Some processes have more than one characteristic length or time scale associated with them, for example

mechanical vibrations with a slowly varying amplitude the slow precession of planetary orbits turbulence-there are various length scales of the turbulent eddies along with the

length scale of objects over which fluid flows surface roughness-the length scale of the surface finish is far smaller than the

overall dimensions of the surface noise in electrical signals-this is a high frequency (short time scale) effect super-

imposed on the signal information.