Divergence aloft occuring over a favorable temperature gradient will induce a synergistic development of the three dimensional disturbance.

A. Divergence yields pressure falls and the creation of a low. The creation of the low in a region of temperature gradient creates temperature advection that acts in the sense of developing the upper wave, which in turn feeds back to the surface disturbance.

B. Counteracting this is the vertical temperature advection of stable air, which is greatest with strong divergence. This will act to lower temperatures and heights at the ridge axis aloft, and raise temperatures and heights at the trough axis, in turn dampening the system and decreasing the upper divergence that was responsible for the initial system deepening.

When (A) exceeds (B), the system develops. When (B) exceeds (A), the system dissipates.

For certain ranges of divergence and zonal temperature gradient, the development will be explosive. For other ranges, no development is possible (either the divergence and vertical dampening is too great or the zonal temperature gradient is too small).

Under the normal conditions of static stability, the wavelength of maximum baroclinic (hydrodynamic) instability is about 4000 km, which is close to the average wavelength of midlatitude synoptic systems. Ther thermal wind required for wave growth is about 4 m s^-1, which implies a vertical wind shear (given normal lower level winds) of acout 8 m s^-1 between 250 mb and 750 mb. This is also commonly observed in middle latitueds for the zonally averaged flow. Therefore the observed behavior of middle latitude synoptic systems is consistent wioth the hypothesis that such systems develop from disturbances in a baroclinically unstable basic current. (from Holton, p. 222).