3. Stability
Table of Contents
- Introduction
- Determining Stability
- Changing Stability
- Introduction
- Heating and Cooling
- Advection
- Vertical Motion
- Vertical Motion of a Layer
- Vertical Displacement of a Dry Stable Layer
- Vertical Displacement of a Moist Stable Layer
- Vertical Displacement of a Layer: Saturated Base / Dry Top
- Vertical Displacement of a Layer: Dry Base / Saturated Top
- Question
- Divergence in the Lower Atmosphere
- Convergence in the Lower Atmosphere
- Vertical Motion vs Divergence/Convergence
- Subsidence Inversions
- Penetrative Convection
- Moistening
- Other Instabilities
Introduction

It has been said that without vertical motions in the atmosphere, there is no weather. Atmospheric stability provides a crucial control on the ability of air to rise or sink and on the resulting weather. For that reason, an assessment of stability is a critical part of the forecast process.
This section provides background on atmospheric stability definitions and concepts, as well as atmospheric processes that act to change stability. The discussion is framed within the context of the skew-T and builds upon what we have learned in the previous sections.
Determining Stability
Determining Stability The Parcel Method
Determining Stability » The Parcel Method Introduction

Nearly all techniques routinely used to analyze the stability of the atmosphere employ the "parcel" method. In this method, stability is determined by lifting or lowering a hypothetical air parcel and comparing its resulting characteristics to those of the surrounding environment. Here the surrounding environment and associated environmental lapse rate are defined by measurements from a sounding device (e.g., rawinsonde), which are then plotted on a thermodynamic diagram.
Using the parcel method, we assume ascending or descending parcels of air experience temperature and moisture changes associated with two primary processes:
- Pressure change
- The release or uptake of latent heat due to condensation or evaporation
We further assume a parcel does not interact with its surroundings. Therefore, the temperature of a parcel changes adiabatically as it is displaced a small distance vertically from its original position. As a consequence, an unsaturated parcel's virtual temperature changes at the dry adiabatic rate. For saturated conditions, an ascending parcel experiences latent heat of condensation and will cool at the saturated adiabatic rate, while a descending parcel warms and immediately becomes unsaturated, warming at the dry adiabatic lapse rate.
The parcel method defines three basic states of atmospheric stability: stable, neutral, and unstable. We will examine them next.
Determining Stability » The Parcel Method Stable
When a lifted parcel has a lower virtual temperature than the surrounding environment, the parcel will be denser than the environment. In this case, the displaced parcel tends to return to its original position and the environment is said to be stable. Under the same conditions, a sinking parcel will have a higher virtual temperature then its surroundings, which will cause it to rise toward its original position.
It is worth noting that the normal condition for the atmosphere is that it is stable, except where certain processes, such as strong surface heating, upward motion, etc. render an unstable condition. Recognizing the impact of such processes on the stability is a critical forecast problem, and is addressed later in the Changing Stability section.
Determining Stability » The Parcel Method Neutral Stability
When a lifted parcel has the same virtual temperature as the surrounding environment, its density will be the same as the surrounding environment. In this case, the displaced parcel tends to remain at its new level and the environmental stability is neutral.
Determining Stability » The Parcel Method Unstable
If a lifted parcel has a higher virtual temperature than the surrounding environment, its density will be less than the environment. In this case, the displaced parcel will accelerate upward away from its original level, and the environment is defined as unstable. Under the same conditions, a sinking parcel will have a lower virtual temperature than its surroundings, and it will accelerate downward away from its original position.
Determining Stability » The Parcel Method Parcel Displacements
A key assumption of the parcel method is that displacements are small, confined to levels adjacent to the parcel level. However, to accurately determine the stability of the real troposphere, the entire profile from ground to tropopause needs to be examined.
The animation shown here provides an example. There is a weak, low-level inversion, and a parcel in that low-level layer is stable. However, a deep unstable layer caps the low-level inversion. Thus, in this case, a parcel lifted to a point above the inversion will become unstable and will accelerate upward.
Determining Stability » The Parcel Method Questions
1. If a dry air parcel is displaced upward in a stable atmosphere, what happens to the parcel when it is released?
2. If a dry air parcel is displaced upward in an unstable atmosphere, what is the virtual temperature virtual temperature of the parcel relative to the ambient virtual temperature?
3. If a dry air parcel is displaced upward in a dry, neutral atmosphere, what is the potential temperature (theta)potential temperature (theta) of the parcel relative to the ambient potential temperature?
Determining Stability Stability Types
Determining Stability » Stability Types Stability Types, Lapse Rates, and Virtual Temperature (Tv)
The skew-T, as with other thermodynamic diagrams, provides a useful tool to apply the parcel method and assess atmospheric stability. With it, you can compare the lapse rate of the virtual temperature (Tv) curve for a given layer with the lapse rate of the corresponding dry adiabat (in the unsaturated parcel case) or saturation adiabat (in the saturated case).
The skew-T, as with other thermodynamic diagrams, provides a useful tool to apply the parcel method and assess atmospheric stability. With it, you can compare the lapse rate of the virtual temperature (Tv) curve for a given layer with the lapse rate of the corresponding dry adiabat (in the unsaturated parcel case) or saturation adiabat (in the saturated case).
In present forecasting practice, the ambient temperature (T) curve is used for these comparisons instead of the more exact Tv curve to allow faster evaluation of stability or instability. It should be noted, however, in very warm, moist environments the substitution of T for Tv can be a source of significant error in evaluating stability.
Determining Stability » Stability Types Stable

