The linear stability of a two-layer front wherein the potential vorticity is uniform inside each layer is investigated as the most natural frontal model formed by two air masses, and also as the most fundamental frontal model. Although there is no pure Rossby mode in each layer owing to the uniformity of the potential vorticity, Kelvin mode with a small wavenumber, where it possesses a Rossby-wave-like feature, plays an important role for causing the instability of this front. In particular, if the basic state is characterized by the most plausible parameter for which the basic velocity outside the frontal zone vanishes, a mode classified as a kind of baroclinic instability is the fastest growing. Rossby-gravity instability is not significant in this model, which is consistent with the result in the continuously stratified model.
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