582 Principles of Weather Forecasting. 



only 17 F., 42 colder. In the northwest HIGH there 

 is a temperature of 10 F., while to the east of the 

 LOW, above 60, or a difference of 70 F., and while a 

 part of this difference is due to difference of latitude, most 

 of it is due to the effect of the storm. 



740. Barometric Changes Connected with Storms. Dur- 

 ing the progress of a storm across a given station the bar- 

 ometer falls more or less gradually until the center has 

 reached the place and then it begins to rise, and may con- 

 tinue to do so until a pressure greater than is normal has 

 been attained. The changes of the barometer, therefore, 

 become indices of the approach, progress and passage of a 

 storm, and so, too, in a less degree, may temperature 

 changes also, during the winter. If the barometer falls 

 faster than usual, if the wind velocity increases rapidly 

 and rapid changes in the wind direction occur, the indica- 

 tions are either that the storm center is approaching at a 

 high rate of speed or that its diameter is small and hence 

 that it is likely to arrive sooner after indications have de- 

 veloped. 



741. Cold Waves. Cold waves in the United States are 

 usually the result of a strongly developed storm which has 

 traversed somewhat slowly the southern and eastern states. 

 When these conditions prevail a HIGH area with clear 

 sky and descending cold air from above forms over Mani- 

 toba, or the northern boundary of the United States, and 

 the strongly developed LOW area, traveling slowly, sets 

 this body of cold air in motion toward it, which often at- 

 tains a velocity of 25 to 40 miles per hour. Under these 

 conditions intense cold is rapidly transported southward 

 and eastward with the speed of an express train, and occa- 

 sionally temperatures even below zero are transported as 

 far south as northern Alabama. 



Besides the extremely cold waves just referred to there 

 are others more common, which are due principally to the 

 first two causes named, and are usually coincident with the 



