CLIMATE. 



CLIMATE. 



th; accumulated heat of the sun rarefies the 

 air, which, ascending into the higher regions 

 of the atmosphere, flows off towards the north 

 and south. To compensate for the loss by this 

 successive flowing off of the heated and rare- 

 fied portion, arid maintain that equilibrium 

 which the barometer informs us always sub- 

 sists in the atmosphere throughout the globe, 

 lower currents of heavier air sweep into the 

 tropical regions from the northward and south- 

 ward. These last have been denominated the 

 polar currents, whilst the uppermost are de- 

 signated as the tropical currents ; and these, j 

 it is well known, do not flow directly north or j 

 south, but slantwise, a fact which is ascribed j 

 to the influence exerted by the motion of the 

 globe upon its axis, and the different velocities 

 existing at different parts of its surface. Owing, 

 therefore, to the combined agencies of solar 

 heat and diurnal rotation, the lower winds in 

 the equatorial region have a slanting direction 

 from the eastward, constituting the trade winds, 

 which blow the year round between the tropics,, 

 except where changed into monsoons by the 

 interposition of some influences by which a 

 change is wrought in their direction during 

 six months of the year. Whilst the winds 

 within the tropics thus blow interminably from 

 the eastward, those without the tropical limits 

 have a prevailing direction from the west. 

 Here then we find the solution of the problem, 

 that in extra-tropical latitudes all countries 

 situated to the eastward of seas or other great 

 bodies of water have milder climates than 

 those occupying the eastern portions of con- 

 tinents. Large bodies of water never become 

 so cold in winter or so warm in summer as the 

 earth. Hence, whenever the predominant 

 winds sweep from the sea, they carry with 

 them the temperature of the water to a greater 

 or less distance inland, and thus obviate ex- 

 tremes. When, however, the prevailing winds 

 pass over large tracts of country, they must 

 necessarily bear with them the greater or less 

 degrees of cold induced by congelation, and 

 still more through radiation, whilst in summer 

 they will convey the accumulated heat ab- 

 sorbed by the earth. This view enables us to 

 understand why the proximity of the Gulf 

 Stream, that mighty lake of warm water, as 

 Major Reynell calls it, not inferior in size to 

 the Mediterranean, does not shed upon the 

 shores of the United States a larger portion of 

 its high temperature, the greatest proportion 

 of the warmth communicated by it to the 

 atmosphere being actually wafted to the distant 

 shores of Europe. 



The celebrated Humboldt, who has devoted 

 so much attention to the investigation of cli- 

 mate, and especially to the laws and agencies 

 concerned in the distribution of heat over the 

 surface of the globe, has formed a system of 

 lines of equal temperature encircling the globe, 

 and passing through places having the same 

 mean temperature, either throughout the year 

 or during particular seasons. Those passing 

 through places having similar annual means 

 are called isothermal hues. As, however, it is 

 frequently found that where the annual tern- 1 

 peratures agree there is a great difference in ' 

 the means of particular seasons, other lines j 



have been drawn to show this, such as pass 

 through places having equal summer tempera- 

 tures being called isotherul, and those represent- 

 ing equal winter means isocheimal lines. 



These lines, which from their generally 

 crooked forms are also called curves, demon- 

 strate to the eye in a striking manner the well 

 known fact, that the distribution of temperature 

 on both sides of the equator is by no means in 

 exact conformity to latitude or distance from 

 the equinoctial line. Let us, for example, take 

 Humboldt's isothermal line drawn through 

 different points around the globe, having a 

 mean annual temperature of 55-40 Fahr., 

 and we shall find it in the eastern part of North 

 America passing near Philadelphia, in latitude 

 39 56'; in the eastern part of Asia, near Pekin, 

 in the same latitude with Philadelphia ; whilst 

 on the western side of Europe it runs near 

 Bourdeaux, in latitude 45 46'; and on the 

 western coast of North America, it is found at 

 Cape Foulweather, a little south of the mouth 

 of the Columbia river, latitude 44 40'. Be- 

 tween the western part of Europe and the 

 eastern portion of North America, the follow- 

 ing differences in the mean temperature are 

 found in similar latitudes, the increase in 

 latitude being attended by a very great increase 

 in the difference of the means : 



Latitude. 

 30 



50 

 60 



Mean temp, of K. 

 tout N-Am. 



54'50 

 37-94 

 2372 



Mean tsmp. W. Difference 

 coast of Europe, luean tern 



7052 3'60 

 63-!4 8-64 

 5iH".i 12-96 

 16-88 



Now all the great varieties in the lines of 

 equal temperature are mainly dependent upon 

 the operation of those extensive natural move- 

 vhich we have styled the great atmos- 

 pheric circulation. 



The climate of the United States is distin- 

 guished by its extremes of heat and cold. It 

 might be naturally expected that the greatest 

 heat would b? registered at the most souther- 

 ly, and the severest cold at the most northern 

 posts. But the exact instrumental observations 

 now furnished prove this not to be the case, 

 especially in the vicinity of the sea, where it 

 would seem the proximity of water tends to 

 moderate the heat of summer in the south, and 

 the cold of winter in the north. It is in some 

 of the western regions, remote from the ocean 

 and inland seas, those, for example, in which 

 forts Snelling, Gibson, and Council Bluffs, are 

 situated, that the mercury rises highest and 

 sinks the lowest On the 15th of August, 1834, 

 at Fort Gibson, two thermometers observed by 

 Dr. Wright of the army, rose in the shade, 

 carefully excluded from reflected or radiated 

 heat, the one to 116, and the other to 117 

 Fahrenheit. 



It is a law applicable to all parts of the 

 world, wherever no inland lakes or seas exist, 

 to interpose a modifying influence, that on 

 leaving the coast and going into the interior, 

 the difference between the mean temperature 

 of summer and winter increases, the climates 

 being more subject to extremes of heat and 

 cold. To show that no exception to this law 

 is furnished in the United States, we may ad 

 duce the instance of Fort Sullivan, Eastpor* 



331 



