Abstract
In this paper customary used equations related to the component of the mean wind direction of gusty wind are classified into the following three types; Dryden type, Karman type and Davenport type. The fundamental properties of these types are surveyed under the theory of isotropic turbulence and a few faults are indicated in Cramer-Davenport's equation which is frequently used as a nondimensional cross spectrum. Theoretically the nondimensional cross spectrum of Karman type is the most reasonable, but this equation is not practical since it is expressed with the modified Bessel functions. Thus its practical expression is proposed. In the case of natural wind which seems to be nonisotropic it may be better to employ the lateral scale of turbulence instead of the longitudinal scale of turbulence as a fundamental factor of the structure of turbulence.
