Determining Atmospheric Stability

The degree of stability or instability of an atmospheric layer is determined by comparing its temperature lapse rate, as shown by a sounding, with the appropriate dry- or moist-adiabatic lapse rates. The dry adiabatic rate is used for air that is not saturated and the moist-adiabatic rate is used for saturated air. The adiabatic process is reversible. Just as air expands and cools when it is lifted, so is it equally compressed and warmed as it is lowered. Hence, stability determinations for either upward or downward moving air parcels make use similar comparisons with the appropriate adiabatic lapse rates.

Unsaturated air

Determining Stability

• Stable: A temperature lapse rate less than the dry adiabatic rate of 5.5° F per 1,000 feet for an unsaturated parcel is considered stable, because vertical motion is damped.
• Unstable: A lapse rate greater than dry-adiabatic (a superadiabatic lapse rate) favors vertical motion and is unstable. Since it is unstable, the air tends to adjust itself through mixing and overturning to a more stable condition. Superadiabatic lapse rates are not ordinarily found in the atmosphere except near the surface of the earth on sunny days. When an unsaturated layer of air is mixed thoroughly, its lapse rate tends toward neutral stability.
• Neutral: In the absence of saturation, an atmospheric layer is neutrally stable if its lapse rate is the same as the dry-adiabatic rate. Under this particular condition, any existing vertical motion is neither damped nor accelerated.

Warming of the lower layers during the daytime by contact with the earths surface or by heat from a wildfire will make a neutral lapse rate become unstable. In an atmosphere with a dry-adiabatic lapse rate, hot gases rising from a fire will encounter little resistance, will travel upward with ease, and can develop a tall convection column. A neutrally stable atmosphere can be made unstable also by advection; that is, the horizontal movement of colder air into the area aloft or warmer air into the area near the surface. Once the lapse rate becomes unstable, vertical currents are easily initiated. Advection of warm air aloft or cold air near the surface has the reverse effect of making the atmosphere more stable.

The term "neutral" stability sounds rather passive, but we should be cautious when such a lapse rate is present. The temperature structure of the atmosphere is not static, but is continually changing. Any warming of the lower portion or cooling of the upper portion of a neutrally stable layer will cause the layer to become unstable, and it will then not only permit, but will assist, vertical motion. Such changes are easily brought about. Thus, we should consider the terms stable, neutral, and unstable in a relative, rather than an absolute, sense. A stable lapse rate that approaches the dry-adiabatic rate should be considered relatively unstable.

Saturated air

Adiabatic Chart

So far we have considered adiabatic cooling and warming and the degree of stability of the atmosphere only with respect to air that is not saturated. Rising air, cooling at the dry-adiabatic lapse rate, may eventually reach the dew-point temperature. Further cooling results in the condensation of water vapor into clouds, a change of state process that liberates the latent heat contained in the vapor. This heat is added to the rising air, with the result that the temperature no longer decreases at the dry-adiabatic rate, but at a lesser rate which is called the moist-adiabatic rate. On the average, this rate is around 3° F per 1,000 feet, but it varies slightly with pressure and considerably with temperature. The variation of the rate due to temperature may range from about 20° F per 1,000 feet at very warm temperatures to about 5° F per 1,000 feet at very cold temperatures. In warmer air masses, more water vapor is available for condensation and therefore more heat is released, while in colder air masses, little water vapor is available.

To determine the degree of stability or instability for a saturated parcel, the same stability terms apply as for unsaturated but the comparison of atmospheric lapse rate is made with the moist-adiabatic rate appropriate to the temperature encountered.

• Stable: A temperature lapse rate less than the moist adiabatic rate is considered stable.
• Unstable: A lapse rate greater than moist-adiabatic is unstable.
• Neutral: In the absence of saturation, an atmospheric layer is neutrally stable if its lapse rate is the same as the dry-adiabatic rate. Under this particular condition, any existing vertical motion is neither damped nor accelerated.

Conditional stability

An atmosphere that has a lapse rate lying between the dry and moist adiabats is said to be conditionally unstable. It is stable with respect to a lifted air parcel as long as the parcel remains unsaturated, but it is unstable with respect to a lifted parcel that has become saturated.

A saturated parcel in free convection loses additional moisture by condensation as it rises. This, plus the colder temperature aloft, causes the moist-adiabatic lapse rate to increase toward the dry-adiabatic rate. The rising parcel will thus eventually cool to the temperature of the surrounding air where the free convection will cease. This may be in the vicinity of the tropopause or at some lower level, depending on the temperature structure of the air aloft.

Layer Stability

Atmospheric stability is affected by vertical movement of both parcels of air or whole layers of considerable horizontal extent of the atmosphere. When an entire layer of stable air is lifted it becomes increasingly less stable. The layer stretches vertically as it is lifted, with the top rising farther and cooling more than the bottom. If no part of the layer reaches condensation, the stable layer will eventually become dry-adiabatic. Occasionally, the bottom of a layer of air being lifted is moister than the top and reaches its condensation level early in the lifting. Cooling of the bottom takes place at the slower moist-adiabatic rate, while the top continues to cool at the dry-adiabatic rate. The layer then becomes increasingly less stable at a rate faster than if condensation had not taken place. A descending (subsiding) layer of stable air becomes more stable as it lowers. The layer compresses, with the top sinking more and warming more than the bottom. The adiabatic processes involved are just the opposite of those that apply to rising air.