Abstract
In news pictures saturating the media last summer there is contained subtle but compelling evidence that tropical hurricanes emit intense and extremely low-frequency tones, originating in the eye. The excellent symmetry of a well-formed eye predisposes the central cavity of a hurricane to resonate in the normal modes of a low-Q nearly cylindrical chamber, due to the slight mismatch in sound speed at the eye wall. Second order perturbation analysis of interaction between eye-modes and steady wind corkscrewing up through the eye has confirmed that positive feedback is generated, the condition for self-excited oscillation. Dimensions and speed of typical hurricanes require that the tones have frequencies on the order of 1/100 of 1 Hz, which is doubtless the reason why they have escaped previous detection. Visual evidence appears in multiple alternating rain and dry bands that originate around the inner circumference of the eye wall, observable in aerial photos by NOAA. These are most easily explained as due to intense acoustically-driven temperature oscillations about the dew point, inside the eye. For a given hurricane, the actual tonal frequencies can be deduced from the spacing of these startups.
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