The kinetic energy of the atmosphere is not spread uniformly over all wavelengths but has certain preferred scales, with gaps in between.

Typically atmospheric structures are either fully three dimensional with horizontal wavelengths of the order of 100 m to several kilometers, such as convection cells (including thunderstorms) and mechanically driven eddies; or they are quasi-two-dimensional with horizontal dimensions of order of thousands of km. The first group of systems derives its energy from Kelvin-Helmholtz and hydrostatic instability, which depends on vertical gradients of wind and temperature; the second group is associated with barotropic or baroclinic instability, which depends on horizontal gradients of temperature and wind.

Aloft, the small-scale systems are relatively less frequent than near the ground; on the other hand, intermediate-scale systems seem to be more common.

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1 On leave from the University of Munich.