On this plot, the red line (line T-T’) depicts a simple, stable observed temperature profile from the surface (near 960 hPa) to 400 hPa. If a parcel of air at the surface, with temperature T and dewpoint Td, is lifted, it will cool following the dry adiabat until it reaches saturation at Point T1. If the parcel is lifted further, it will then cool following the saturation adiabat to Point T2. At every point in its ascent, the parcel is colder than the surrounding air (line T-T’). Thus, the air along the path T-T1-T2 will always be cooler and denser than the surrounding air—meaning the parcel will always tend to return to its equilibrium state. This air is said to be stable.
Note a parcel lifted to T2 would not subsequently sink back down to its original position. The parcel became saturated in the lifting process and cooled moist adiabatically (along the saturation adiabat) above the LCL. When the stable parcel at T2 descends, it warms dry adiabatically to a point where its temperature intersects the observed temperature curve and the parcel will stop descending. This occurs at about the 730 hPa level.
Determining Stability » Stability Types Absolutely Unstable

On this plot, the red line (line T-T’) depicts an observed temperature profile from the surface (near 960 hPa) to 850 hPa. If a parcel of air at the surface, with temperature T and dewpoint Td, is lifted, it will cool following the dry adiabat until it reaches saturation at Point T1. At every point in its ascent, the parcel is warmer than the surrounding air (line T-T’). As a result, the air along path T-T1 will be warmer and less dense than the surrounding air. Consequently, it will continue to rise of its own accord. This air is said to be "absolutely" unstable. The sounding lapse rate from T-T’ exceeds dry adiabatic and is therefore commonly referred to as a superadiabatic lapse rate.
Note that absolute instability is a relatively rare condition and usually confined to a shallow layer near the surface under conditions of strong surface heating (for example, solar heating or very cold air over warm water).
Determining Stability » Stability Types Conditionally Unstable - 1

On this plot, the red line (line T-T’) depicts an observed temperature profile from the surface (near 960 hPa) to 400 hPa. If a parcel of air at the surface with temperature T and dewpoint Td is lifted, it will cool following the dry adiabat until it reaches saturation at Point T1. If the parcel is lifted further, it will then cool following the saturation adiabat to Point T3. During its ascent and prior to reaching Point T2 (at about 770 hPa), the parcel is colder and denser than its surroundings, and, thus, below 770 hPa (along the section of the line T-T1-T2) the atmosphere is stable. However after passing Point T2, the parcel is always warmer and less dense than its surroundings and is therefore unstable. We refer to this point (T2) as the level of free convection (LFC)level of free convection (LFC). The air is said to be "conditionally" unstable. "Conditional" means the lifted parcel is stable if unsaturated and unstable (above the LFC) if saturated. In the case of downward displacement, the temperature of both initially saturated and unsaturated parcels would warm following the dry adiabat.
Determining Stability » Stability Types Conditionally Unstable - 2

Conditional instability is commonly used in two ways: the lapse-rate definition (just described) and the available-energy definition, examining the positive buoyant energy of a parcel (to be described here).
Schultz et al. (2000) described a state of conditional instability as a statement of uncertainty with regard to stability. Not all conditionally unstable atmospheres lead to unsettled weather. Because moisture is not accounted for in assessing conditional instability, some measure of the moisture profile is needed to refine the classification of stability. Thus, the concept of available energy was introduced by Normand (1938), who further subdivided conditional instability into additional classifications based on what we now term convective inhibition (CIN) and convective available potential energy (CAPE)convective inhibition (CIN) and convective available potential energy (CAPE). When a conditionally unstable atmosphere is unsaturated, CAPE must be evaluated to determine the degree of instability.
As the sounding above shows, if the conditionally unstable layer is surmounted by a stable layer such that there is no LCL and the CAPE is zero, no vertical displacement of parcels, however large, will produce any positive buoyancy.
Determining Stability » Stability Types Neutral Equilibrium

