When you think of the word “stable,” you typically think of an object that is unlikely to change or something that is balanced. The opposite is true with something that is “unstable”. An unstable object is likely to fall or change position with time. The same is true with clouds. When you see a fluffy cumulus cloud, you might notice them changing shape from one minute to the next. Such clouds are in a constant state of change, and thus represent the atmosphere in an unstable state.
Stability can be understood with the given diagrams below where –
Stable : if the ball push right or left, it will come back to it’s original position.
Neutral : if the ball pushed right or left it will move to that point only.
Unstable : if the ball pushed right or left it will move further even if we remove the force.
- In case of clouds, it is their upward movement in case of instability, for stable atmosphere clouds will come back to it’s original position.
Adiabatic Process : Air parcel movement is considered as the Adiabatic process, meaning that the air will not mix or exchange the energy with the surroundings while rising or down movement.
“Air parcel cools as it rises pressure decreases leads to less collision between the particles and warms as it sinks because pressure increases leads to more collision between the particles”
Lapse rate : Lapse rate is the decrease in the temperature with increase in height. It is one of the important aspect for determination of the stability of the atmosphere. There are three types of Lapse rate –
- Environmental Lapse Rate (ELR) : 6.50C/km, change in the temperature of the surrounding environment.
- Dry Adiabatic Lapse Rate (DALR) : 100C/km, change in the temperature of air which is dry.
- Moist/Saturated Adiabatic Lapse Rate (MALR/SALR) : 60C/km, change in the temperature of the air which is moist.
We will see stability in terms of the air pressure if the air is colder (low T) than environment that means air will generate high pressure and come to the downward, while is the air temperature is warmer (High T) than the environment air will have low pressure and it will rise further. Now temperature we have in terms of ELR, DALR, SALR explained above and will use this to understand types of stability –
- Absolute Stability : when DALR > SALR > ELR. In this case temperature of the air will always be less than as it rises. so will always come back to the original position.
- Absolute instability : when ELR > DALR > SALR. In this case temperature of the air always be more than atmosphere. so the air will always move up without any effort.
- Conditional Stability : when DALR > ELR > SALR. In this case the 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.
REFERENCES :
- https://atoc.colorado.edu/~saraht/atoc1050/Lecture_Notes/chapter6-notes.pdf
- https://faculty.kutztown.edu/courtney/blackboard/physical/17stability/stability.html
- https://pressbooks-dev.oer.hawaii.edu/atmo/chapter/chapter-5-atmospheric-stability/#:~:text=Stability%20in%20the%20atmosphere%20refers,or%20no%20change%20(neutral).
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