372 CARNEGIE INSTITUTION OF WASHINGTON. 



follows after one cyclone family and precedes the next. The successive 

 families must then follow each other, with the mentioned interval of 5}4 days. 



The regularity of the system is broken by the irregular distribution of oceans 

 and continents, by the topography of the latter, and by the seasonal changes of 

 thermal conditions. This may cause one or more of the polar currents to be 

 for some time cut off before they reach the trades; thereby two or more of the 

 advancing discontinuities may for some time be joined to a single "polar 

 front," while in general we have to count with four distinct polar fronts, or 

 advancing left flanks of the polar currents. It may happen also that one of the 

 moving polar currents is checked for some time in its eastward propagation 

 at the western coast of a continent, producing thus a stationary current feed- 

 ing the trades, while the rest of the system continues its regular propagation 

 eastward. 



Irregularities of this kind can not, however, hide the grand regularity 

 underlying the system; and this regularity is of an extreme importance to 

 weather forecasting. As a matter of fact, the Norwegian Weather Service 

 has begun to supplement the ordinary forecasts for the next day with long- 

 range forecasts, giving the general weather conditions for the next period of 

 five days. These long-range forecasts are based upon the knowledge, which 

 the weather telegrams obtained from the Atlantic, and in the latest time also 

 from America, give of the situation and the probable movement of the next 

 cyclone family. As the movements of the cyclone families are far more regular 

 than those of the individual cyclones, these forecasts succeed rather better than the 

 short-range forecasts for the next day, although the telegrams from the Atlantic 

 and America are not yet at all adapted to the needs of this service of long- 

 range forecasts. 



Similar long-range forecasts ought to succeed at least equally well in the 

 eastern parts of North America, as there is good opportunity to observe in due 

 time the invasion of the cyclone families from the Pacific to the Western 

 States. And with sufficient marine messages from the Pacific and from 

 Japan, they should succeed even for the entire North American continent. 



