the soil is exposed to the full effects of 

 radiation during the long winter nights, and as a 

 consequence, the temperature rapidly fans. In the 

 centre of Siberia, the January temperature falls 

 to 40, which is 9 colder than the coldest part 

 of the American continent ; and this centre of 



greatest cold lies near the eastern part of the con- 

 tinent of Asia. On the other hand, in July the 

 interior of continents is much warmer than their 

 western parts. Hence the interior and eastern 

 parts of Asia and America are characterised by 

 extreme climates, and the western parts by equable 

 climates. Thus, at Yakutsk, in Siberia, the July 

 temperature is 02-2, and the January -43'8, the 

 difference being io6x>; whilst at Dublin these are 

 respectively 6<?-& and 38-s, the difference being 

 only 22'3. This constitutes the most important 

 distinction of climates, both as respects vegetable 

 and animal life. On man especially the effect is 

 very great the severity of the strain of extreme 

 climates on his system being shewn by the rapidly 

 increasing death-rate as the difference between 

 the July and January temperatures increases. 



Winds and Storms. Winds are classed as 

 Constant, Periodical, and Variable. The Trade- 

 winds and Return Trades constitute the first 

 class ; Sea and Land breezes, and Monsoons form 

 the periodical class, and both have been con- 

 sidered under the general movements of the atmos- 

 phere. 



Variable winds depend on purely local or tem- 

 porary causes, such as the nature of the ground,, 

 covered with vegetation or bare; the physical 

 configuration of the surface, level or mountainous ; 

 the vicinity of the sea or lakes ; and the passage 

 of storms. The hot, suffocating wind peculiar to 

 Africa and Western Asia is known by the name 

 of the Simoom ; on the coast of Guinea it is 

 called the Harmattan. A similar wind in Sicily 

 and Italy is called the Sirocco, and in Spain the 

 Solano. The East Winds which prevail in the 

 British Islands in spring are part of the great 

 polar current which at that season descends over 

 Europe through Russia. Their origin explains 

 their dryness and unhealthiness. It is a prevalent 

 notion that the east winds in this country are damp. 

 It is quite true that many easterly winds are 

 peculiarly damp ; all that prevail in the front part 

 of storms are very damp and rainy, and soon 

 shift round to some westerly point. But the 

 genuine east wind, which is the dread of the 

 nervous and of invalids, does not shift to the west^ 

 and is specially and intolerably dry. Deaths from 

 brain-diseases and consumption reach the maxi- 

 mum in Great Britain during the prevalence of 

 east winds. The Etesian Winds are northerly 

 winds which prevail in summer over the Medi- 

 terranean Sea. They are caused by the great heat 

 of North Africa at this season, and consist in a 

 general flow of the air of the cooler Mediterranean 

 to the south, to take the place of the heated air 

 which rises from the sandy deserts. The Mistral 

 is a steady, violent north-west wind, felt particu- 

 larly at Marseille and the south-east of France,, 

 blowing down on the Gulf of Lyons. 



Lord Bacon remarked that the wind most fre- 

 quently veers with the sun's motion, or passes 

 round the compass in the direction of N., N.E., 

 E., S.E., S., S.W., W., and N.W., to N. This 

 follows in consequence of the influence of the 

 earth's rotation in changing the direction of the 

 wind. Professor Dove* of Berlin has the merit of 

 having first propounded the Law of the Rotation 

 of the Winds, and proved that the whole system 

 of atmospheric currents the constant, the peri- 

 odical, and the variable winds obey the influence 

 of the earth's rotation. 



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