CHAPTER XXIV 

 WINDS AND WEATHER 



In the phenomena we have studied thus far the rela- 

 tions between causes and effects have been rather simple. 

 They have been cases in which certain causes produce 

 certain effects which are not difficult to predict. But 

 with wind and weather we shall find that it is different. 

 In fact you already know that weather is a difficult thing 

 to predict, for it is a result not of one but of many causes. 

 It is so important to man to know what the weather is 

 going to be that millions of dollars are spent by govern- 

 ments each year in doing all that science can do to foretell 

 it. Yet their predictions often go wrong. 



The work of a weather bureau consists, first, in securing 

 daily reports from all over the country as to local weather 

 conditions. Then, with such data in hand, it is possible 

 to make forecasts (predictions) which are fairly accurate. 

 For weather, like all other phenomena of nature, is not 

 accidental, it is the result of certain definite causes. The 

 difficulty in its prediction is that these causes can be ascer- 

 tained only in a general way, and local weather conditions 

 may be determined by purely local causes which conflict 

 with the general causes. We shall see first what the gen- 

 eral causes are, and then consider some of the local causes. 



Weather Is the State of the Atmosphere. We speak 

 of it as warm or cold, as wet or dry. If we stop to consider, 

 it is evident that we mean by this that the atmosphere 

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