Facts in Meteorology. 133 



Of Climate, as connected with the Atmospheric and Oceanic Currents. 



The term climate, expresses that particular combination of tem- 

 perature and moisture, which exists in the atmosphere of any greater 

 or less extent of country. 



The temperature of the sea has an effect upon the winds which 

 pass over it, and this temperature being more equable than that of 

 the land, tends to equalize the temperature of an island or a mari- 

 time country. The temperature of the ocean, contiguous to any 

 country, also depends much upon the position of that country in re- 

 lation to the great oceanic currents. 



' It is the character of the prevailing winds which chiefly, and more 

 immediately, affects the climate of any country situate without the 

 tropics. The currents of the atmosphere, like those of the ocean, 

 form a system of continued circuits, by which the accumulated 

 warmth of one region is often conveyed to another, and by this 

 means important modifications of climate are produced. 



Mountains also affect climate in more ways than one. By causing 

 the condensation of aqueous vapor they occasion copious rains. They 

 also afford shelter from winds ; and by their position modify or con- 

 trol the currents of the lower atmosphere, and sometimes occasion 

 great diversities in the climate of countries and places near to each 

 other. 



The climate of the United States and Canada strongly illustrates 

 the influence of these causes. The tropical current or trade wind, 

 being deflected by the Mexican elevations, enters the great basin of 

 the Mississippi and sweeps freely over the extensive country lying 

 east of the Rocky mountains. Here, by change of latitude, the 

 diurnal motion of the surface becomes less than that of the super- 

 incumbent fluid, which therefore necessarily assumes the form of 

 westerly winds, and passes back to the Atlantic, to be in due time 

 again merged in the north-easterly trades. When this tropical cur- 

 rent keeps sole possession of the surface, which it often does for 

 days together, extraordinary heat prevails, extending frequently 

 through the entire basin of the St. Lawrence, and sometimes rais- 

 ing the thermometer on the borders of that river, at Montreal, to 

 98 degrees of Fahrenheit. But in winter, when the locality of this 

 great circuit is changed to a more southern region, and when its cur- 

 rent is entirely displaced from the surface of the great interior pla- 

 teau by the cold winds of the interior, which come down from the 



