DYNAMIC METEOROLOGY. 423 



fresh or brisk, will generally prevent or nullify the severity of the frost. 

 Areas of cold dry air frequently flow southward and spread as northers 

 over large regions of the country, confining themselves especially to the 

 low lands and river valleys. Within these areas of clear, cold, dry air, 

 and especially on their advancing fronts, the process of freezing is often 

 accelerated by the combination of convection with radiation, and in 

 such cases the destruction to vegetation is particularly severe; there- 

 fore, in the prediction of frosts, attention must be especially directed 

 to such areas. The rate at which these advance isof course greater when 

 the cold air flows down a gentle incline, such as that of the Mississippi 

 Valley. When such cold areas reach the Gulf of Mexico the lowest 

 stratum of air moves more rapidly, and after overrunning a large por- 

 tion of the Gulf, banks up the atmosphere on the Mexican shore, and 

 causes extensive rains, which sometimes initiate cyclones, and return 

 as such to the northward. This phenomenon was clearly perceived and 

 verbally explained to my fellow-laborers in 1871-'72 in the Signal office ; 

 the parallelism between the Norther, the Bore, the Pampero, and sim- 

 ilar winds was also then dwelfe on ; the flow of cold air and the forma- 

 tion of gulf storms were even then clearly predicted. 



{h) Rain. — For convenience in establishing general rules for the pre- 

 diction of rains, they may be divided into four classes, as follows : 



(1) Merely local rains depending almost wholly upon the rapid ascent 

 of small masses of over-heated moist air; these occur almost entirely 

 between 9 A. m. and 6 p. m., and are oftentimes simultaneous in isolated 

 localities over a great extent of the country. They are liable to occur 

 whenever the 7 A. m. map shows a high dew-point and clear or nearly 

 clear weather, and when the winds are or are about to be such as will 

 carry the surface air up a gentle slope; they occur with southerly winds 

 in the Gulf States or southeast winds on the Atlantic coast; they occur 

 on the latter coast rarely with winds between west and northwest, 

 since such winds are carrying the air downwards toward the sea and 

 warming it by compression. 



(li) Local rains, on a larger scale, generally accompanied by light- 

 ning, and known as thunder storms, occurring in the afternoon between 

 3 and 10 o'clock, and due to the uplifting of large masses of moist air 

 from the low lands by the underflow of denser {i. e. cooler or drier) air. 

 This convective process begins most easily in mountainous regions, 

 where the afternoon radiation, during the declining sun, proceeds more 

 rapidly than in the low lands ; in fact the dowuflow of cold dry air from 

 the mountains is a periodical i>henomena tending to take place regularly 

 at all times of the year. The decision of the question as to whether rain 

 will follow or not depends upon the balance between the mass of cooled 

 air over the mountains and the adjacent moist air. The question can 

 generally be decided by considering the time of day at which the for- 

 mation of heavy cumuli begins. If, e. </., at Washington this occurs 

 later than "> p. m. rain is not likely, if before 3 p. m. rain is probable. 



