From Glossary of Meteorology
A nondimensional number occurring in the study of periodic or quasiperiodic variations in the wake of objects immersed in a fluid stream.
The phenomena are due to the more or less regular formation of vortices or vortex streets in the wake (especially at Reynolds numbers around 100 in the case of circular cylinders):
where n is a frequency, D a representative length, and U a representative velocity of the stream. Common examples are aeolian sounds such as those produced by electric power or telephone lines in a wind.