If the temperature curve of a sounding is parallel to a saturation adiabat, then a saturated parcel displaced upward would be neither aided nor hindered by the surrounding atmosphere and would tend to remain at the level to which it is displaced. In this case, the upward displaced saturated parcel is in neutral equilibrium with the environment.
Similarly, if the T curve of the sounding parallels a dry adiabat, an unsaturated parcel displaced upward would also tend to remain at the level to which it is displaced. Therefore, the upward-displaced unsaturated parcel is in neutral equilibrium with the environment.
Note the adiabatic assumption inherent in the parcel method requires downward displacement of all parcels to be dry adiabatic, whether the parcel is saturated or unsaturated. Therefore, all downward-displaced parcels will be in neutral equilibrium if the sounding exhibits a dry adiabatic lapse rate.
Determining Stability Lapse Rates
Determining Stability » Lapse Rates Determining Stability from Lapse Rates
In addition to tracing the path of air parcels, the stability can also be determined by comparing the environmental lapse rate to the dry and saturated adiabatic lapse rates.
Absolute stability occurs when the environmental lapse rate is less than the moist (saturation) adiabatic lapse rate.
Absolute instability occurs when the environmental lapse rate is greater than the dry adiabatic lapse rate. We refer to such lapse rates as superadiabatic.
Neutral equilibrium occurs when the environmental lapse rate equals the dry adiabatic lapse rate in an unsaturated environment or when the environmental lapse rate equals the saturated adiabatic lapse rate in a saturated environment.
If the environmental lapse rate falls between the dry and moist (saturation) adiabatic lapse rates, then the environment is conditionally unstable. To assess the stability for this case, we need to know something about the moisture content of the parcel:
- If the parcel is saturated, it will exhibit moist absolute instability.
- If the parcel is unsaturated, then the stability will depend on its CAPE:
- If CAPE = 0, then the parcel is absolutely stable.
- If CAPE > 0, then the parcel becomes unstable when lifted beyond the level of free convection (LFC).
Determining Stability » Lapse Rates Superadiabatic Lapse Rates
Superadiabatic lapse rates rarely persist for any length of time in the environment. Any vertical perturbation will spur the release of the instability. Genuine superadiabatic lapse rates frequently occur within the first 300 meters of the surface (due to surface heating). However, when absolutely unstable lapse rates are reported in the troposphere above the boundary layer, they are often rejected as erroneous data. Such lapse rates are typically ascribed to measurement defects or spurious effects. However, there is evidence to indicate that valid cases of superadiabatic lapse rates can occur through a fairly shallow layer sandwiched between stable layers in the troposphere, well above the surface layer.
Two processes can explain the occurrence of such real superadiabatic lapse rates:
- Significant destabilization due to rapid lifting at a saturated/dry-air interface within the lifted layer
- High rate of evaporation at the top of a cloud layer
The simultaneous occurrence of both processes is also a possibility.
Thus, when a superadiabatic lapse rate is included in a RAOB report, you should not disregard it as bad data. Pay close attention to the synoptic conditions occurring in the area of the RAOB ascent. When rapid lifting of tropospheric layers is likely, due to either the rapid movement of a cold front or orographic lifting, there is a significant probability of finding a shallow layer with a superadiabatic lapse rate. Altocumulus castellanus reports on the surface chart are often noted in the area of such activity.
Determining Stability » Lapse Rates Autoconvective Lapse Rates

There are superadiabatic lapse rates where the temperature decrease with altitude is so great that the density of the air is either constant or increases with altitude. The density of air is constant with height when the lapse rate equals 34.2°C per km. This is known as the autoconvective lapse rate or autoconvection gradient. When this lapse rate is exceeded, the denser air above will spontaneously sink in the less dense air below. This spontaneous overturning of the atmosphere is called autoconvection because the self-convection of air starts without any external impulse. Autoconvection usually occurs with instances of strong surface heating and is confined to a few meters above the earth's surface.
Determining Stability » Lapse Rates Isothermal Lapse Rates

Isothermal lapse rates occur when the temperature does not change with height. Because any vertically displaced air parcel will cool or warm according to the appropriate adiabatic lapse rate, isothermal lapse rates are a special case of a stable lapse rate.
Determining Stability » Lapse Rates Inversion
An inversion occurs when the temperature increases with increasing height. Because any air parcel that is displaced upward will cool according to the appropriate adiabatic lapse rate, inversions are a special case of a stable lapse rate.
Inversions form in response to several processes including (but not confined to) the following:
- Radiative surface cooling
- Subsidence aloft
- Frontal passage
Each of these processes is discussed in more detail in the appropriate part of the Changing Stability section.
Determining Stability » Lapse Rates Question

This this skew-T diagram shows several hypothetical lapse rates for a location at 1000 hPa and 0°C. For each lapse rate (labeled A-H), choose the term that most specifically describes the lapse rate.
Determining Stability Potential Instability
Determining Stability » Potential Instability Introduction

