Tutorial 18:
Atmospheric Stability
Concepts:  Atmospheric stability determines whether or not air will rise and cause storms, sink and cause clear skies, or essentially do nothing.  Stability is dependent upon the Dry and Saturated Adiabatic Lapse Rates and the Environmental Lapse Rate.  We will eventually be able to compare a measured ELR with both the DALR and SALR and determine the atmospheric stability condition.  First, however, we must become familiar with basic forms of stability.  Other forms will be derived from these later.
 

Basic Forms of Stability: There are essentially three types of stability (we will introduce special conditions later).  These three are actually rather easy to understand and remember.  They are shown below.

In each instance we have a parcel in contact with a surface.  Remember the rule: "Air takes on the characteristics of the surface beneath it?"  In each case, then, a parcel is formed with the same temperature characteristic as the underlying surface.  To accurately determine the stability condition, you must follow this rule:

Always compare the parcel's temperature to that of the surrounding environment and not the other way around!!

A) If an air parcel is warmer than its surrounding environment, then it will be less dense than its surroundings and will rise like a hot air balloon.  This is Unstable Air and has the potential for creating storms.

B) If an air parcel is cooler than its surrounding environment, then it will be denser than its environment and will sink.  Think of this situation as a pebble sinking in water.  This is Stable Air which generally leads to clear skies.

C) If an air parcel is the same temperature as its surrounding environment, then the parcel will not move of its own accord.  This is Neutral Air.
 

Determining Stability Conditions Using DALR, SALR, and ELR:  We introduced lapse rates in Tutorial 13.  You may wish to go back and refresh your memory as to the values of the DALR and SALR as we will need them shortly.  We also showed how the adiabatic lapse rates plot on a diagram.  If we add the ELR to such diagrams, we may easily determine the atmospheric stability condition.  Let's see what such a diagram would look like and also find out tricks to interpreting the diagram.  In the following diagrams, a dashed line is used to indicate the ELR.
Remember that the DALR and SALR are fixed rates and will always plot as shown.  The ELR, on the other hand, is extremely variable and may plot in many different positions yielding different stability conditions.   In this example, the ELR just so happens to fall between the SALR and DALR.  We must compare the parcel lines to the ELR to determine stability conditions.
Diagram Features:  There are a set of features you need to become familiar with.  The ELR always has "Warm" and "Cool" sides to it.  Here is what I mean.  Temperature increases to the right along the X-axis.  I want you to consider the region to the right of the ELR to be the"Warm Region" and the left region to be the"Cool Region".  We may prove this idea.  Read up the Y-axis to an arbitrary altitude (in this example, where the green line is).  We will find out how much dry and saturated air and the environment will have cooled by this altitude.  Read straight across along the green line and drop lines down to the X-axis where the green line hits the DALR, SALR, and the ELR.  Note that dry air would have cooled the most, the environment the next greatest amount, and saturated air the least. The dry air "D" is on the left of the ELR "E" while the saturated air "S" is on the right of "E."  Let's apply this to the first diagram.  Here, dry air will cool more rapidly and thus be cooler than the environment at any given level.  According to our first diagram, dry air would be Stable.  Saturated air would cool more slowly than the environment and therefore remain warmer than the surrounding environment.  The first diagram shows this situation as being Unstable.

Rules:  A parcel line plotting to the right of the ELR indicates unstable conditions for that parcel type.  A parcel line plotting to the left of the ELR indicates stable conditions for that parcel type.
 

Specific Stability Conditions:  The following diagrams depict the various forms of stability that you will be expected to know.

Absolute Stability:
Whenever both parcel plots are on the same side of the ELR, we refer to the condition as being "Absolute."  In this case, both plots are on the left or "cool" side of the ELR.  That is, both are on the stable side.  This is called Absolute Stability.  Air of any relative humidity (RH) will cool faster than the surrounding environment and will therefore not rise.  Air would tend to sink and create clear skies.
Notation: DALR > SALR > ELR.

Absolute Instability:
In this instance, both parcel lines are on the right or "warm side" of the ELR.  This means that air of any RH will cool more slowly than the environment and therefore always be warmer than the surrounding environment.  Air parcels will be buoyant and rise like hot air balloons.  Air would tend to rise and create storms.
Notation: ELR > DALR > SALR.

Conditional Stability:
Here the ELR falls between the DALR and SALR.  Note here that the SALR is on the "warm" side and the DALR is on the "cool" side of  the ELR.  This means that saturated parcels will be unstable and dry parcels will be stable.  The overall picture is called Conditional Stability because one must know the condition (RH) of a parcel to determine if it will be stable or unstable.
Notation: DALR > ELR > SALR.

Wet Neutral:
Here the ELR matches the SALR.  Saturated parcels will thus be neutral.  Notice, however, that dry parcels, being on the left or "cool" side of the ELR will be stable.
Notation: DALR > SALR = ELR.

Dry Neutral:
The ELR now matches the DALR meaning that dry parcels will be neutral.  Note that saturated parcels, being on the right or "warm" side of the ELR will be unstable.
Notation: DALR = ELR > SALR.

Extreme Stability:
For the first time, we have a Negative ELR!  This indicates that the layering of the lower atmosphere is such that warmer air lies on top of cooler air.  This is, of course, a Temperature Inversion!  Note that both parcel lines are on the same, "cool" side of the ELR.  This is a special form of Absolute Stability.  In this situation, no air parcel may rise.  This often has negative effects upon pollution dispersal (read about Donora, PA in the text!!).
Notation: ELR is negative!!

A Helpful Technique:  In Tutorial 12, I mentioned that you may use your right hand to mimic the lapse rate diagram.  Let's do it again.  Hold your right hand with your palm facing you.  Now fold all fingers but your index and middle fingers.  There are the DALR and SALR.  Now, use your thumb as the ELR.  This is your handy-dandy grapher!  Try and mimic the stability diagrams immediately above.

A Quick Example:  What would the stability condition of a saturated parcel be if the ELR was +6°C / 1 km?  Take out your handy-dandy grapher.  Where will the ELR plot?  You must now recall from Tutorial 12 what the DALR and SALR are numerically.  Once you know what they are per 1 km increments, you will be able to plot the given ELR.  In this situation, the ELR will plot somewhere between the DALR and SALR; this is called Conditional Stability.  The question is, however, what the condition of a saturated parcel will be.  The SALR is on the "warm" or unstable side of the ELR so the answer is unstable.  You will need to be able to do these kinds of problems on exams.