370
CLIMATE OF NORTH AMERICA.
This particular veering of the wind is easily accounted for.By turning back to Plate 2, it will be seen that the wind wassouth-east at Key West on the morning of the 9th Novem-ber. The air was then flowing towards the line of low baro-meter, west of the Mississippi. Indeed, the wind at KeyWest only formed part of the vast aerial current which wasraising the temperature, and lowering the barometer from theGulf of Mexico to Lake Superior. It has been shown thata warm southerly current was propagated from west to eastover the United States in November. In accordance withthis fact, we find the wind at Key West gradually veeringround from the south-east on the 9th, to south on the 11thand 12th (see Plate 3), when the hot and moist wind fromthe tropic was sweeping the coast of the United States. Thewind changed to north or north-west at Key West, shortly afterthe cold west current cleared the State of Florida of thewarm southerly winds (see Plate 4).
In the centre of the Gulf of Mexico the temperatureand pressure of the air in winter vary comparatively little.The warm and moist south winds of the United States, beingsupplied from the trade or tropical winds, create great dis-turbances in winter. Thus, after they raise the temperature,and cause a diminished pressure to the west of the Missis-sippi, the air to the east of the Mississippi will be forcedwestwards, as south-east winds, in obedience to the law thatair flows from a high towards a low barometer.
When the south-east wind blows from the Gulf of Mexicoover the South-Eastern States, it becomes warmer, and as soonas it becomes as warm as the south wind, which at the samemoment is to the westward, it will have no farther tendencyto blow in that direction, and will naturally become a southwind. It is for this reason, that after the south-east windblows for some time it veers round to the south or south-west, and becomes a part of the southerly current, which,by being lighter, in its turn draws the air on its eastern banktowards it from the south-east. Hence the manner in which
is evidently erroneous, for the terminating cold wind in the winter storms ismore northerly along the shores of the Gulf of Mexico than in any other part ofthe United States.