The stability criteria for small parcel displacements, while generally applicable and widely used, are not indicative of what might happen when layers or parcels are given larger vertical displacements. Such displacements would cause whole layers to change their type of stability over a broad area, or would cause parcels to cool adiabatically to saturation and to perhaps penetrate deeply into layers having different stability.
As a result, a number of procedures have been proposed to apply the parcel theory to this problem of large vertical displacements. One such approach to the lifting problem involves the concept of potential instability, in which the effect of bodily lifting any layer is considered.
Determining Stability » Potential Instability Determination of Potential Instability
In the ear1y 1930s, Rossby introduced a criterion for the instability or stability of a layer resulting when it is lifted as a whole, as at a front or mountain. This criterion came to be known as "potential instability," also known as "convective instability." To determine potential instability, one compares the lapse rate of the wet-bulb temperature to the moist (saturation) adiabatic lapse rate:
- Layers in which the wet-bulb lapse rate is greater than moist (saturation) adiabatic are potentially unstable.
- Layers in which the wet-bulb lapse rate is less than moist (saturation) adiabatic are potentially stable.
Determining Stability » Potential Instability Question

In this figure, and those that follow, the blue line between the temperature and dewpoint profiles represents the wet-bulb temperature profile.
Examine this skew-T diagram to answer the following.
How would you characterize the temperature lapse rate from the surface to 500 hPa?
Determining Stability » Potential Instability Lifted Example

To apply this concept of potential instability to vertical displacement of the layer, let's examine the same initially stable layer as the previous example. First lift the layer by 100 hPa, and then examine the resulting lapse rate. The lifted layer becomes conditionally unstable because the lift causes the bottom of the layer, where there is greater moisture content, to saturate and cool at the moist (saturation) adiabatic lapse rate while the drier top of the layer cools at the dry adiabatic rate—the result: the layer's lapse rate steepens. Thus, the unlifted layer is potentially unstable.
Determining Stability » Potential Instability Effects on Weather
The idea of potential instability and stability appears to be very simple, and the criteria for them are certainly very easy to use. But the relation of these states to the resulting weather is very complex and difficult to predict. The classic example of the release of potential instability in severe convection is when an air mass characterized by a dry layer overlying a surface moist layer is lifted. This is similar to the example we just examined.

There are, however, many other layers with less-marked potential instability, and the assessment of the type of weather that lifting will produce remains a challenge. For example, the sounding above shows a shallow potentially-unstable layer with deep layers of potentially-stable air both above and below. When lifted to saturation, this layer may produce any of the following:
- A solid stratiform deck of cloud
- Scattered shallow cumulus clouds
- Mixed cumulus and altostratus clouds
- Deep cumulus penetrating into the higher, stable layers with or without precipitation
Determining Stability » Potential Instability Factors that Influence Weather Effects
Factors that Influence Weather Effects
The kind of weather that actually occurs depends on a variety of factors, such as the following:
- The amount of lifting beyond that which is just sufficient to start condensation
- The steepness of the wet-bulb temperature lapse rate
- The degree of stability of the adjacent layers
- The speed and spatial uniformity of the lifting
For many practical purposes there may be little difference in the clouds and weather that result from lifting a shallow potentially stable layer as compared to those from lifting a shallow potentially unstable layer. Therefore, there may be instances of potential instability in which the "instability" aspect is of trivial significance.
Determining Stability » Potential Instability Superadiabatic Lapse Rates from Layer Lifting

In evaluating the effect of lifting potentially unstable layers, one sometimes finds cases where a superadiabatic lapse rate appears to result. This may occur when the moisture content decreases sharply with height, as in "dry-type inversions." These conditions are frequently associated with subsidence and trade-wind inversions.
On this sounding, click the layer that you think is most likely to become superadiabatic with 100 hPa of lifting, as might occur with the passage of a fast-moving cold front.
If you lift the layer 100 hPa, the saturated bottom cools at the moist (saturation) adiabatic lapse rate, and the unsaturated top cools at the dry adiabatic rate, resulting in a superadiabatic lapse rate in the 700-650 hPa layer.
Determining Stability » Potential Instability How Much Lift is Needed for Release of Instability?
Determining Stability » Potential Instability Processes That Change the Potential Instability

In general, those processes that increase the moisture content of the lower levels and/or decrease the moisture content at higher levels tend to create or increase potential instability.
This graphic shows an example over the southern U.S. with advection of warm, moist air from the Gulf of Mexico at low levels and advection of drier air to the lee of the Rocky Mountains in southwesterly flow aloft. These conditions substantially increase potential instability leading to initiation of severe thunderstorms over the Southern Plains, especially in the spring season.
It is important to note that all the previously discussed effects that change the lapse rate can indirectly change the vertical distribution of potential stability and instability at a given location.
Determining Stability Potential Errors
Determining Stability » Potential Errors Potential Errors Caused by the Effect of Virtual Temperature (Tv)

The use of temperature in place of virtual temperature virtual temperature for stability determinations may occasionally lead to a misrepresentation of the stability. The discrepancy will be greatest where a layer of high moisture content is adjacent to a dry one. In these cases, use of the temperature curve will affect stability determinations as follows:
- If the dewpoint decreases rapidly with height, using the T curve will indicate too much stability.
- If the dewpoint increases rapidly with height, the T curve will indicate too little stability.
Frequently, the dewpoint decreases rapidly with height across a warm front. The sounding above shows one such example. Through the 800-850 hPa layer, the dewpoint decreases rapidly with height. Consequently, the lapse rate indicated by the temperature curve is approximately moist (saturation) adiabatic, and the lapse rate indicated by the virtual temperature curve is nearly dry adiabatic. Thus, the stability indicated by the temperature curve is greater than the stability indicated by the virtual temperature curve.
In most instances, stability determination using the temperature curve will provide a reliable assessment of stability. However, it is worthwhile to be aware of the limitations of this approximation.
Determining Stability » Potential Errors Non-Adiabatic Warming and Cooling

