UM 



METEOROLOGY. 



[OA8EOCS ABSORPTION' OF BEAT. 



by convection, to the various parts of the globe, a large 

 proportion would be again radiated into space, and be 

 loot, so far as our earth it concerned. This provision is 

 twofold : namely, that of the absorption effected by the 

 olid crust of the earth, and that by the aqueous vapour 

 which floats over its surface. It is particularly the latter 

 provision that we wish here to investigate. 



In our article on the Refraction of Heat, we pointed 

 out that certain bodies have a property by virtue of 

 which they either absorb or transmit that forco ; and 

 that they vary much in the amount of this power : in 

 fact, just as we noticed, that, with respect to light, some 

 bodies are transparent, translucent, and opaque ; so, 

 similarly, some bodies hnve a precisely identical, or, at 

 all events, analogous effect on radiant heat.* The 

 general law of absorption was fully investigated in 

 the article on that subject, a re-perusal of which will 

 be useful to the student in connection with these 

 remarks, f 



It must be borne in mind, that considerable difference 

 subsists between these two classes of absorbers. Those 

 which refract heat have a general relation to certain 

 similar properties in respect to light. Thus, rock-salt 

 and alum will transmit and refract Tight ; yet, whilst the 

 former permits the passage of heat-rays with the greatest 

 facility (exceeding that of plate-glass), a plate of alum, 

 transparent to light, entirely obstructs the passage of 

 non-luminous heat-rays. Similarly, we find that solid, 

 good absorbers, such as lamp-black, &c. , have a mutual 

 relation to heat and light ; but in this case they agree, 

 for they absorb equally the luminous and calorific rays 

 impinging on them. 



Now, in nature, these two classes of absorbers and 

 radiators are met with on the large scale. Every tract 

 of laud more or less absorbs the rays of heat, and also 

 radiates them again into space. But above this solid 

 matter is the atmosphere, which, by its heterogeneous 

 nature, exercises a most important action on the heat 

 reflected and radiated from the surface of the earth. 

 Referring to the account of Dr. Tyndall's experiments 

 (given at page 38, in the Section on Heat), we find that 

 the mixture not a chemical compound forming our 

 atmosphere, the carbonic acid generally found in it, and 

 the aqueous vapour (also usually present), have each a 

 definite and greatly varying relationship to, and action 

 on, the rays of heat incident on, and transmitted by 

 them. Here, again, we notice that certain qualities of 

 bodies, such as transparency and translucency, are in 

 connection alike with the forces of both light and heat ; 

 and the law of diathermancy seems to comprise a par- 

 ticular condition in regard to caloric, equivalent to 

 transparency in relation to light. X 



Aqueous vapour possesses a wonderful power of 

 absorbing the rays of heat, and thus preventing their 

 passage from the earth into space. We have already 

 noticed this when speaking of dew ; as we then stated 

 that a cloudy night entirely prevented its deposition; 

 the clouds, of course, being composed of aqueous vapour 

 in a partially condensed state, but still in minute 

 divisions. But, in this case, the comparative opacity of 

 the clouds would naturally suggest a certain amount of 

 interference ; and a supposition might be encouraged 

 that they must act as a screen between the radiating 

 surface of the earth and the open space beyond the 

 limits of the atmosphere. 



It is not, however, in the form of clouds alone, or even 

 specially, that aqueous vapour becomes a retainer of 

 terrestrial heat. Suspended invisibly, as in the clear air 

 of a hot summer's day, it acts powerfully in absorbing the 

 heat transmitted from the sun, and reflected or radiated 

 from the earth, and that to an extent enormously beyond 

 the similar power of our atmosphere when in a normal 

 state ; that is, considered as a mixture of oxygen and 

 nitrogen gases simply. The moisture constantly present 

 in the atmosphere has thus a most important influence 

 oa the temperature of most climates ; indeed, all 



SM StcUon on Jlial. p. . 



4 ;w., p. M. i n<4.. p. M. nn., p. m 



I Set mto, pp. 1180, ud USJ, US*. H Set nti, p. UM. 



in which aqueous vapour is generally prevalent in the 

 atmosphere. 



