Influencing Factors

In an effort to simplify motion in the atmosphere, all of our discussions regarding stability up to now were based on three assumptions:

1. The atmosphere is in hydrostatic equilibrium.
2. As a parcel of air rises and sinks, there is no compensating motion in the displaced environmental air. The environment around the parcel is static.
3. The rising and sinking parcel is isolated from the environment such that the rising and sinking air and the environmental air do not mix.

However, the atmosphere is somewhat more complex. In reality, on the meso- and microscale, the atmosphere is rarely in hydrostatic equilibrium. Perhaps a heated parcel is located within a layer that experiences orographic lifting up a mountain side. In additon, the rising parcel displaces the air around and above it so that the environment near the rising parcel is not even static. Also, this non-static environmental air does mix with the rising air. Next, we will look at how these actions influence the stability of the atmosphere.

As a parcel rises the air around and above it is displaced. The slice method of lifting parcels to determine stability attempts to compensate for the effects of the non-static environment. Though this method has weaknesses of its own, a brief overview of this method does provide insight into how the displaced environmental air influences atmospheric stability. The slice method looks at a slice of the atmosphere with several saturated parcels rising through it. This method assumes that for every unit of mass being lifted, a unit of mass of environmental air is sinking. So, as the parcels are rising, air is sinking all around them. The air between the parcels, at a given point in time, is actually displaced air that has descended from above. Keep in mind, the rising parcels are saturated and, therefore, are cooling according to the moist adiabatic lapse rate. The descending environmental air is not saturated and is warming much faster according to the dry adiabatic lapse rate. The environmental air adjacent to a parcel is warmer than the air would be if the environment was static. By the equation of state, the environment should be in a state of equilibrium. As it descends, the environmental air warms in response to the pressure change. Here is the net effect of all of this motion. According to the slice method, there is not as much of a temperature difference between the parcel and the environment as there would be if the environment was static. This causes the parcel to be less buoyant than we might expect in a static environment and thus reduces the amount of instability. In our previous discussion in the unstable atmosphere, instability is exaggerated. We will see next that the entrainment of environmental air into a parcel of rising air also has the effect of reducing instability.

Not only is the environment in motion around rising air parcels, the environment and the air parcels are not isolated from each other, as if the parcel had an invisible skin surrounding it. The two do mix and interact with each other. Based on the theories of hydrodynamics, a current, or a jet, within a fluid, like the jet stream, draws or entrains some of the surrounding fluid into itself. A cumulus cloud, which is a saturated air parcel, is essentially a small jet, and likewise draws into it some of the surrounding air. Because the surrounding air is dryer than the parcel (i.e., the surrounding air is not saturated), as air is drawn into the cloud, the mixing ratio of the cloud decreases. The entrained dry air evaporates liquid water within the cloud (produced by condensation) until the dry air too becomes saturated. This process of evaporation has a cooling effect within the cloud in much the same way our bodies cool off when water evaporates from our skin. As the air temperature of the cloud decreases, the cloud (or parcel) becomes less buoyant again reducing the amount of instability.

We must keep in mind when trying to determine atmospheric stability that neither of the methods we have discussed are perfect in their treatment of the atmosphere. Atmospheric motion is extremely complex, especially in the planetary boundary layer (PBL). The concepts outlined by these methods do, however, give us good insight into some of characteristics which are common to different conditions of stability, as well as some of the factors which influence stability in the atmosphere.