The typical geometry of an airmass interface, or front, resembles a wave form (Figure 9.7). Similar wave patterns are, in fact, found to occur on the interfaces between many different media; for example, waves on the sea surface, ripples on beach sand, eolian sanddunes, etc. Unlike these wave forms, however, the frontal waves in the atmosphere are usually unstable; that is, they suddenly originate, increase in size, and then gradually dissipate. Numerical model calculations show that in middle latitudes waves in a baroclinic atmosphere are unstable if their wavelength exceeds a few thousand kilometres. Frontal wave cyclones are typically 1500 to 3000 km in wavelength. The circu lation of the upper troposphere plays a key role in providing appropriate conditions for their development and growth, as shown below.
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