CHAPTER #6
Stability and Cloud Development





Atmospheric stability: the property of the environmental (ambient) air that either suppresses (stable) or enhances (unstable) vertical motion of an air parcel. It depends on the vertical temperature profile of the ambient air and whether the parcel is saturated or unsaturated.

If the rising air parcel becomes cooler (denser) than the ambient air or the sinking air parcel becomes warmer (less dense) than the ambient air, the parcel is forced back to return its original position. The parcel is stable with the environment.

If the rising air parcel becomes warmer (less dense) than the ambient air or the sinking air parcel becomes cooler (dense) than the ambient air, the parcel continues its upward or downward motion. The parcel is unstable with the environment.


Absolute instability: Property of an ambient air layer that is unstable for both saturated and unsaturated air parcels. It occurs when the temperature of the ambient air drops more rapidly with altitude than the dry adiabatic lapse rate.

Figure 6.8, page #150 (Ahrens)

Absolute stability: Property of an ambient air layer that is stable for both saturated and unsaturated air parcels. It occurs for the following three cases: i) the temperature of the ambient air drops more slowly with altitude than the moist adiabatic lapse rate, ii) the temperature does not change with altitude (isothermal), and iii) the temperature increases with height (temperature inversion).

Figure 6.3, page #148 (Ahrens)

Conditional stability: Property of an ambient air layer that is unstable for saturated air parcels and is stable for unsaturated air parcels. It occurs when the temperature of the ambient air drops more slowly with altitude than dry adiabatic lapse rate, but more rapidly than moist adiabatic lapse rate.

Figure 6.8, page #151 (Ahrens)

When an air parcel rises along dry adiabatic lapse rate, it is neural for unsaturated air, but unstable for saturated air.

When an air parcel rises along moist adiabatic lapse rate, it is neural for saturated air, but stable for unsaturated air.

Figure 6.10, page #152 (Ahrens)


Changes in Atmospheric Stability:

i) local radiative heating and cooling: Radiative cooling of the ground stabilizes the overlying air over a clear and calm night, whereas intense solar heating of the ground destabilizes the overlying air during the day.

ii) air mass advection: An air mass flowing over a colder surface is stabilized, whereas it is destabilized as if flows over a warmer surface.

iii) large scale ascent or descent of air: An air layer undergo more compressional warming in the upper portion than in the lower portion during descending (sinking) motion so that the ambient temperature profile becomes steeper and air layer stability increases. The reverse condition is true during ascending (rising) motion so that the ambient temperature profile becomes less steeper and air layer stability decreases.

Figure 6.12, page #153 (Ahrens)


Lifting processes:

i) Convection: the vertical air motion in which warm air rises and cool air sinks. Lifting by convection is driven by solar heating of the ground as heated air rises, expands, and cools. Eventually the air becomes so cool and dense that it sinks back to the surface where it is heated again.

ii) Front: a narrow zone of transition between two air masses that differ in temperature and humidity. Warm front is the leading edge of an advancing warm and humid (less dense) air mass as it rides over a cold and dry (dense) air mass, while cold front is the leading edge of an advancing cold and dry (dense) air mass as it slides under a warm and humid (less dense) air mass. The replacement of one air mass by another air mass is the frontal lifting of air, which, in turn, leads to expansional cooling.

Figure 6.15, page #156 (Ahrens)


iii) Orographic lifting: the forced rising of air up the slopes of a hill or mountain. As the air is forced to rise along mountain's windward slopes (upwind side), it expands and cools, which increases its relative humidity. Meanwhile, On the mountain's leeward slopes (downwind side), air descends and warms, which reduces its relative humidity.

Mountain ranges induce two contrasting climatic zones: a moist climate on the windward slopes and a dry climate on the leeward slopes. For example, the windward slopes of Mount Waialeale, Kauai island, Hawaii, receives annual average rainfall of 1199 cm (39.3 ft.), the rainiest location on Earth. Heavy rainfall is triggered by orographic lifting of warm, humid air by persistent northeast trade winds. By contrast, average annual rainfall on the leeward slopes of the same volcanic mountain is less than 50 cm (20 in.).