This will be readily noticed if we compare the V 

 and lower latitudes of the earth, or even its mountain 

 elevations. The nearer the poles, or the higher the ele- 

 vation above the surface of the earth, the less the moisture 

 that is constantly present in the superincumbent atmo- 

 sphere -,11 and on the other hand, the nearer we approach 

 the equator, the greater becomes the constituent moist- 

 ure : as a general and consequent law, therefore, the 

 temperature is directly as the moisture in the air, under 

 certain circumstances. In our own country, the con- 

 trast between the dry, cold, north-east wind, and that 

 from the south-west, which is of a warm and genial tem- 

 perature, and flowing from the Gulf -stream, is constantly 

 recognised. For similar reasons, we shall find, in most 

 countries, whilst ascending lofty mountains, which, at 

 their base, have a climate of a high temperature, that we 

 experience increasing cold until we arrive at a region 

 where every particle of moisture is always congealed into 

 the solid state, which will be at a point situated near 

 what is called the snow-line. The folio plates give a 

 general view of the chief elevations in the eastern and 

 western hemispheres, and are accompanied with a scale, 

 indicating the heights of mountain ranges, &c. There ;iro 

 also marked certain altitudes attained by Mr. Gluisher, 

 Gay Lussac, and Humboldt, whose observations verify 

 the remarks just made. A little above and below the 

 snow-line, the annual variation of temperature either 

 melts the congealed water in summer, or freezes it as 

 winter conies on. The former process, together with the 

 precipitation of moisture (as rain), gives rise to rivers, the 

 chief of which are also noted, with a scale of their lengths, 

 in the plates just referred to. 



Thus we find, that when the atmosphere is destitute 

 of suspended moisture, intense cold prevails, chiefly 

 owing to the immediate radiation of the heat received 

 with the luminous solar rays. On this principle may be 

 further explained the fact noticed by Mr. Glaisher, that, 

 as he ascended in the air, a blackcned-bulb thermometer 

 ceased to show an increased heat, when exposed to the 

 full rays of the sun, beyond that indicated by those kept in 

 the shade. We have alraady anticipated, in some re- 

 spects, the solution of this at a previous page ;^f but may 

 here add, that a blackened-bulb thermometer, similarly 

 exposed at the sea-level, will occasionally rise even 40 

 to GO above the temperature shown by a thermometer 

 shaded from the rays of the sun at the same time and 

 place. In the latter cose, the absorbed heat will, 

 according to the researches of Dr. Tyndall, be prevented 

 from immediate radiation by the moisture resident in the 

 surrounding hot atmosphere. 



The average altitude of the snow-line does not depend 

 entirely on latitude, but is greatly affected by the amount 

 of moisture, &c., in the atmosphere. Thus, some por- 

 tions of the Himalayas have the snow-line at 1,500 feet 

 greater elevation than that common at the equator : and 

 comparing, again, Sierra Nevada in Spain, in 35 north 

 latitude, with some of the Himalayan mountains in 

 Thibet, in the same latitude, we find that the snow-line 

 of the Spanish mountain is at 9,500 feet, whilst that of 

 the Himalayan ranges from 15,000 to 2tyOOO feet, above 

 the sea-level varying, in a distance of sixty miles, to the 

 extent of 5,000 feet. This is chiefly due to the absence 

 of moisture a circumstance that prevents the formation 

 of the snow, simply because there is literally nothing of 

 watery matter to freeze. For similar reasons, the 

 altitude of the snow-range on these mountains rises 

 uniformly, in proceeding northward from India, and 

 consequently from the equator, up to a north latitude of 

 35. The rapid radiation of heat, and c< 

 equally rapid evaporation even of snow, in some parts 

 of the Himalayan range, is the chief cause of this in- 

 creased average altitude of the snow-line, which is fre- 

 quently greater on the north than on the south side. 



The following table gives its height above the sea-lovel, 

 for many latitudes, both north and south of the equator, 

 and will form a kind of supplement to the illustrations 

 afforded in the folio plates. It will bo noticed, that, 