We use the parcel method to assess stability because the results correlate nicely with the observed weather. However, because the parcel method relies solely upon adiabatic processes, it fails to account for many processes that affect stability. For example, if we look at a convective environment, we can find several processes that result in the transfer of heat and moisture between the rising updraft and the surrounding environment. These processes include the following:
- Horizontal mixing of the convective updraft or cumulus cloud with its environment. Mixing the saturated updraft with the drier surrounding air cools the updraft through evaporation and reduces its water content. This decreases the buoyancy of the updraft, particularly in the outer parts of the convective column.
- Vertical mixing, both within the updraft and between the updraft and the environment at the top. In cumulus clouds this leads to downdrafts, which redistribute condensed water and heat. Consequently, lapse rates may depart from moist (saturation) adiabatic.
- Latent cooling from melting of falling precipitation. For instance, the lapse rate in rising saturated air that is being cooled by melting of falling snow or hail can substantially exceed the moist (saturation) adiabatic lapse rate.
Determining Stability » Potential Errors Other Effects that Modify Vertical Motion

Several non-thermodynamic effects occur that retard or enhance the buoyancy-driven vertical motion of parcels, particularly in and around convective storms. The parcel method fails to account for these processes. Among those processes are the following:
- Friction and drag between the rising thermal or cloud and the surrounding winds. These processes have their greatest effect when there is strong vertical wind shear in the environment.
- A reduction in buoyancy due to the weight of condensed water. This effect retards updrafts and enhances downdrafts.
- Drag from falling precipitation. This is another process that retards updrafts and enhances downdrafts.
- Compensatory subsidence near a convective updraft. Some of the surrounding air is drawn downward to replace the rising and expanding updraft.
Determining Stability » Potential Errors The Net Effect of Non-Adiabatic Processes
Since the parcel method does not take into account the effects just discussed, we must use it with some degree of caution. Generally, the parcel assumption of continuous adiabatic ascent within clouds results in much higher values for updraft speed, water content, and temperature than we would typically observe. Nevertheless, empirical forecast techniques that use sounding analyses were derived by correlating actual weather events with parcel-method-derived parameters. For example, parameters such as CAPE and stability indices such as lifted index, both of which are derived using parcel methods, have empirically derived thresholds for assessing the strength and severity of convection. Therefore, these techniques indirectly incorporate these diabatic effects.
Determining Stability Exercise

In this sounding, the Tv curve is shown as the dashed line from 1000 to 800 hPa. Above 800 hPa, Tv nearly equals T.
Determine the stability for each of the following layers using both temperature and virtual temperature:
Changing Stability
Changing Stability Introduction

Several processes in the atmosphere act to modify lapse rates. As a forecaster, you will need an understanding of these processes to obtain the best analysis of stability through a forecast period. Five basic kinds of physical processes can change the stability at a point or in a given local vertical layer.
- Diabatic heating and cooling
- Advection
- Vertical motion of a layer
- Penetrative convection
- Moistening
In the following sections we will examine each of these processes individually.
In practice several of these processes usually operate simultaneously at the same location, and it may be difficult or impossible to evaluate their effects separately. Therefore, you will need to consider all of these processes when analyzing a sounding.
Changing Stability Heating and Cooling
Changing Stability » Heating and Cooling Diabatic Heating and Cooling

