50 The Ottawa Naturalist. [May 



From the greater heat at the equator the mass of air there rises 

 to a greater altitude than at the pole, and in consequence we have 

 a current of air in the upper regions moving from the equator to 

 the pole. But just as soon as this flow has commenced there is 

 a decrease in the actual weight of the atmosphere at the equator, 

 and a counter current sets in along the surface of the earth from 

 the pole towards the equator. Vertical currents connect these 

 two horizontal currents ; the one at the equator being ascend- 

 ing and that in the region of the pole descending. 



From the revolution of the earth on its axis, there is a torce 

 arising from this rotation which causes a free moving body to 

 depart to the right of its original direction in the northern hemi- 

 sphere, and to the left in the southern hemisphere. It is this 

 condition which mainly gives to the existing circulation of the 

 atmosphere its complex character. In the vertically moving air 

 masses dynamic heating and cooling take place as a conse- 

 quence of the compression and expansion of the air. We have 

 in British Columbia and extending to the east slope of the 

 Rocky Mountains a phenomenon — the Chinook wind — depend- 

 ent on this property. The Chinook is similar to the Foehn of 

 the Alps. Dry air in passing over a mountain range would not 

 differ in temperature on the two sides of the range. As the air 

 ascended it would be cooled dynamically. As it descended it 

 would be warmed just as much. But if the air is moist, in 

 ascending it cools, and the moisture is condensed and falls as 

 rain or snow. The latent heat released by the condensation 

 raises the temperature of the air, and in descending to other side 

 of the mountain it is warmed up dynamically still more. This 

 is the action of the Chinook wind and the explanation of its 

 warmth and dryness, the moist warm winds from the Pacific 

 being Especially favorable for its creation. ' 



Atmospheric disturbances are easily divided into two 

 classes— cyclonic or low area storms, and anti- cyclonic or high . 



