CON'TRIBDTIONS TO METEOROLOGY. 



13 



oliait from Novciiibci- 4, 5 a. M., to Novoiiihor 0, it A. M. ; tlic <lir('(!tioii of the. wiiiil for t'ii<;li lioiir 

 is sliown at tlie bottom oi' tlic cliart; and tlie rainfall is shown on the lower i)art of the chart as 

 measured at intervals of three hours. The teini)erature is show n in centigrade degrees on the 

 right margin of the cliart, and also the relative humidity. 



15. The term loir, as used in the preceding ])ages, is to he understood in a relative sense, and 

 does not necessarily indicate that tiie barometer is below its mean height. The characteristic 

 feature of an area of low pressure is a general nioveinent of the winds inwaiil, and at the same 

 time circulating from right to left about the low center. Such a movement of the winds is found 

 to prevail in the violent cyclones of the West India Islands, and such a system of winds is called 

 a cyclonic system, or simply a cycloiu^; and an area of low barometer over which such a system of 

 winds i)revails, is called an area of cycdonic winds, or simply a cyclonic area. The barometer at 

 the center of such an area may stand as high as '.M inches, and occasionally it stands as high as 

 .30.1, or 3(1.2, or even higher. Plate VI shows the isobars for the United States on the morning of 

 January 5, 1.H82, at which time the barometer stood above 30 inches over nearly the whole of the 

 Unitf^d States, with an area of high pressure (30.8 inches) over the river Saint Lawrence. Near 

 latitude ii)° the isobars 30.4 and 30..5 were separated by an interval of over 800 miles, and between 

 them was an area nearly 400 miles in diameter, within which the pressure was less than 30.4 inches. 



On the morning of January 4 there was an area of low pressure over Arkansas (the pressure 

 at the center being a little below 30 inches) and it was surrounded by a distinctly marked system 

 of circulating winds. During the next twenty-four hours this low area advanced about 450 miles 

 toward the northeast, and during this time the barometer had been continually rising, and the 

 system of circulating winds was generally supplanted by feeble winds froui some northern quarter. 

 At only one station on the morning of January 5 was the wind within this area reported from the 

 south. The isobar of 30.4 inches included a region over which the pressure was lower than the 

 pressure immediately surrounding it, that is, the pressure was relatively low, but there remained 

 only a slight trace of the system of circulating winds which had previously prevailed. The term 

 cyclonic area, when applied to a system of circulating winds with jiressure above 30 inches, is more 

 descri]itive than the term low area; but both of these terms are in common use. 



10. A comparison of Plates 1 to VI shows that we may have deep cyclones as seen in Plates 

 I and III, or shallow e.^ clones as seen in Plate VI, and there is a corresi)onding difference in the 

 velocity of the winds in the two cases. The winds shown in Plate I were very strong, particularly 

 on the northwest side of the low center, being 48 miles an hour at Dodge City, 36 miles at Yankton, 

 32 miles at Leavenworth, and 31 miles atEscanaba. The winds represented on Plate III were still 

 more violent, five vessels having reported the force of the wind as rising to 10 on Beaufort's scale, 

 which is considered to be equivalent to a velocity of C5 miles per hour; and one vessel rejjorted 

 the force of the wind as 12 on the same scale, indicating a velocity of 90 miles pei' hour. Within 

 the low area represented on Plate VI the highest wind reported was at Nashville, C miles per hour, 

 while at Cohnnbus and Louisville the velocity was 4 miles per hour; at Cincinnati and Indianapolis, 

 3 miles per hour; and at Knoxville only 1 mile per hour. 



17. When an area of low juessure is very much elong.ated we frequently (iud two cyclonic 

 centers included within the same area of low pressure. Plates I, II, and III show only one low 

 center, but Plate IV shows two centers of cyclonic movement within the same area of low pressure, 

 besides three less important centers on the lower iiart of the chart, and each of them is attended 

 by a system of feeble winds circulating about it. When there are two cyclonic centers within the 

 same area of low pressure these centers are generally of unequal depth ; but sometimes they are 



