4 THE WEATHER 



a study of tracks of storms in the United States. It indicates that, 

 in general, there are two sets of tracks running westerly and easterly, 

 one set over the northwestern boundary, the Lake region, and the St. 

 Lawrence Valley ; the other set over the middle Rocky Mountain dis- 

 tricts and the Gulf States. Each of these is double, with one for the 

 " highs " and one for the " lows." Furthermore, there are lines cross- 

 ing from one main track to another, showing how storms pass from one 

 to the other. The transverse broken lines show the average daily 

 movement. On the chart the heavy lines all belong to the tracks of 

 the " highs," and the lighter lines to the " lows." Let us trace them 

 somewhat in detail. A " high " appearing on the California coast 

 may cross the mountains near Salt Lake, and then pass directly over 

 the belt of the Gulf States to the Florida coast ; or it may move farther 

 northward, cross the Rocky Mountains in the State of Washington, up 

 the Columbia River Valley, then turn east, and finally reach the Gulf 

 of St. Lawrence. The paths are determined by the laws of the general 

 circulation of the atmosphere and the configuration of the North 

 American continent. This movement of the " highs " from the 

 middle Pacific coast to Florida or to the Gulf of St. Lawrence is con- 

 fined to the summer half of the year April to September, inclusive. 

 In the winter months, on the other hand, the source of the " highs " 

 is different, though they reach the same terminals. In the months 

 October to March, inclusive, many " highs " enter the United States 

 near the one hundred fiftieth meridian and move south along the 

 Rocky Mountain slope into the southern circuit, and thus reach the 

 South Carolina coast ; or else they turn more abruptly eastward and 

 move in the northern circuit over the Lakes to Newfoundland. The 

 chief difficulty in the art of forecasting is to decide which of these paths 

 will be pursued and the probable rate at which the movement will take 

 place. 



The weather map. 



The daily maps of the Weather Bureau show stations in the United 

 States and Canada that make telegraphic reports of the weather each 

 day at 8 a. m. and 8 p. m., seventy-fifth meridian time. The reports 

 consist of observations of the barometer and thermometer, the veloc- 

 ity and direction of the wind, amount, kind, and direction of move- 

 ment of clouds, and amount of rain or snow, and the 8 a. m. reports 



