Forecasting the Weather and Storms 259 



an idea of how the charts are made for 

 the study of the forecast official. From 

 these he gets a panoramic view, not only 

 of the exact conditions of the air over 

 the whole country at the moment of 

 taking the observations one hour before, 

 but of the changes that have occurred 

 in those conditions during the preceding 

 12 and 24 hours. As fast as the reports 

 come from the wires they are passed to 

 the Forecast Division, where a reader 

 stands in the middle of the room and 

 translates the cipher into figures and 

 words of intelligible sequence. A force 

 of clerks is engaged in making graphic 

 representations of the geographical dis- 

 tribution of the different meteorological 

 elements. On blank charts of the United 

 States each clerk copies from the trans- 

 lator that part of each station's report 

 needed in the construction of his par- 

 ticular chart. One clerk constructs a 

 chart showing the change in tempera- 

 ture during the preceding 24 hours. 

 Broad red lines separate the colder from 

 the warmer regions, and narrow red lines 

 inclose areas showing changes in tem- 

 perature of more than 10 degrees. The 

 narrow lines generally run in oval or 

 circular form, indicating (as will be 

 shown subsequently) that atmospheric 

 disturbances move and operate in the 

 form of great progressive eddies ; that 

 there are central points of intensity from 

 which the force of the disturbance di- 

 minishes in all directions. 



A second clerk constructs a chart 

 showing the change that has occurred 

 in the barometer during the past 24 

 hours. As in the construction of the 

 temperature chart, broad, heavy lines 

 of red separate the regions of rising 

 barometer from those of falling barom- 

 eter. Narrow lines inclose the areas 

 over which the change in barometer has 

 been greater than one-tenth, and so on. 



Here, for instance, throughout a great 

 expanse of territory, all the barometers 

 are rising — that is to say, the air cools, 

 contracts, becomes denser, and presses 



with greater force upon the surface of 

 the mercury in the cisterns of the in- 

 struments, thereby sustaining the col- 

 umns of liquid metal at a greater height 

 in the vacuum tubes. Over another 

 considerable area the barometers are 

 falling, as increasing temperature rare- 

 fies and expands the volume of the air, 

 causing it to press upon the instruments 

 with less force. This chart is extremely 

 useful to the forecaster, since, in con- 

 nection with the general weather chart, 

 it indicates whether or not the storm 

 centers are increasing or decreasing in 

 intensity, and, what is of more impor- 

 tance, it gives in a great measure the 

 first warning of the formation of storms. 



A third clerk constructs two charts, 

 one showing the humidity of the air 

 and the other the cloud areas, with the 

 kind, amount, and direction of the 

 clouds at each station. It is often in- 

 teresting to observe at a station on the 

 cloud chart high cirrus clouds composed 

 of minute ice spiculae moving from one 

 direction, lower cumulo-stratus com- 

 posed of condensed water vapor moving 

 from another direction, and the wind 

 at the surface of the earth blowing from 

 a third point of the compass. Such 

 erratic movements of the air strata are 

 only observed shortly before or during 

 rain or wind storms. 



A fourth clerk constructs a chart 

 called the general weather chart, show- 

 ing for each station the air temperature 

 and pressure, the velocity and direction 

 of the wind, the rain or snow fall since 

 the last report, and the amount of 

 cloudiness. The readings of the barom- 

 eter on this chart are reduced to sea- 

 level, so that the variations in pressure 

 due to local altitudes may not mask and 

 obscure those due to storm formation. 

 Then lines, called isobars, are drawn 

 through places having the same pres- 

 sure. By drawing isobars for each dif- 

 ference in pressure of one-tenth of an 

 inch the high and the low pressure 

 areas are soon inclosed in their proper 