Diabatic heating and cooling effects are generally important only near the ground surface and within some clouds. Radiative heating and cooling lead to the formation of daytime low-level instability and nocturnal low-level stability, respectively. Radiative processes in the free air and at cloud tops, however, are slow and their effect on the lapse rate are generally minimal. The release of the latent heat of condensation has important local effects, potentially leading to deep convection. In addition, evaporation and melting effects have a significant impact on lapse rates locally in instances of heavy precipitation.
Changing Stability » Heating and Cooling Instability from 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.
Changing Stability » Heating and Cooling Stability from Surface Cooling
Pure nocturnal radiative cooling in calm air results in a shallow surface-based inversion. The depth of the inversion increases with greater duration of the cooling, while the strength of the inversion increases with the degree of cooling.
Wind complicates the effects of cooling of the ground on the low-level lapse rate. This is true for both radiative cooling and passage of air over colder ground. Turbulent mixing of the air cooled at the ground with warmer air above tends to establish a surface layer with an adiabatic lapse rate capped by a turbulence inversion. Intermediate conditions and combinations between the ground inversion and the turbulence inversion can occur, depending on the relative degrees of wind and cooling.
Evaporative cooling of precipitation can produce significant cooling in an unsaturated boundary layer, resulting in a more stable environment.
Changing Stability » Heating and Cooling Surface Cooling and Fog Formation
If sufficient surface cooling takes place, saturation results, leading to fog or stratus formation. However, unlike the case of condensation in cumulus clouds, the latent heat released in fog or stratus is usually too small to greatly increase the mixing depth. On the other hand, the formation of fog or stratus greatly reduces or stops the further radiative cooling of the ground. However, radiative cooling at the top of the fog or stratus tends to maintain the inversion near the top of fog or cloud.
A detailed explanation of fog processes is beyond the scope of this skew-T module. More details regarding fog formation and sounding analyses may be found in the following COMET modules:
Radiation Fog
Forecasting Radiation Fog
Dynamically Forced Fog
Changing Stability » Heating and Cooling Question
Do the following processes make conditions more stable or less stable?
Nocturnal radiative cooling at the earth's surface:
Cold air flowing over a warm lake surface:
Warm air flowing over a snow surface:
Solar heating of the earth's surface:
Changing Stability Advection
Changing Stability » Advection Introduction
Advection, both at the surface and aloft, has a strong influence on the lapse rate through a given region of the atmosphere. Before making a forecast from a sounding, consider the lapse rate changes that will result from the effects of advection during the forecast period.
The advection effects may be visualized as two processes:
- Uniform advection of an air mass with a different lapse rate
- Differential advection of temperature due to vertical wind shear
Changing Stability » Advection Uniform Advection
In the case of uniform advection, an air mass with a different lapse rate may move into the forecast area. The most obvious example of this phenomenon is frontal passage. It's generally safe to assume that the imported air mass will have the typical weather associated with the imported lapse rate.
This sequence of soundings comes from a location along the California coast and spans the passage of a landfalling cold front. Looking at the lower levels over the period, we see a stable, moist, prefrontal air mass replaced by a less stable, drier, postfrontal airmass.
Changing Stability » Advection Question

This image shows a map of the Coterminous U.S. (CONUS) with a hypothetical cyclone and associated warm and cold fronts. Idealized soundings (1000-700 hPa) are shown for air masses in relation to the fronts. Use the graphic to answer the following questions:
With passage of the cold front, will the low-level atmosphere become more or less stable?
With passage of the warm front, will the low-level atmosphere become more or less stable?
Changing Stability » Advection Differential Advection
Differential advection is less obvious than uniform advection and also more difficult to evaluate on synoptic charts. Differential geostrophic temperature advection is simply advection of different stability into the sounding from an upwind (geostrophic) source, but ageostrophic advections can be very important, as well. Since observed or model-derived wind trajectories contain the ageostrophic wind component, we may use them to estimate temperature advection, rather than geostrophic winds computed from height fields. By examining the temperature advection at multiple levels, we can assess the effects of differential advection on the local lapse rate.
Changing Stability » Advection Question

In practice, many forecasters estimate the total advective change of stability on a given sounding by estimating the temperature advection at several levels in the troposphere using the actual winds and isotherms from upper air analyses. This procedure takes into account the combined lapse rate advection and shear effects, if both are present.
These plots show streamlines and isotherms at 850 and 500 hPa. Based strictly on advection, will the stability increase or decrease in the 850-500 hPa layer at St. Cloud, MN (circled in red)?
Changing Stability » Advection Frontal Inversions

Fronts mark a boundary between two air masses, which sometimes may be seen in soundings, most often as a frontal inversion.
Observational studies of cyclones and frontal zones have determined that there are two primary types of cold frontal zones: katafronts and anafronts. With a katafront the component of flow perpendicular to the frontal zone is downward toward the surface of the earth, while with an anafront air flows up the face of the frontal surface. A cold anafront is akin to a warm front in reverse and reveals a similar sounding pattern.
This plot shows two idealized soundings for an anafront and a katafront. The anafront sounding, on the left, shows a weak frontal inversion and a small dewpoint depression, indicating a deep layer of high relative humidity. The katafront sounding, on the right, shows a rather strong inversion aloft, which has been enhanced by subsidence along the frontal zone. This subsidence also creates a substantial layer of dry air, shown by the large dewpoint depression above the inversion.
Typically, passage of an anafront is marked by a sharp drop in temperature at the surface, but not humidity, while passage of a katafront is marked by a sharp drop in humidity, but not temperature.
Changing Stability Vertical Motion
Changing Stability » Vertical Motion Vertical Motion of a Layer

