From AMS Glossary
If the measurements are tower-based, the longitudinal (x) component is commonly along the mean wind direction, the lateral (y) component is perpendicular to the longitudinal component on the horizontal plane to the left when facing the downwind direction, and the vertical (z) component is upward, perpendicular to the longitudinal and lateral components. For aircraft measurements, the gustiness components may also be in the airplane coordinate system, with the longitudinal component along the longitudinal axis of the aircraft, the lateral perpendicular to the longitudinal in the horizontal plane of the aircraft, and the vertical upward, perpendicular to the horizontal plane of the aircraft.
- The ratios, to the mean wind speed, of the average magnitudes of the component fluctuations of the wind along three mutually perpendicular axes.
For a mean wind speed U, the gustiness components along the x axis (longitudinal or downwind gustiness), the y axis (lateral or crosswind gustiness), and the z axis (vertical gustiness) are given byHereare the average magnitudes of the eddy velocities along the x, y, and z axes, respectively.
- (Or intensity of turbulence.) The ratios of the root-mean-squares of the eddy velocities to the mean wind speed:for the longitudinal component, and similarly for the lateral and vertical components.
The gustiness components are sometimes defined as the square of the above expressions.