DYNAJIIC METKOKOIJXJV. 423 



treyh or brisk, will uciicially picxciit or iiuilil'v (lie severity of tlici'rost. 

 Areas of cold dry air rre(|iieiitly li(»\v .southward and .s[)rea(l as uortbers 

 over larj^'e re.uionsol the eomitry. eonliiiinj;- themselves especially to the 

 low lands and river valleys. Within these areas of clear, (;old, dry air, 

 and especiallj" on their advaiicinj^- fronts, the process of freezing is often 

 accelerated by the combination of convection with radiation, and iu 

 such cases the destrnction to vegetation is partictdarly severe; there- 

 fore, in the prediction of frosts, attention mnst be especially ^ire(;ted 

 to snch areas. The rate at which these advance isof conrse greater when 

 the cold air tlows down a gentle incline, snch as that of the Mississippi 

 Valley. When snch cold areas reach the Gnlf of Mexico the lowest 

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

 tion of the Gulf, banks uj) 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 iu the Signal office; 

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

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

 tion of gulf storms were even then clearly predicted. 



(/;) Iiain. — 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 snuill masses of over-heated moist air; these occur almost entirel}^ 

 between *.> A. m. and r. m., and are oftentimes simultaneous in isolated 

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

 whenever the 7 a. :\i. map shows a high dew-point and clear or nearly 

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

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

 in the Gulf States or southeast winds on the Atlanti(; 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 undertiow of denser (/. e. cooler or drier) air. 

 This convective process begins most easilj' in mountainous regions, 

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

 rapidly than in the low" lands ; in factthedowntlow of cold dry air from 

 the mountainsis a periodical phenomena teiulingto 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 (juestion can 

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

 mation of heavy (tumuli begins. If, 6'. .^/., at Washington this occurs 

 later than a p. m. rain is not likely, if before 3 P. M. rain is probable. 



