154 GENERAL SCIENCE 



Air Density. If the earth's surface had the same 

 temperature at every point, the air near this surface 

 would have the same density. With the varying degrees 

 of temperature of the surface, the air becomes heated in 

 certain places and then being less dense is forced up by 

 the heavier cold air around it, just as a cork released at 

 the bottom of a basin of water is forced up by the heavier 

 substance surrounding it. Air also becomes lighter as it 

 absorbs moisture or water vapor, since steam has a lower 

 density than air. 



For measuring the pressure of the atmosphere, the 

 barometer is used. This instrument has been described 

 in Chapter V. It should be observed that the barom- 

 eter does nothing more than indicate the air pressure, 

 and this with other facts enables the forecaster to predict 

 changes in the weather. 



Isobars. In order to forecast weather conditions, it 

 is necessary to compare barometric readings from many 

 different parts of the country. Since barometric readings 

 vary with the elevation and the temperature, in order 

 to make intelligent comparisons it is necessary to make 

 corrections for these differences in elevation and tempera- 

 ture. By the aid of prepared tables this is readily done, 

 all readings being reduced to the corresponding reading at 

 the elevation of sea level and the temperature of the 

 freezing point of water. After the corrections are made 

 lines are drawn connecting points having equal atmos- 

 pheric pressure. These lines are called isobars. A map 

 showing lines of equal pressure is called an isobaric map. 

 On the daily weather map isobars (continuous black lines) 

 are drawn representing variations in pressure of one tenth 

 of an inch. See Figure 132. 



An examination of several daily weather maps will 



