WEATHER INDICATIONS 5 



are furnished to nearly one hundred stations of the Weather Bureau 

 for use in the preparation of maps and bulletins. 



On the weather maps solid lines, called isobars, are drawn through 

 points that have the same atmospheric pressure, a line being drawn 

 for each one-tenth of an inch in the height of the barometer. Dotted 

 lines, called isotherms, are drawn through points that have the same 

 atmospheric temperature, a line being drawn for each ten degrees of 

 temperature. Heavy dotted lines are sometimes used to inclose areas 

 where decided changes in temperature have occurred during the pre- 

 ceding twenty-four hours. The direction of the wind at each station 

 is indicated by an arrow which flies with the wind. The state of the 

 weather — clear, partly cloudy, cloudy, rain, or snow — is indicated by 

 symbols. Shaded areas are used on the maps issued at Washington, 

 and at several stations, to show areas within which precipitation in 

 the form of rain or snow has occurred during the preceding twelve 

 hours. The tabular data give details of maximum and minimum 

 temperatures, and 24-hour temperature changes, wind velocities, and 

 amount of precipitation during the preceding twenty-four hours. The 

 text printed on the maps presents forecasts for the state and the sta- 

 tion, and summarizes general and special meteorological features that 

 are shown by the Unes, symbols, and tabulated data. 



The weather indications. 



The centers of areas of low barometric pressure, or general storms, 

 are indicated on the map by the word " low," and the centers of 

 areas of high barometric pressure by the word " high." The gen- 

 eral movement of " lows " and " highs " in the United States is from 

 west to east, and in their progression they are similar to a series of 

 atmospheric waves, the crests of which are designated by the " highs " 

 and the troughs by the " lows." These alternating " highs " and 

 " lows " have an average easterly movement of about six hundred to 

 seven hundred miles a day. The " lows " usually move in an easterly, 

 or north of east, direction, and the " highs " in an easterly, or south 

 of east, direction. 



In advance of a " low " the winds are southerly or easterly, and 

 are, therefore, usually warmer. When the " low " passes east of a 

 place, the wind shifts to westerly or northwesterly with lower tempera- 

 ture. The eastward advance of " lows " is almost invariably preceded 



