WEATHER FORECASTING 237 



ing over the mountains to the west. On the mountains 

 and high plateaus, however, there is a considerable fall of 

 rain, as the winds are cooled sufficiently in passing over 

 these to deposit their remaining moisture. In most of 

 this region, as also in southern California, irrigation must be 

 resorted to if agriculture is to succeed. The fall of rain 

 on the mountains and high plateaus supplies rivers of 

 sufficient size to furnish water for extensive irrigation, 

 and so a considerable part of the area which is now prac- 

 tically a desert will in the future be reclaimed for the use 

 of man. The government is at present engaged in extensive 

 irrigation work in this territory. 



From about the 100th meridian to the Atlantic Ocean 

 there is a varying rainfall, but it is as a rule sufficient for 

 the needs of agriculture. It gradually increases toward 

 the east, moisture being supplied plentifully from the Gulf 

 of Mexico and the Atlantic Ocean by the southerly and 

 easterly winds. The rainfall is well distributed through- 

 out the year and averages from thirty to sixty inches. 



Weather Forecasting. The data necessary for fore- 

 casting the weather are telegraphed to the Weather Bureau 

 stations every day, and a record of them placed on the 

 weather map. The observations recorded on these maps 

 furnish the forecasters with all the information obtainable 

 as to what the weather of the future is to be. It has al- 

 ready been stated that the dominant cause of our weather 

 conditions, in middle latitudes, is the eastward movement of 

 cyclones and anti-cyclones. 



If the direction and rate of motion of these can be deter- 

 mined the weather of those places which are likely to come 

 under their influence can be foretold with a good deal of 



