CAUSES OF UNUSUAL WEATHER PERIODS 267 



and ascends. Figure 189 shows the average paths along which 

 the winds move from HIGHS toward a LOW. 



293. The Paths of Highs and Lows. We have seen that 

 HIGHS and LOWS usually move across the United States in a 

 general direction from west to east. Observation of many 

 HIGHS and LOWS shows that they have a tendency to follow 

 certain paths (Fig. 190). 



Exercise 58. Plotting the Paths of Highs and Lows 



Obtain a month's maps from a Weather Bureau station. On a 

 single blank map draw the paths of all the LOWS that crossed the 

 country during the month, making a dot or small circle for the posi- 

 tion of the center on each day. On another map plot the paths of 

 the HIGHS in similar manner. On a third map plot, from day to day, 

 in the same manner the paths of both the HIGHS and LOWS appear- 

 ing on the weather maps as they are received. 



Exercise 59. Making a Chart of the Month's Weather 



Prepare a chart like Fig. 191, showing the weather of your locality 

 ' for one month, using your class weather record. Make the chart as 

 complete as practicable. The daily observations, especially of tem- 

 perature, should be taken as nearly as pqssible at the same hour each 

 day (Art. 227)- 



Exercise 60. Explaining the Causes of the Daily Weather 

 Using the daily weather maps, for the month covered by Ex. 59, 

 explain the causes of the principal weather changes (of wind, tempera- 

 ture, cloudiness, rain, or clearing) that came to your locality from day 

 to day. 



VIII. CAUSES OF UNUSUAL WEATHER PERIODS 



294. Further Control of Weather by Pressure Areas. 

 Figures 192 and 193 show the average tracks of LOWS and 

 HIGHS in the United States during 10 years; and Fig. 194, 

 the average tracks of LOWS for the principal regions of the earth 

 where they occur. Within these regions, there is considerable 

 variation in the paths and behavior of LOWS and HIGHS. We 

 have seen that in the temperate zone most of the weather 

 from day to day is controlled by passing LOWS or HIGHS. 



