3. Stability

Changing Stability » Heating and Cooling Instability from Surface Heating

Sounding animation showing instability caused by surface heating

The ground absorbs solar radiation, causing the surface temperature to rise. This in turn heats the surface air parcels by conduction. These heated parcels tend to organize into large "bubbles" that ascend by virtue of their buoyancy relative to the surroundings. If the lapse rate is already adiabatic (or superadiabatic), the "bubbles" rise rapidly until a stable region is reached, which resists further rising motions.

On the other hand, if the initial lapse rate is stable, then the rise of the surface-heated parcels is delayed. "Bubbles" will start to rise either when some of them become sufficiently warm to spontaneously rise or when some are impelled upward by mechanical turbulence. The rising parcels will then penetrate some distance into the overlying stable region.

Through such penetrative convection, heated "bubbles" warm the lowest atmosphere. With time, warm air rises to higher and higher altitudes in the stable layers above, thereby slowly extending a dry-adiabatic lapse rate from the surface to greater altitudes. Thus, surface heating creates instability indirectly through the intermediate mechanism of convective mixing. This process is limited by the amount of heat absorbed by the ground and conducted to the air. The animation shown here depicts the evolution of the sounding for this process.

The inversion and CINCIN are wiped out or reduced by surface heating as the surface temperature rises.