METEOROLOGY. 



the task ; and a chart for the year, and one for 

 each of the two months of July and January, 

 published in his Handy Book in 1868, are the first- 

 fruits of his labours. Ten more charts are to 

 follow, so as to exhibit the progress of the annual 

 variation, with its anomalies, throughout the year. 

 It is easy to conceive that such a system of lines 

 must throw great light on all inquiries regarding 

 prevailing winds, the varying temperature, and 

 the rainfall over the world. 



The insight into the causes of atmospheric 

 changes afforded by these charts, may be judged of 

 from what Mr Buchan says in describing that for 

 January : 'Over the North Atlantic occurs an exten- 

 sive diminution of pressure, which deepens north- 

 wards till the greatest depression, 29-5, is reached 

 in Iceland, or perhaps in a slightly lower depres- 

 sion nearly midway between that island and 

 Spitzbergen. The widening of the isobarometric 

 curves of 29-6 and 297 inches to the westward 

 over Greenland, and to the eastward over the 

 north of Norway and Russia, is an interesting 

 feature of this area of low pressure. The low 

 pressure of this region is due to the saturated 

 state of its atmosphere and to the copious rainfall 

 resulting from it. The flow of the Gulf-stream 

 north-eastwards through the Atlantic to at least 

 beyond Spitzbergen, and the larger amount of 

 vapour poured into the atmosphere from its 

 warmer waters, tends still further to lower the 

 pressure. It is this low pressure over the North 

 Atlantic, together with the high pressure to the 

 eastward over Asia, which forms the key to the 

 explanation of the winter climate of Europe.' 



Distribution of Terrestrial Temperature. The 

 temperature of the earth differs not only in differ- 

 ent regions of its surface, but at different eleva- 

 tions above or below the surface. In ascending 

 a mountain or into the air in a balloon, the ther- 

 mometer, as a rule, falls. The rate generally 

 allowed is i of Fahr. for every 300 feet.* The 

 increase of cold at high elevations arises from 

 several causes. One cause is the rarefac- 

 tion of the atmosphere, which takes place on 

 ascending (see No. 15). Under ordinary cir- 

 cumstances, when a gas is allowed to expand, its 

 temperature falls. This used to be explained by 

 saying, that a gas when rarefied has a greater 

 capacity for heat, and therefore requires a greater 

 absolute quantity to keep it at a certain tempera- 

 ture. There is really, however, no change of 

 specific heat, for it is possible, under certain 

 conditions, to rarefy a gas without any loss of 

 temperature. The real cause is that the gas in 

 expanding performs work; it puts itself in motion 

 to occupy the wider space, and no motion can be 

 caused without expending some form of energy. 

 The temperature of the gas depends upon some 

 kind of oscillating motion among its molecules ; 

 part of this molecular motion is converted into a 

 motion of translation, and the energy or heat 

 of the gas is diminished by the amount thus 

 expended. Another cause is found in the fact, 

 that the sun's rays have little effect on the atmos- 

 phere, especially when dry, and only give out 

 their full heat when they strike solid objects. It 

 is thus the lower strata of the air that are in 



* According to the experiments made by Mr Glaisher in 

 balloons, the diminution of temperature is 7-2* F. for the first 

 thousand feet, but only 5-3" F. between the first and second 

 thousand. From 14,000 to 15.000, it is reduced to 2-1*. 



contact with the warm earth that derive most heat 

 from the sun's rays. In addition to all this, the 

 radiation, or loss of heat, goes on more rapidly, 

 there being no solid objects around to return it, 

 and the rarer air opposing less obstacle to its 

 escape. At considerable elevations, too, there is 

 proportionally less vapour in the air ; and as it is 

 the chief obstructor of radiation, objects in high 

 regions are exposed naked, as it were, to the cold 

 of the outer universe. At a certain height over 

 every place, water will freeze, and if a mountain 

 rise to this height, it will be covered with snow. 

 The height over any place where water must be 

 frozen at all seasons is called the snow-line, the 

 altitude of which is greatest at the equator, and 

 diminishes as the latitude increases. At a certain 

 high polar latitude, it reaches the mean sea-level ; 

 that is to say, the ground at that level is eternally 

 clad with snow.* 



The law, however, of loss of temperature by 

 elevation is subject to important exceptions. In 

 winter, the earth is losing more heat by radiation 

 than it derives from the sun's rays ; therefore, 

 in a dry, calm, clear night, the surface rapidly 

 loses heat, and the stratum of air in contact with 

 it is thus chilled below the general temperature of 

 the atmosphere. The effect is most marked at 

 short heights above the surface. The air in con- 

 tact with the ground is frequently 15 or 20 below 

 the air four feet above the ground ; above four feet 

 the differences are comparatively small It is 

 evident from all this, that in comparing thermom- 

 eters, it is important that they be placed at the 

 same height above the surface. 



The effect of this chilling of the lower stratum 

 of the atmosphere is modified by the configuration 

 of the surface. From off heights and slopes, the 

 chilled air flows down into the low-lying grounds, 

 and there accumulates. 



' This explains,' says Mr Buchan, ' why vapour 

 becomes visible so frequently in low places, whilst 

 adjoining eminences are clear ; and the same fact 

 instinct has made known to cattle and sheep, 

 which generally prefer to rest during night on 

 knolls and other eminences. Along most of the 

 water-courses of Great Britain, during the memor- 

 able frost of Christmas 1860, laurels, araucarias, and 

 other trees growing below a certain height were 

 destroyed, but above that height they escaped.' 



The variations of temperature below the surface 

 belong rather to Geology and Physical Geo- 

 graphy. 



We have now to consider how heat is distributed 

 over the earth's surface horizontally. The causes 

 of the varying quantities of heat enjoyed by dif- 

 ferent places have already been described in a 

 general way. The results of these causes, as 

 modified by the varying relations of land and 

 water, are made visible to the eye by means of 

 charts having lines drawn through all places 

 having the same temperature. These lines are 



The snow-line is found at various heights, according to 

 latitude, proximity to the sea, and other causes, which affect the 

 general climate of the region. In the Himalaya and Andes, it a 

 found at an elevation of about 17,000 feet ; in the Swiss Alps, at 

 8500 feet ; and in the Scandinavian range, at 3500 feet. Gener- 

 ally, in those countries which are near the equator, the snow-line 

 is found about 16,000 feet, or three miles above the sea-level : 

 about the 45th parallel in either hemisphere, it occurs at an eleva- 

 tion of 9000 feet ; under 60' of latitude, at 5000 feet or thereby ; 

 under 70" latitude, at 1000 feet ; and under 8p', the snow-line 

 comes down to the mean sea-level : for countries which are 10* 

 distant from the poles are covered with snow all the year round. 



