THE IMPENETRABLE SEA 



to the west of the continent, in a zone which extends 

 northward to Alaska. 



On the continent itself is another zone of disturbance, 

 east of the Rockies, where warm, moist winds from the 

 Gulf meet cool Pacific ones, dried by their passage over 

 the mountains. There are other zones of disturbance, so 

 that the Asiatic jet, receiving its main ''drive" from the 

 sun, is affected by auxiliary forces arising in the other 

 zones. Complicated though it becomes, the process con- 

 tinues rhythmically in a kind of "wind routine" which 

 can be used as one of the permanent factors in the data 

 studied by experts in forecasting the world's weather. 



Water-vapour is generally present in the air in greater 

 or lesser quantities. If pure, dry air — that is, air from 

 which all dust and traces of electricity have been removed 

 — is mixed with pure water-vapour, and the resulting 

 mixture cooled below the temperature of saturation, 

 condensation does not take place. But if fine dust is 

 injected into the pure mixture, without altering its 

 temperature or pressure, a fine mist is at once developed. 

 A charge of electricity introduced into the mixture will 

 also cause condensation. The colder the air, the less 

 water it can hold in the form of invisible vapour. A 

 pound of air at ninety degrees can hold as much as half 

 an ounce of water-vapour, but at freezing point it can 

 only hold a sixteenth of an ounce : the air is fully saturated 

 at that temperature with that amount of moisture. 



The air may be chilled as it passes up the surface of a 

 mountain, or as a current rising from a warm surface, 

 or it may be forced up the sloping surface of a mass of 

 cooler, heavier air. If no dust particles were present 

 the rising air might become overloaded (or "super- 

 saturated") with water molecules without cloud forming 

 — but the dust particles are almost always present in 

 vast numbers. When the saturated air becomes too cold 

 to sustain the number of water molecules present, these 

 converge on the dust particles to form droplets — but still 



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