202 NINTH ANNUAL REPORT OP 



?»IETEOROLOGY. 



Third Lecture. 



When northwest winds prevail over the United States in winter, the 

 air is very dry and cold ; so long as the wind remains in this quarter 

 there is no termination to the cold weather. It does not moderate, at 

 least the thermometer does not rise, until the moisture is increased. I 

 suppose it is admitted by meteorologists that the cold spells ot weather 

 in the United States are first felt in the west and northwest, and gradu- 

 ally extend over the country to the southeast. This direction, it will 

 be obsei'ved, corresponds very nearly with the course of the current, 

 which is so constant in the upper stratum of the atmosphere in this 

 region. But it is a curious fact, well worthy of the attention of mete- 

 orologists, that the warm and moist surface air which precedes storms 

 and cold weather travels also from northwest to southeast ; at all 

 events, it apparently does so. Sometimes a belt of moist air, several 

 hundred miles in breadth, and extending from the mouth of the Missis- 

 sippi to the lakes, and probably much further north, seems to advance 

 from the west, while there is very cold weather both behind and in 

 front of it. The storms of February, 1842, described by Professor 

 Loomis in the Transactions cf the American Philosophical Society, 

 were of this character. On the morning of the 10th November last, 

 similar phenomena were presented : the eastern portion of the Missis- 

 sippi valley, and all the States south of the lakes and west of the Alle- 

 ghany range, were under the influence of a comparatively moist and 

 warm stratum of air, at the very moment that the temperature was 

 about 20^ lower from latitude 35° to 45° on the Atlantic seaboard, and 

 between the same parallel of latitude in the western States west of 

 longitude 92°. By the night of the 10th November, the weather be- 

 came very moist and w^arm in the Atlantic seaboard States, more so 

 than it had been in Ohio the previous day. 



The moisture which was found in the broad band could not origin- 

 ally have come from the west, northwest, or north, because these winds 

 are always exceedingly dry ; nor could it have come firom the Atlantic 

 ocean, because the weather remains cold on the coasts when it is much 

 warmer in Ohio. Indeed the storm was at hand before the east wind set 

 in, and this is a usual occurrence. There can be but little doubt, therefore, 

 that this moisture is originally derived from the Gulf of Mexico, and 

 that it is spread over the middle and eastern portion of the United States 

 by the southerly current, which, as we have before remarked, is, in our 

 opinion, a deflected portion of the trade winds. 



In a former lecture, I pointed out the fact, abundantly proved by 

 Profisssor Coffin, that these southerly winds prevail more in summer than 

 in winter ; and an important point here suggests itself for future discus- 

 sion. I think it very probable that a broad band of southerly winds, 



