WEATHER MAPS 285 



it is inferred that the pressure at all points within this isobar is 

 less than 29.5, though nowhere so low as 29.4. West of the low 

 the pressure increases. The pressure in the high near the Pacific 

 coast is not so great as that in the high over the Hudson Valley. 



Most weather maps show both lows and highs, or at least one 

 of each. Since this is the case, the atmospheric pressures are gen- 

 erally unequal in different parts of the country. 



2. Winds. Wherever barometric pressures are unequal, 

 isobaric surfaces are uneven. The arrows on a weather map (Fig. 

 239), show the direction of the winds, which blow as the arrows fly. 

 Their positions are based on the reports received Bt the map-making 

 offices, from the stations of the Weather Bureau. On January 12, 

 1899 (Fig. 239), winds were blowing away from the highs in the 

 east and west, respectively, and toward the low in the northwest. 

 The movements of air out from an area of high pressure constitute 

 an anticyclone, and the movements in toward an area of low pres- 

 sure constitute a cyclone, A cyclone is one type of a storm. The 

 winds in a cyclone are not always strong rarely strong enough to 

 be destructive. The violent wind-storms, popularly called cy*- 

 clones, are here called tornadoes. 



Winds do not blow straight out from the anticyclone centers, 

 nor straight in toward the cyclonic centers. They may start 

 straight out from the center of each high, but in the northern 

 hemisphere they are turned (deflected) toward the right (right 

 hand half of Fig. 240), as most of the arrows about the anticyclones 

 (Fig. 239), show. Similarly, the winds which blow toward the 

 cyclonic centers do not blow straight toward them, but are deflected 

 a little to the right in the northern hemisphere (left part of Fig. 

 240), as most of the arrows about the lows (Fig. 239) show. 

 In the southern hemisphere, the winds are turned to the left instead 

 of to the right (Fig. 241). 



The strength of the winds at various points may be inferred 

 from the map. The distance from the center of the high in the east 

 (Fig. 239) to Lake Michigan is about 800 miles. The difference 

 in pressure is about .5 inch. This would cause a wind of about 12 

 miles an hour a fresh breeze between these points. In general, 

 the gradient is high and the winds strong, where isobars are crowded. 



