46 ELEMENTS OF GENERAL SCIENCE 



or other special means. The eastern slope, the lake region, 

 and the Mississippi Valley as far west as the middle of the 

 Dakotas and Nebraska are well watered, but the great plains 

 and mountains of the West have very scanty rainfall excepting 

 on the westward slopes of mountains near the Pacific Ocean. 



The rainfall on the Pacific coast is due mainly to the effect 

 of the mountains upon the prevailing westerly winds. The 

 scantiness of rainfall in the mountains and on the plains 

 directly east of them is due to the same thing. The larger 

 rainfall of the Mississippi Valley occurs principally in con- 

 nection with the cyclones and is condensed from the air 

 which is sweeping northward from the Gulf of Mexico into 

 the cyclones. Farther east the Atlantic Ocean contributes 

 in the same way. 



43. The United States Weather Bureau. The importance of 

 the weather to the people of the country is so great that the 

 government has established a bureau for the purpose of study- 

 ing the weather and forecasting it as far as possible. While it 

 is not possible to foretell the weather with certainty, the pre- 

 dictions that are made are fairly accurate and have been of 

 great value. 



All forecasting of weather is based upon the weather map. 

 The map is made up each Jay from the reports telegraphed 

 from the observing stations all over the country. The predic- 

 tions are based almost wholly on knowledge of the character 

 and actions of cyclones. The usual path of cyclones and the 

 rate of motion have been carefully studied, and it is possible 

 to estimate quite closely what a cyclone will do within the 

 next twenty-four hours, and therefore how weather conditions 

 will change. For instance, if it were found that within twenty- 

 four hours a cyclone that is now in the West could be expected 

 to approach to within a short distance of a certain place, it 

 would be safe to predict clouds and rain for that place. On 

 the other hand, the approach of a high-pressure area would 

 justify the prediction of clear weather. 



