412 Eighth Annual Report of the 



winter winds are northwesterly, a moderate variation in tlie in- 

 tensity of the southern high or the northern low pressure area 

 is snfificient to modify their direction very materially. For ex- 

 ajnple, in January, 1890, the average pressure over the south- 

 eastern States was 0.2 inch above the normal value, while to the 

 north of this State there was about an equal deficiency ; and con- 

 sequently the prevailing winds were southerly, raising the mean 

 temperature for the month 10 degrees to 12 degrees above the 

 normal.* 



Every cyclonic storm which passes over or north of the State 

 causes an increase of temperature, due, in part, to the southerly 

 "vvinds which flow towards the depressed area. Such events 

 occur^ on an average^ five or six times during each of the winter 

 months; so that by platting the temperature of the State for 

 each day^ a curve is obtained similar to that shown in plate 1, 

 which represents the actual condition of pressure and mean daily 

 temperature which obtained during the year 1893. The cyclonic 

 systems give northwesterly winds in the rear of the storm cen- 

 ters, so that a considerable fall of temperature usually occurs after 

 their .passage; and this effect is frequently increased by the anti- 

 cyclonic areas which follow, bringing masses of cold dry air from 

 the interior of the continent to the eastern coast. The average 

 difference between the extremes of mean daily temperature 

 occurring in advance and in the rear of winter storms passing north 

 of New York is about sixteen degrees^ as determined by an exami- 

 nation of fifty individual cases; the variation being greatest in the 

 northern and least in the southern section of the State. Storms 

 passing to the southward of New York are also usually preceded 

 by an increase of temperature in the region of the Grreat Lakes, 

 and the southern and southeastern sections; while north of the 

 Mohawk valley only northerly winds are felt^ and therefore no rise 

 of the temperature results. A depression which passes eastward 

 over the center of the State may cause a great difference between 



• An opposite effect is produced by an unusually low pressure over the north Atlantic, 

 when in conjunction with a strong development of the continental " high; " in which cas» 

 the northwesterly circulation is strengthened, and the low temperature of the interior extends 

 to the eastern States. 



