Man's physical universe

xanabras

WEATHER-FORECASTING 269

before turning to the Great Lakes. There are many exceptions to this

general tendency, depending on the season and other factors.

Cyclones travel variable distances; some are dissipated before they

travel far, whereas others traverse entire continents. In February,

1925, a low was observed to travel completely around the earth, a

distance of 21,379 miles, in about a month's time, before it finally

broke up as it started on its second lap.

Fig. 88. Weather map retouched to show lines of equal pressure and equal

temperature. The heavy lines are lines of equal pressure, while the broken lines

are lines of equal temperature. The shaded areas indicate rain. Note the

relation between the areas of rain and the cyclones and anti-cyclones.

Though it is uncertain just how cyclones are formed, they seem to

involve the contact of large air masses from different sources. An average

of about 112 cyclones affect the United States annually. Nearly

45 originate over the North Pacific, 20 over Northwest Canada, 10 over

the Rocky Mountain Plateau, 12 over the Colorado Rockies, 15 over

Texas and the Western Gulf region, 3 over the Ohio River Basin, and

7 over the Eastern Gulf states.

Cyclones originating in the tropics are usually more violent than

those in the other zones.

Hurricanes and typhoons are tropical cyclones,

100 to 200 miles in diameter ; they originate over warm oceans near

the boundary between the doldrums and the trade-wind zones. They

move westward or northwestward as long as they remain in tropical

latitudes.

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