Shortwave radio Shortwave radio Shortwave radio is radio transmission using shortwave radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), and generally extends from ; from the high end of the medium frequency band (MF) just above the mediumwave AM broadcast band, to the end of the HF band. Radio waves in the shortwave band can be reflected or refracted from a layer of electrically charged atoms in the atmosphere called the ionosphere. Therefore, short waves directed at an angle into the sky can be reflected back to Earth
Atmospheric tide The latitudinal structure of the tides is described by the "horizontal structure equation" which is also called "Laplace's tidal equation": with "Laplace operator" using formula_30, formula_31 and "eigenvalue" Hence, atmospheric tides are eigenoscillations (eigenmodes)of Earth's atmosphere with eigenfunctions formula_33, called Hough functions, and eigenvalues formula_34. The latter define the "equivalent depth" formula_35 which couples the latitudinal structure of the tides with their vertical structure. Longuet-Higgins has completely solved Laplace's equations and has discovered tidal modes with negative eigenvalues ε (Figure 2). There exist two kinds of waves: class 1 waves, (sometimes called gravity waves), labelled by positive n, and class