174 FIRST YEAR SCIENCE 



weather changes of this kind are due to atmospheric con- 

 ditions related to areas of low pressure. 



86. 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 ob- 

 tainable as to what the weather of the future is to be. It 

 has already been stated that the dominant cause of our 

 weather conditions 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 

 accuracy. If a cyclone were central over the lower Mis- 

 sissippi valley with an anti-cyclone to the west of it, we 

 should expect that the southerly and southeasterly winds 

 and rains to the east and southeast of the Mississippi would 

 gradually change to fair weather and westerly winds with 

 increasing cold, as the cyclonic area was replaced by the 

 anti-cyclonic. 



The rate at which the change would take, place would 

 depend upon the rapidity of the movements of the two 

 areas of high ancl low pressure, and the order of change in 

 the direction of the winds would depend, for any place, 

 upon the directions taken by the centers of these areas. 

 The direction of movement and the rapidity of movement 

 of the cyclonic areas are, therefore, two of the chief factors 

 which enter into the prediction of the weather. There is 

 usually an increase in the intensity of the storm as the 

 Atlantic coast is approached. 



87. Climate. The average succession of weather changes 

 throughout the year, considered for a long period of 

 years, constitutes the climate. Thus, if the average tern* 



