Medium frequency travel longer distances via skywave propagation, in which radio waves radiated at an angle into the sky are reflected (actually refracted) back to Earth by layers of charged particles (ions) in the ionosphere, the E and F layers. However at certain times the D layer (at a lower altitude than the refractive E and F layers) can be electronically noisy and absorb MF radio waves, interfering with skywave propagation. This happens when the ionosphere is heavily ionised, such as during the day, in summer and especially at times of high solar activity, At night, especially in winter months and at
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