>> Atmospheric Stability <<

 



This lecture includes an overview of another vital concept in meteorology - atmospheric stability.  Below you will find a basic outline and some illustrations associated with this topic.

 
 

 >> Lecture Outline <<

 


Stability Terminology

>> Hydrostatic Equilibrium: Vertical PGF and Gravity in balance
>> Lapse Rate: Rate of decrease of temperature with elevation

Figure 1: The image above shows the dry and moist adiabatic lapse rates and their relative (not to scale) orientation with respect to height.


In the atmosphere, rising or sinking air will only cool or warm at one of two rates - the dry adiabatic lapse rate or the moist adiabatic lapse rate.  The first, the dry adiabatic lapse rate, is the rate an unsaturated parcel of air warms or cools when moving vertically through the atmosphere.  The dry adiabatic lapse rate is approximately a 5.5 degree Fahrenheit change in temperature for every 1000 feet of vertical movement.

The moist adiabatic lapse rate, on the other hand, is the rate at which a saturated parcel of air warms or cools when it moves vertically.  This lapse rate is approximately 3.3 degrees Fahrenheit for every 1000 feet of vertical movement.

Rising air cools, sinking air warms- no exceptions!

The actual stability of an air parcel is determined by the orientation of the environmental lapse rate in comparison with either the dry or moist adiabatic lapse rates.  The environmental lapse rate is simply what it says- the rate of change of the temperature of the environment (atmosphere) with changing altitude.  It is important to relaize that because the atmosphere (environment), on average, is not rising or sinking, the environmental lapse rate can look much different than the dry or moist adiabatic lapse rates.  In fact, it is those differences that allow us to determine whether a particular part of the atmosphere is stable or unstable.  The dry and moist adiabatic lapse rates can be seen in Figure 1 to the right.

>> Unstable Equilibrium or "Unstable": Parcel accelerates away from initial position

Figure 2: This image illustrates the concept of unstable equilibrium.  In this case, the environmental lapse rate is greater than both the dry and moist adiabatic lapse rates.

 

 

 

 


The atmosphere is considered to be unstable if a rising parcel cools more slowly than the environmental lapse rate.  This causes the air parcel to remain warmer and less dense than its surroundings and, therefore, continue to accelerate upward.  The orientation of an unstable environmental lapse rate can be seen to the left in Figure 2.

 

 

 



>> Stable Equilibrium or "Stable": Parcel resists vertical motion

Figure 3: This image illustrates the concept of stable equilibrium.  In this case, the environmental lapse rate is less than both the dry and moist adiabatic lapse rates.

 

 

 

 


The atmosphere is considered to be stable if a rising parcel cools faster than the environmental lapse rate.  This causes the air parcel to be cooler and more dense than its surroundings and, therefore, lose its buoyancy.  Vertical motions tend to be restricted when the atmosphere is in stable equilibrium.  The orientation of a stable environmental lapse rate can be seen to the right in Figure 3.

 

 

 

 

>> Temperature Inversions

Figure 4: The figure above shows a typical temperature inversion.  An inversion occurs when temperature increases with height, a situation "inverted" from the general temperature decrease with altitude in the troposphere.

 

 

 


A temperature inversion occurs when the temperature increases with height.  The environmental profile associated with a temperature inversion is the most stable type of environment.  A temperature profile featuring an inversion can be found to the left in Figure 4.  The inversion is at the top, where temperature increases with height.

 

 

 

Stability and Vertical Mixing

>> Unstable Environment: Promotes mixing through the rising and sinking of bubbles of air

Figure 5: This figure illustrates the vertical mixing process that occurs in an unstable atmosphere.


When the environment is unstable, air mixes readily in the vertical.  This vertical mixing can have a profound effect on various atmospheric phenomena as diverse as air quality, wind speed and cloud type.  Vertical mixing in an unstable environment helps bring cleaner air from above down to the surface, while transporting polluted air aloft.  Also, stronger winds from above (where there is little friction) can be transported (mixed) toward the ground when the atmosphere is unstable.  This is why it is often breezy on a sunny afternoon, and often quite calm in the morning, when vertical mixing is restricted.  An example of this vertical mixing can be seen in Figure 5 to the right.

 



If there is sufficient moisture in the atmosphere the water vapor in the rising bubbles of air will condense into clouds if the parcel rises high enough to cool to the dewpoint.  In an unstable environment these convective motions work to produce cumuliform clouds.

>> Stable Environment: Inhibits vertical mixing, Stratification

Figure 6: This image depicts a typical stable, or stratified, environment where no mixing occurs.

 

 

 


The atmospheric motions that occur in a stable environment are fundamentally different than those found in an unstable environment.  The most profound difference between these two types of environments is the inhibition of vertical mixing in the stable environment.  The lack of vertical mixing leads to a "stratified" atmosphere, where many atmospheric variables are separated into layers instead of being well-mixed.  The stratification of the atmosphere when stable leads to, for instance, pollution episodes and drastic changes in wind speed and direction over short vertical distance.  An example of a stratified and stable environment can be seen in Figure 6 to the left.

 

 

 

Another atmospheric consequence of a stable and stratified atmosphere involves the process of cloud formation.  Assuming that there is sufficient moisture present in the atmosphere, stratiform clouds can form in a stable environment.  This can only occur if the stable air is forced upward either through the convergence of air into a low pressure center or through the orographic lifting.  An example of each of these processes can be found below in Figure 7 and Figure 8.

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Figure 7: This figure shows how air converges into the center of a low pressure system and is forced to rise. Figure 8: The demonstration above shows how air can be forced to rise due to a change in topography.

>>Temperature Inversions: The "ultimate" in stability

As mentioned above, a temperature inversion is the most stable environmental profile possible.  This type of atmospheric temperature profile can occur pretty much anywhere throughout the atmosphere, aloft or near the ground- each having a different impact on the weather. 

At the surface, temperature inversions can form during both the daytime and the nighttime, depending on the conditions and location.  The more common type of inversion is the nocturnal temperature inversion that occurs due to the radiational cooling of the Earth's surface after the sun sets.  A temperature inversion can also form over cool bodies of water during the daytime in the summer.

Temperature inversions also occur aloft in the stratosphere (the layer directly above the troposphere).  Inversions in this layer of the atmosphere limit the vertical development of thunderstorms and their anvils (hence the name, stratosphere).

The last type of temperature inversion is called a subsidence inversion.  This type of temperature inversion is a result of the sinking and warming of air parcels.  In this case, however, the top of the air parcel descends and warms more than the bottom of the air parcel.  This type of temperature inversion is generally found to the east of high pressure systems well above the ground and well below the stratosphere.

Clouds

>> If formed in a stable environment, they are flat in appearance (stratiform)
>> If formed in an unstable environment, they are puffy with good vertical development (cumuliform)
>> The difference between water and ice clouds can be seen on the edges

Houze's Cloud Atlas

Cloudman's Mini Cloud Atlas

One can tell the difference between clouds composed of primarily water and clouds composed of ice by looking at the edges of the cloud.  If the cloud has distinct, sharp edges, it is most likely a water cloud.  If the cloud is more diffuse, it is generally an ice cloud.  The presense of ice or water in a cloud says nothing, however, about the stability of the atmosphere near the cloud.  For stability clues, look at the vertical extent of the cloud.  In order to obtain a better understanding of the different cloud types, explore one or more of the Cloud Atlases provided on this page.

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2004-2017 by Thomas P. Harrington