| Popis: |
Summary Of the waves which propagate in the atmosphere at acoustic velocity in the period range from 10 to 100 s, one type has been classified by triangulation as arising principally from mountainous regions. These signals were first described as ' northwesters ' or ' 310 ers ' by the NBS Geoacoustics Group under R. K. Cook at Washington, D.C., from the predominant direction of arrival. Subsequent operation of an observatory at Boulder, Colorado by Vernon Goerke gave a source region by triangulation in the Pacific Northwest, primarily in Montana and Alberta. Installations of observatories at College, Alaska (Wilson) and Pullman, Washington-Moscow, Idaho (Craine and Thomas) enlarged the data base available, and triangulation showed the principal source areas to be along the coast of British Columbia and in the inland Rocky Mountains of the British Columbia-Alberta border. This paper discusses the presently known characteristics of this class of infrasonic waves, locates the triangulation areas, reviews selected events, and suggests that certain of these waves are produced as aerodynamic sound. The paper shows a correlation between the 500 mb jet stream velocity and direction in these mountainous regions, and the detection of these atmospheric pressure waves. Infrasonic waves-10- to 100-s periods There are a number of atmospheric wave structures travelling at acoustic velocities that appear in the 10- to 100-s period range. An observatory, through four pressure sensitive transducers, provides basic information on the amplitude variations of pressure with time, from which the waveform, period, and time sequence of the event at an individual microphone can be obtained. By overlaying the charts and visually correlating the waveforms from several transducers, a signal can be identified in the noise, and arrival time differences measured. The azimuth of the direction of arrival, and the horizontal velocity across the transducer array, can then be calculated, assuming a plane wave. Detection of signals is often difficult, since local wind noise may obscure the desired signal, and changes in waveform caused by frequency dispersion of the signal or the effects of other signals arriving from different directions may make the waveform vary at each transducer. We define an ' event ' as a series of signals that appear to be related. |
