414 Eighth Annual Keport of the 



During the winter months elongated depressions, or *' troughs " 

 of low pressure, are frequently" formed over the eastern States; 

 their longest diameter commonly extending from the region of the 

 Gulf of Mexico to the Great Lakes, and including portions of the 

 Ohio and Miss.issippi valleys. Such systems often derive their 

 moisture both from the Mexican gulf and the Atlantic ; and many 

 of the heaviest rain and snow storms of the winter are due to con- 

 ditions of this nature, especially in the vicinity of the lake region 

 and on the highlands adjacent to the Atlantic coast. 



The character of our wintere depends very largely upon the 

 number and general course of the auticyclones. As previously 

 staited, the usual course is somewhat south of the east across the 

 States toward the permanent Atlantic ''high;" but in many 

 cases the intense cold waves originating in British America move 

 directly eastward along the Canadian border to the coast, and 

 thence pass southward. Under such conditions the northern 

 part of New York experiences the full efifects of the cold waves, 

 their severity in the more southern sections being usually some- 

 what decreased by the influence of the Great Lakes. Statistics 

 as to the relative frequence and amount of temperature changes 

 in various parts of the State will be found on pages 463, 464. 



In the spring, and especially during April and May, the in- 

 creased amount of heat received from the sun brings about a 

 rapid modification and s'hifting of pressure systems, which are 

 then less shai'ply contrasted than at other seasons. The winds 

 decrease greatly in velocitj', and their direction is variable, al- 

 though the southerly component which is characteristic of sum- 

 mer becomes well defined duiring May. The pressure conditions 

 of March are essentially those of a winter month, the high 

 pressure systems over central British America and the southern 

 Atlantic coast being still in force, while the cyclone of the north 

 Atlantic continues to give northerly winds over eastern Canada 

 and the adjacent States. In April and May the pressnre has 

 decreased over nearly the whole extent of North America, the 

 barometer being relatively higher over the central and south- 

 eastern States, thus giving a condition similar to that described 



