376

CLIMATE OF NORTH AMERICA.

from the Gulf of Mexico in the same manner as the sea-breezeor monsoon occurs during the hot season.

The winds of the West India Islands being often fromthe south-east, the thunder and hurricane clouds float in them,and the atmospheric disturbances have thus a progressionfrom south-east to north-west. At the same time, however,it must be borne in mind that the hurricane clouds could notdrift in such a current before it was established over theUnited States, and before the disturbed equilibrium took placeas far as the south wind prevailed.

The thunder and tornado clouds drift in the south windover the States bordering on the Gulf of Mexico, and thuspropagate the disturbances from south to north.

On the other hand, in the Northern States and Canada,the thunder and tornado clouds drift in the westerly uppercurrent which prevails constantly from the west in the MiddleStates and from the north of west in the Northern States andCanada. Hence the rains during thunder and tornadoestravel from west to east at the same time that the south wind,which supplies the moisture, is blowing at the surface of theearth.

In the United States, rains only occur in summer afterthe south winds have blown for some time and furnishedmoisture. The rains are thus chiefly derived from the Gulfof Mexico.

The winter rains are produced by the south wind firstblowing over Texas and the country west of the Mississippi.The south wind occasions a great rise of temperature, andconsequently a rarefaction of the air which causes the baro-meter to stand low in a long line from north to south.

The existence of the long line of low barometer west of theMississippi causes the south wind to blow with greater strengththe longer it blows, as the air flowing as an under current isalways becoming warmer, and causing a greater fall of thebarometer.

The temperature and pressure of the air vary compara-tively little in the Caribbean Sea in winter; and as the baro-meter is constantly higher there than in the States swept