24 



Symposium on Microseisms 



SlBHIJXiQSL FOR 90 KNOT STog i^ 



Figure 4. Amplitude chart for 90 knot storms in range of Miami 



widely spaced on the other sides. Each storm 

 passing a station inside the 10 mm line devel- 

 ops the same characteristic type of storm mi- 

 croseisms and will fit one of the amplitude 

 charts, depending only on the intensity of the 

 storm and its distance from the station. Simi- 

 lar storms, in passing over the same area as a 

 previous storm, will generate point by point 

 the amplitude of microseisms as shown on the 

 chart. Microseisms recorded simultaneously 

 at four stations located between 190 and 740 

 miles from the storm center produce storm mi- 

 croseisms of a uniform character, regardless 

 of the direction of the storm from the station, 

 Figure 5. Any similar storm in the same lo- 

 cation will duplicate these microseisms at the 

 four stations, provided the instruments are 

 maintained in the finest of calibration and at a 

 standard magnification. 



Use of Amplitude Charts — The Fleet Weather 

 Centrals at Guam and Miami direct aircraft to 

 fly reconnaissance into and around tropical 

 storms, when they are far from land. Some 

 reconnaissance planes are now equipped with 



radar for tracking storms at night, but be- 

 cause of various interferences these radar re- 

 ports are not always reliable. Valuable time is 

 consumed in alerting the planes, flying to the 

 storm location and transmitting reports. Occa- 

 sionally a storm far from an airfield cannot be 

 tracked, other than by extrapolation, because 

 the planes have been grounded for various rea- 

 sons : chiefly, mechanical trouble. Ship reports 

 are often just as valuable, when available, but 

 once the storm is located ships are warned to 

 stay clear of the storm area and consequently 

 give little additional information. 



The Fleet Weather Centrals at Guam and 

 Miami have been using the data from micro- 

 seismic records for several years as a helpful 

 aid in forecasting the intensity and movement 

 of tropical storms. There are many specific 

 cases in which microseisms have given valuable 

 information that could be obtained in no other 

 way. 



The type of pertinent storm information 

 furnished by the amplitude chart is adequately 