Figure 6.22, page #160 (Ahrens)


Cloud street: the clouds aligned in rows due to strong vertical shear in horizontal wind speed and direction.

Mountain-wave clouds: stationary clouds situated downwind of a prominent mountain range and caused by the disturbance of the wind by the mountain range. A cloud that forms over the summit of the mountain peak is called banner cloud.

Figure 6.24, page#161 (Ahrens)


Lee-wave clouds: lens-shaped clouds formed in the crest of a standing wave downwind from a prominent mountain range. Altocumulus lenticularis is a type of lee-wave cloud.

Figure 6.25, page #164 (Ahrens)


Billow clouds: wave-type clouds formed along the interface between two layers of air that are moving horizontally at different speeds.

Figure 6.27, page #165 (Ahrens)


iv) Wind convergence: occurs near the ground, generates upward motion as it lifts surface air to the certain level within the atmosphere.


Convective condensation level (CCL): the altitude at which condensation begins to occur through convection. It is the altitude of convective cloud base, typically between 1000 and 2000 meters (3600 to 6600 feet). For practical application, the height of the CCL in meters is given as

height_CCL = 125 (temperature - dew point temperature)

Level of free convection (LFC): the altitude at which the temperature of a surface parcel that is lifted adiabatically first becomes warmer than the temperature of the environment.

Equilibrium level (EL): the altitude at which a rising saturated parcel encounters negative bouyancy, parcel is cooler than the surrounding.


Convective available potential energy (CAPE): the amount of energy available to a parcel as it freely rises between the level of free convection (LFC) and the equilibrium level (EL). The CAPE is non-zero only if LFC exists. It is expressed as Joule/kg. CAPE of value of 0 indicates the stable atmopshere, while CAPE of less than 1000 shows marginally unstable conditions. CAPE of 1000-2500 refers to moderately unstabe conditions, CAPE of 2500-3500 indicates very unstabe conditions. CAPE of larger than 3500 is the extremely unstable conditions.

Convective inhibition (CIN): the amount of energy that must be supplied to a parcel for it to rise to the elvel of free convection (LFC). Convective inhibition is non-zero only if an LFC exists. It is expressed as Joule/kg. CIN of less than 50 refers to weak cap, while CIN of 50-200 is teh moderate cap, and CIN of larger than 200 is the strong cap.



Lifted Index (LI): a measure of the air mass thunderstorm potential taking into account the low level moisture. It is the difference between the parcel temperature that was raised from the surface to 500 mb and ambient temperature at 500 mb. It is it positive, thunderstorm is unlikely, if it is between 0 and -2, thunderstorm is possible, if it is between -2 and -5, thunderstorm is possible, if it is less than -5, thunderstorm is severe.

K Index (KI): a measure of the air mass thunderstorm potential taking into account the low level moisture. It is defined as

K_index = temperature at 850 mb - temperature at 500 mb + dew point at 850 mb - temperature at 700 mb + dew point at 700 mb

If K_index is less than 20, thunderstorm is unlikely, if it is between 20 and 25, isolated thunderstorm is likely, if it is between 26 and 35, widely spread thunderstorm is likely, if it is above 35, severe thunderstorm is possible.

difference between the parcel temperature that was raised from the surface to 500 mb and ambient temperature at 500 mb. It is it

Total totals Index (TTI): a measure of the air mass thunderstorm potential taking into account the low level moisture. It is defined as

TT_index = temperature at 850 mb + dew point at 850 mb - 2 x temperature at 500 mb

If TT_index is less than 44, thunderstorm is unlikely, if it is above 44, thunderstorm is likely, if it is above, severe thunderstorm is possible.