WEATHER BUREAU 



0230 



WEATHER BUREAU 



little thought as to the means by which they 

 are obtained. Back of these reports is an 

 elaborate system of observation that may be 

 compared to a single observer's obtaining a 

 bird's-eye view of the country as a whole. Sup- 

 pose such an observer were to ascend in an 

 airship every twelve hours, to a point where 

 he could see all the weather conditions from 

 the Canadian border to the Rio Grande, and 

 from the Atlantic to the Pacific. The reports 

 he would give of his surveys and the predictions 

 he would make would'be accepted by every one. 

 The government Weather Bureau attempts 

 something of the same kind by working 

 through a number of observers, all acting in 

 unison. 



Scattered throughout the country are more 

 than 200 telegraphic observing stations, and 



tion of the amount of moisture in the atmos- 

 phere, and noting of such phenomena as thun- 

 derstorms, smoke, halos and fog. In addition, 

 the temperature is read from the thermometer 

 and the extremes since the last observation are 

 noted. 



Wiathcr maps showing conditions over the 

 country are constructed twice a day at the 

 Washington office. A definite system of figures, 

 letters and symbols to indicate various condi- 

 tions has been adopted, as shown in the wcuthrr 

 map, following. Small circles indicate the 

 various observing stations. If the entire area 

 of the circle is black it means prevailing cloudi- 

 ness; partly cloudy conditions are shown by a 

 half-blackened circle, and a clear circle indi- 

 cates clear skies. An "R" in the circle means 

 rain falling at the time of observation, and 



BLUE 



WEATHER FLAGS 



(ay White flag, indicates clear or fair weather; (&) blue flag, forecasts general rain or snow; 

 (c) white and blue flag, local rain or snow; (d) black triangular flag, temperature indication; (e) 

 white flag with black square in center, heralds the approach of a cold wave. When d is placed above 

 a, 6 or c it is a prophecy of warmer weather (temperature higher) ; when below any of the three, 

 colder weather is announced ; when the temperature flag is not displayed no great change is ex- 

 pected. During late spring and early fall the cold wave flag announces threatened visits of frost; 

 in the winter, of suddenly lowering temperature. 



every twelve hours, exactly at 7:45 A. M. and 

 7:45 P. M. (seventy-fifth meridian time, near 

 Washington, D. C.), the observation and the 

 recording of weather conditions in the territory 

 of each station take place. The results of his 

 survey are then summarized by each observer, 

 condensed into a cipher message of four or five 

 words and transmitted to the proper tele- 

 graphic circuit. When translated, these brief 

 code messages contain from thirty to fifty 

 words. Though all are finally collected at the 

 central office in Washington, D. C., they are 

 translated at various forecasting stations along 

 the route. 



Though the work of observing conditions and 

 of summarizing data is always completed at 

 each station by eight o'clock, the notations and 

 calculations are comprehensive. They include 

 observation of sky and classification of clouds; 

 reading of the barometer to ascertain atmos- 

 pheric pressure, and maKing corrections to ob- 

 tain corresponding readings for the sea level ; 

 observation of direction and velocity of wind; 

 measurement of rainfall or snowfall ; calcula- 



similarly "S" indicates a fall of snow. Arrows 

 indicate the direction in which the wind is 

 blowing. The small figures at the side of sta- 

 tions show barometer readings, temperature, 

 velocity of wind in miles per hour, depth of 

 rainfall or snowfall. 



The prominent solid lines on the map are 

 isobars, or lines of equal barometer readings. 

 The}' pass through points of equal air pressure, 

 and are drawn for each tenth of an inch of 

 variation. To illustrate, the line marked "30.0" 

 at each end (see Gulf of Mexico and Atlantic 

 areas) passes through those points where the 

 barometer readings are exactly thirty inches. 

 The readings on one side of this line are higher 

 than thirty inches, lines being drawn for each 

 tenth of an inch increase until a crest or center 

 is reached; such a center is marked "High." 

 A center of depression on the other side of line 

 "30.0" is located and marked "Low," lines be- 

 ing drawn for each tenth of an inch decrease. 

 The value of isobars rests chiefly in the fact 

 that they outline great atmospheric whirls, or 

 eddies, and thus indicate centers of action. In 



