250 THE WEATHER 



and will construct the weather map as it is done at a weather 

 station, though the steps will be in a different order. 



Exercise 55. Making the Weather Map of 8 a.m. (Eastern Time), 



March 2, 1904 



1. Using the foregoing table of weather reports, write the tempera- 

 ture of each place in small figures at its point on the map. Next, 

 draw a light line with blue pencil through all points having a tem- 

 perature of 40. Draw other lines for 50, 30, 20, and 10. Label 

 each line at its ends. These lines are called isotherms. Note their 

 trend. They loop northward over the middle plains, showing that a 

 temperature of 40 extended farther north over the plains than else- 

 where. We should expect cold in the north and warm in the south 

 with the isotherms running east and west somewhat like parallels of 

 latitude. Why? Also, if the isotherms ran east and west like the 

 parallels of latitude, any two of them should be about the same dis- 

 tance apart all along their course. But the map shows the isotherms 

 far apart in the eastern states and crowded near together over 

 Nebraska and South Dakota; compare the temperature of western 

 Nebraska with that of South Carolina. There must be a special 

 cause for such peculiar temperature conditions. The next step 

 will explain. 



2. On the same map, place the wind direction at each city, using 

 arrows pointing with the wind, i.e., for a wind coming from the south- 

 east make the arrow point toward the northwest. Note that the 

 winds in South Carolina and South Dakota are from northerly points, 

 while from the Gulf Coast to Nebraska they blow from the south. 



Notice also that the warm temperatures extend far to the north in 

 the region of south winds, and cold temperatures extend southward 

 where the north winds blow. Does this explain most of the bends 

 in the isotherms? Does it explain the crowding together of the 

 isotherms over Nebraska and South Dakota? The bending of the 

 isotherms seems to be caused by the winds, blowing from different 

 directions. But why should the winds blow in different directions 

 at the same moment in different parts of the country? The next 

 step answers. 



3. Chart the barometer readings at each city in the same manner. 

 We already know that air has weight and rests upon the earth with 

 a pressure of nearly 15 Ib. per sq. in., and that the pressure, at any 

 moment, is not the same in all sections of the country (Art. 211). 

 After charting the barometer readings on your map, draw a line in 

 red pencil through all the places having a reading of 29.9 in.; another 