The vertical motions that affect the lapse rate may be of any scale or type, from small-scale turbulence to the large-scale mean vertical-motion field associated with synoptic features. As different principles apply to the vertical motion of different scales or types, we customarily consider the vertical motions of different scales as somewhat distinct phenomena. Accordingly, we will discuss separately ascent and subsidence of whole layers and penetrative convection. The lifting effects of mountains and fronts involve combinations of these phenomena.
Changing Stability » Vertical Motion Vertical Displacement of a Dry Stable Layer
Consider the case of a thin layer within some air mass. The air in the layer is dry (unsaturated) and has a uniform lapse rate. We then lift that layer with no divergence or convergence, so that the horizontal area of the layer doesn't change. As we lift the layer, the pressure drops, so its density decreases, and its volume increases correspondingly. To accommodate the increased volume, the layer deepens. Now the question is, did the layer become more or less stable when we lifted it?
To answer the question, let's first look at the temperature at the bottom of the layer. As the base of the layer rose, its temperature dropped, following the dry adiabatic lapse rate. Meanwhile, at the top of the layer, the temperature also dropped by the dry adiabatic lapse rate. However, the top of the layer ascended more than the bottom of the layer because the layer depth increased as the layer ascended. Therefore, the temperature at the top decreased more than the temperature at the bottom. The lapse rate moved toward the dry adiabatic lapse rate. If we lift the layer more and more, its lapse rate will progressively approach dry adiabatic. Thus, in the absence of divergence or convergence, the stability of a dry, ascending layer decreases, while the stability of a descending layer increases.
Changing Stability » Vertical Motion Vertical Displacement of a Moist Stable Layer
Similar to the previous case, if a saturated stable layer ascends, it will become less stable with the lapse rate tending toward moist (saturation) adiabatic.
Changing Stability » Vertical Motion Vertical Displacement of a Layer: Saturated Base / Dry Top
If an ascending layer is saturated at the bottom and unsaturated at the top, the bottom and top cool at the moist (saturation) adiabatic and dry-adiabatic lapse rate, respectively, and the layer is destabilized.
Changing Stability » Vertical Motion Vertical Displacement of a Layer: Dry Base / Saturated Top
In contrast to the previous example, if an ascending layer is unsaturated at the bottom and saturated at the top, the layer becomes more stable.
For a more quantitative description of the effects of convergence, divergence, and vertical motion on stability, see the sidebar: The Continuity Equation.
Changing Stability » Vertical Motion Question

