Hough function
The Hough Functions are the eigenfunctions of Lapalace's tidal equations which govern fluid motion on a rotating sphere. As such, they are important in geophysics and meteorology where they form part of the solutions for atmospheric and ocean waves.
Each Hough mode is a function of latitude and longitude and are orthogonal over the sphere in the continuous case. Thus they can also be thought of as a generalized Fourier series in which the basis functions are the normal modes of a resting atmosphere.