Summary For four years continuous recording of infrasonic signals in the frequency range 0.1 to 1 Hz, known as microbaroms, has been conducted at Palisades, New York. The microbaroms we recorded are radiated into the atmosphere by interfering ocean waves in the North Atlantic as far as 2000 km away. A characteristic diurnal variation in the amplitude of the received signal has been noted, independent of any variation in the source. We conclude that the variation is due to variations of the factors affecting atmospheric sound propagation, namely wind and temperature.
In winter a semidiurnal variation in signal amplitude is observed, with maximum reception around 11 : 00 and 22; 00 local time. Reference to wind and temperature observations in the literature shows that at these times the lowest level of reflection of the vertically propagating signal occurs between 100 and 110 km due to the presence of strong east winds. At 18 : 00, time of minimum amplitudes, the reflection level rises to about 115 km because of a change in tidal wind phase. Viscous dissipation associated with the changed reflection height can account for the observed signal weakening. A third maximum, a less regular effect, is found to be related to more variable winds between 95 and 105 km.
In summer, reflection is found to occur from about 50 km due to the presence of stratospheric easterlies. The summer diurnal variation, different from that of the winter, exhibits only a weak minimum about 20 : 00. This appears to result from a diurnal temperature variation superimposed on a diurnal wind variation. Abnormally high microbaroms were recorded at times that can be related to an atmospheric event known as a stratospheric warming. Microbaroms thus provide a continuously available natural mechanism for probing the upper atmosphere. We conclude that the establishment of microbarom observation systems could give a comprehensive technique for monitoring several upper atmospheric parameters.