The lapse rate in the 900-800 hPa layer may be described as:
(Choose all that apply then click Done.)
The temperature is constant within the layer, so the lapse rate is isothermal. An isothermal lapse rate is also an absolutely stable lapse rate.
T′ = °C
Since conditions are saturated, the temperature follows a saturation adiabat from 900 to 800 hPa.
T" = °C
Since conditions are unsaturated, the temperature follows a dry adiabat from 800 to 700 hPa.
Is the layer more stable or less stable after being lifted?
Before lifting, the layer was isothermal and hence stable. After lifting, the temperature decreases 6°C over 100 hPa and the layer is conditionally unstable.
Changing Stability » Vertical Motion Divergence in the Lower Atmosphere
Now let's look at the effects of divergence and convergence on stability. In order to simplify the discussion, we will assume that the elevation of the layer remains constant while divergence or convergence occurs. We can make this assumption for the special case of a surface-based layer.
First, let's look at the case of divergence. With divergence, we horizontally stretch an air mass, so its area increases. In response, the surface-based layer thins. As the top of the layer descends, it warms at the dry adiabatic lapse rate. This change acts to stabilize the layer.
Changing Stability » Vertical Motion Convergence in the Lower Atmosphere
Conversely, convergence will act to destabilize a dry, surface-based layer.
Changing Stability » Vertical Motion Vertical Motion vs Divergence/Convergence
In the preceding pages, we demonstrated that ascent without divergence and convergence without vertical motion both act to destabilize a layer. Similarly, descent without convergence and divergence without vertical motion both act to stabilize a layer.
This fact has a practical significance in the lower atmosphere, where, because of the constraining boundary at the earth's surface, divergence must always accompany descent and convergence must always accompany ascent. Therefore, in this region it is customary to regard the effects on the lapse rate of divergence-combined-with-descent (or of convergence-combined-with-ascent) to be of the same kind and essentially inseparable.
Changing Stability » Vertical Motion Subsidence Inversions
A subsidence inversion is produced by the adiabatic warming of air as it sinks. If the initial, pre-subsidence lapse rate is substantially less than dry adiabatic, an inversion quickly forms at the base of the subsident layer. Within the inversion, temperatures rise and dewpoints fall with increasing height. Above the inversion, the lapse rate approaches dry adiabatic.
Subsidence inversions are usually associated with anticyclones and/or stable air masses.
Changing Stability Penetrative Convection
Changing Stability » Penetrative Convection Introduction
Unlike the vertical motion of a layer in the atmosphere, penetrative convection consists of local vertical currents having cross-sections of the order of a few meters (feet) to a few kilometers (miles) across. Most of these vertical motions are due to thermal convection in an unstable layer, which leads to neighboring updrafts and downdrafts within the layer. These updrafts and downdrafts develop momentum that carries them beyond the unstable layer so that they penetrate into the adjacent stable layer. The effect is called penetrative convection and tends to destabilize the adjacent stable layer.
To see how penetrative convection destabilizes a stable layer, let's examine a fairly typical case of a saturated convective updraft rising through a conditionally unstable layer. At some height, a stable layer will cap the conditionally unstable layer. The rising updraft is warmer than the surrounding environment and it will remain warmer until it reaches the equilibrium level. The equilibrium level sits within the stable layer at some elevation above the unstable/stable boundary. Momentum carries the updraft past the equilibrium level. Now the updraft is cooler than the ambient air, so it slows and eventually falls back to the equilibrium level. So we can see that the rising convective updraft has penetrated a significant distance into the overlying stable layer. As it does, it warms the base of the stable layer through mixing, conduction, and radiation. This warming increases the lapse rate through the stable layer, effectively destabilizing it.
Changing Stability » Penetrative Convection Factors that Affect Penetrative Convection
Factors that Affect Penetrative Convection
The speed with which the penetrative convection changes the lapse rate varies greatly. Some of the factors that affect the rate include:
- The duration of the convection
- The resistance (stability) of the layers affected
- The size spectrum and pattern of convective cells
The rate of change is also modified by mixing between the updrafts and their environment, as well as by any compensating subsidence. This subsidence may be spread over a much larger area than that affected by the updrafts. Widespread, continuous penetrative convection can render whole layers completely unstable. This frequently happens in layers near strongly heated ground or in layers lifted by a front.
Changing Stability Moistening
Changing Stability » Moistening Introduction
It is fairly simple to understand how warming the surface temperature of a sounding renders the atmosphere more unstable. However, increasing the surface dewpoint can also increase the instability. In this animation, increasing the surface dewpoint lowers the LCL so parcels do not have to be lifted as far to become unstable.
Changing Stability » Moistening Question 1
Changing Stability » Moistening Question 2
Other Instabilities
Other Instabilities MAULs
Other Instabilities » MAULs Moist Absolutely Unstable Layers (MAULs)
Bryan and Fritsch (2000) posed that saturated layers may exist in the lower to middle troposphere that have lapse rates that exceed the moist-adiabatic lapse rate and are therefore moist absolutely unstable layers (MAULs). A MAUL may be better understood by considering the evolution of the sounding in this animation. The initially unsaturated 900-600 hPa layer is lifted 100 hPa and becomes saturated. The resulting lapse rate of the saturated 800-500 hPa layer is steeper than the moist-adiabatic lapse rate, hence the formation of a MAUL.
Other Instabilities » MAULs Characteristics
Many MAULs can exceed 100 hPa in depth, extending hundreds of kilometers horizontally. While convective overturning may quickly neutralize many of these MAULs, there may be instances where processes creating the unstable state may act to maintain the layer instability for periods longer than 30 minutes. Such persistent MAULs may occur in the vicinity of the inflow region of some mesoscale convective systems and may help to explain some of the dynamic features observed with these systems.
Other Instabilities CSI
Other Instabilities » CSI Conditional Symmetric Instability (CSI)

Thus far, the discussion of stability has focused on vertical displacement of the air parcel. Bennetts and Hoskins (1979) proposed the intriguing concept of conditional symmetric instability (CSI) to explain instability brought about by slantwise displacements of parcels in regions otherwise stable to vertical (convective stability) and horizontal (inertial stability) displacements. The release of CSI results in a slantwise convection and precipitation bands oriented parallel to the geostrophic shear or thermal wind vector. Conditions favoring CSI are often found in the overrunning zones of extratropical cyclones.
Other Instabilities » CSI Favorable Conditions

Simple 2-dimensional CSI theory assumes geostrophic flow with unidirectional shear and variations occurring only in the direction normal to the geostrophic shear or thermal wind vector. One can qualitatively use an observed or model-derived sounding, in combination with surface and upper air analyses, to infer environments potentially favorable for CSI, given the following:
- Conditions at or near saturation
- Strong, preferably unidirectional, vertical wind shear
- Weak conditional stability, usually a sounding slope close to moist adiabatic
- An environment favorable for lift
- Weak inertial stability, generally on the anticyclonic shear side of an upper-level jet
If the above conditions are met, the environment may be favorable for formation of precipitation bands oriented parallel to the thermal wind or thickness lines. In this case, you may wish to further evaluate the potential for CSI by analyzing cross sections.
Other Instabilities » CSI Caveats / Other Resources

While CSI analyses based on observations or model output cannot be used to diagnose or forecast the precise location of precipitation bands, they can be used to infer the character of the precipitation (e.g., broad and uniform vs. organized bands). It is important to note that when assessing CSI, one must first check for convective or potential instability. If potential instability exists, upright convection will dominate and the CSI analysis is moot.
A detailed explanation of CSI and its analysis is beyond the scope of this skew-T module; however, more details regarding CSI and cross-section analysis techniques may be found in the following COMET modules:
Slantwise Convection
The Use and Misuse of Conditional Symmetric Instability
Mesoscale Banded Precipitation