The Weather 9 



over which there is marked a " low/' and southward toward a 

 region marked " high." A relatively low temperature is regis- 

 tered in association with the " highs " ; as, for instance, on the 

 map of December 24, the area marked " Colder 20 or more " 

 is associated with the " high " over southern Idaho and eastern 

 Oregon. 



Consider the isotherms curving northward in the region of 

 Maine and Canada, over which there is a " low." With this 

 low the temperature is relatively higher than that of lower 

 latitudes, as in the upper Mississippi valley. Is it to be ex- 

 pected that the isotherms follow parallels of latitude? Give 

 reasons for your answer. 



8. Isobars. The heavy continuous lines on the maps are 

 drawn through places which report the same atmospheric 

 pressure. This is measured in inches of mercury (page 16) 

 with intervals of tenths or hundredths of an inch. The num- 

 bers are printed at the ends of the lines or near circles repre- 

 senting pressures, such as 30, 29.9, 29.65. The lines are some- 

 times nearly straight, but usually curved so that they resemble 

 the letter " U," or run in loops or circles. In each complete 

 circle is printed " high " or " low." " High " appears in 

 circles with a pressure above 30, as 30.2 or 30.7. " Low " is 

 printed in circles with pressures below 30. 



What is the average pressure given on any of the maps? 

 How far is it from thirty inches ? What is the greatest pressure 

 on the map of December 24? What is the lowest pressure? 

 Where are these areas located? How many " lows " are there 

 on this map? How many " highs "? Compare the tempera- 

 tures associated with the low-pressure areas with those as- 

 sociated with the high-pressure areas. 



The lines connecting different places having the same pres- 

 sures on a given date are called isobars. The word is composed 

 of two Greek words, isos meaning equal, and baros meaning 

 weight or pressure. The word baros forms the first syllable of 

 the word barometer, which means weight-measure, the name of 



