AGU journal highlights -- Sept. 6, 2007( 1. Solving the mystery of booming s...)

Booming noises in deserts can sometimes be heard after a natural slumping event or during a sand avalanche generated by humans sliding down the slip face of a large dune. This sound is composed of one dominant audible frequency ranging between 70 and 105 hertz, and several higher harmonics. Previous studies have sought to explain this noise by suggesting that the frequency is a function of the average size of sand grains. Vriend et al. challenge this finding and instead demonstrate through field measurements that the booming frequency results from a natural waveguide, where sound waves channeled through the dune amplify as they constructively interfere. They also show that during slipping events, the surficial layer of dry, loose sand interacts with the air above it, behaving as a loudspeaker. With their observations, the authors were able to develop a mathematical relationship that predicts the frequency and harmonics of the booming emission of dunes. They determine that this frequency is dependent on compressional wave velocities and the depth of the surficial sand layer.

Title:
Solving the mystery of booming sand dunes

Authors:
Nathalie M. Vriend, Melany L. Hunt, Christopher Earls Brennen, Katherine S. Brantley, and Angel Ruiz-Angulo: Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California, U.S.A.;
Robert W. Clayton: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, U.S.A.

Source:
Geophysical Research Letters (GRL) paper 10.1029/2007GL030276, 2007, http://dx.doi.org/10.1029/2007GL030276

2. Reductions in the northward incursion of the South Asian monsoon since AD 1400

Variations in the Asian monsoon, one of the Earth's largest seasonal weather patterns, can bring drought or torr
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