Wind and Weather 145 



and rests on the earth with a pressure averaging nearly 15 pounds 

 per square inch, and that such pressure varies in different parts 

 of the country at any given time. After indicating the pressure 

 data or 'barometric readings on the map, draw a line with a col- 

 ored pencil connecting all places having a barometric reading 

 of 30 inches. Draw another line through places with a barometric 

 reading of 30.1 inches, and similar lines for each tenth of an inch 

 variation. These lines are called isobars; they pass through 

 places where the pressure or barometric reading is the same at a 

 given time on a given date. Write the word "Low" where you 

 find the lowest pressure reading and the word "High" where you 

 find the highest. 



The main cause of changes in weather in our latitudes is the 

 movement across the continent of cyclones (lows) and anti-cy- 

 clones (highs). The Weather Bureau finds out the directions and 

 the rates of motion of these cyclones and thus is able to forecast 

 quite accurately the weather conditions of all places over which 

 they are expected to pass. Since the growth and the harvesting 

 of farm crops, the success of many industries and occupations, 

 and even the health of people depend to a large extent on the 

 weather conditions, the importance of the work of the Weather 

 Bureau is apparent. 



Climatic Regions. Although the United States lies in the 

 temperate zone, there is considerable difference in the climate of 

 the various sections. In a general way it may be divided into 

 several climatic regions but, even in these, local causes often 

 bring sudden changes of weather. It is difficult to find a place 

 where a perfectly satisfactory climate prevails the year round, that 

 is, a climate which is always best for the health and comfort of 

 those who live there. In seeking a satisfactory climate, it is neces- 

 sary to consider the average temperature, the relative humidity, 

 and the changes which occur in both temperature and humidity, 

 the nature and changes of the prevailing winds, the amount of 

 sunshine and of cloudy weather, the amount of rainfall and 

 times of year when it occurs, the altitude of the section, and 

 whether the atmosphere is free from dust and other undesirable 

 matter. It seems hardly possible to find a place where all these 



