38 



ELEMENTS OF GENERAL SCIENCE 



of the country that the temperature of the north rises unsea- 

 sonably high, producing a period of very warm weather which 

 in this country is usually called a hot wave. In the western 

 part of the Mississippi Valley the crops are sometimes seri- 

 ously damaged by hot winds coming off the deserts of the 

 southwestern part of the United States. Such a hot and 

 dry wind is often called a sirocco, though in this country it 

 is more usually referred to as a hot wind. 



FIG. 25. An unusually large " high " area 



Note the low temperatures throughout the country. United States Weather Map 

 for January 12, 1914 



If a high-pressure area remains for a while in the northwest, 

 the cold air which comes from it may spread over the greater 

 part of the country (fig. 25) ; and if the fall in temperature 

 is rapid and of considerable amount, it is called a cold wave 

 (figs. 26 and 27). The presence of low pressure over the 

 Gulf of Mexico or over the Atlantic Ocean near our southern 

 coast will usually assist in the progress of the cold wave. It 

 occasionally happens that the freezing temperatures reach the 



