506 SCIENCE OF HOME AND COMMUNITY 



high areas are usually accompanied by clear weather and 

 lower temperatures. 



The broken lines are called isotherms and show places of 

 equal temperatures. The line marked 40 degrees passes 

 through all places with 40 degrees temperature. These are 

 made for every 10 degrees. Arrows are used to indicate 

 the direction of the wind. These fly with the wind, just 

 opposite to the way a weather vane points. For example, 

 an arrow pointing like this >- means that the wind is 

 blowing towards the east, and we call this a west wind be- 

 cause it blows from the west. The circles at the ends of 

 the arrows indicate the state of weather. Their meanings 

 are given in the explanation accompanying figure 199. 



On the complete maps shaded areas show precipitation of 

 .01 inch or more during the last 24 hours. A table at the 

 ] 3wer right-hand corner gives the maximum and minimum 

 temperatures, the wind velocity in miles per hour, and the 

 precipitation in inches during the last 24 hours. In the 

 lower left-hand corner are given the forecasts for the next day. 



How figures are obtained. These facts regarding the 

 weather are obtained daily from observers situated in about 

 200 stations in various parts of the United States and Canada. 

 Each morning at eight o'clock these observers telegraph 

 to Washington and to other leading cities the temperature, 

 the air pressure, the precipitation, the direction and velocity 

 of the wind, and the condition of the sky, whether clear or 

 cloudy. 



Weather instruments. In order to measure these con- 

 ditions accurately instruments are used, the chief ones being 

 the barometer, the thermometer, and the rain gauge. 



Air pressure. The pressure of the air is measured by the 

 barometer. The air that surrounds us has weight (as we 

 have already seen in Chapter IV) , although we do not or- 

 dinarily feel it. But when a strong wind is blowing, we get 

 some idea of the reality of this weight. Some simple ex- 



