1. Stability & Skew-T Diagrams
ATS 351 Lecture 6
Stability & Skew-T Diagrams

2. Air Parcel To demonstrate stability, a parcel of air is used
Expands and contracts freely
Always has uniform properties throughout

3. Air Parcel Movement: Why does rising air expand and cool?
Lift parcel: pressure lowers  air molecules push outward  EXPANDS
Energy is used to expand so molecules slow down  COOLS
Lower parcel: pressure increases  COMPRESSES parcel
Compressing increases molecular energy  WARMS

4. Adiabatic Process
Adiabatic Process: when a parcel expands and cools or compresses and warms WITHOUT exchange of heat with the surrounding environment.
In unsaturated air, a parcel of air cools or warms at the Dry Adiabatic Rate (about 10ºC/km)‏
The dew point also decreases as a parcel is raised “Dry Adiabatically”
Dew Point Lapse Rate: 2ºC/km

5. Moist Adiabatic Process
As the parcel rises, temperature and dew point get closer together and are eventually equal  condensation
Td decreases at a slower rate than T
Since latent heat is released inside the parcel during condensation, the temperature will now decrease at a slower rate
Moist Adiabatic Lapse Rate: ~6ºC/km
Bullet 2: LH release offsets the adiabatic cooling that is occurring because of the rising motion - temperature decreases at a slower rate.

6. Stability Stable Equilibrium Unstable Equilibrium Neutral Equilibrium
If the ball is displaced it will return to it’s original position
Unstable Equilibrium
If the ball is displaced it will accelerate away from the equilibrium point
Neutral Equilibrium
If the ball is displaced it will stay in it’s new location.

7. Stability in the Atmosphere
At any height, if the temperature of the parcel is greater than the environment, the parcel will rise (and vice versa).
Temperature profile of the environment is received from radiosonde data.
We can look at the lapse rate of the environment to see what an air parcel will do if it is displaced
In a stable atmosphere: a displaced parcel will return to its initial position.
In an unstable atmosphere: a displaced parcel will continue to move in the initial direction of motion.

8. Conditions for Stability
Absolutely Stable
Environmental lapse rate is less than moist adiabatic lapse rate.
Lapse rate < 6ºC/km
Absolutely Unstable
Environmental lapse rate is greater than dry adiabatic lapse rate.
Lapse rate > 10ºC/km
Conditionally Unstable
Environmental lapse rate lies between moist and dry lapse rates.
Lapse rate between 6- 10ºC/km

9. Stable Atmosphere
The parcel of air is colder than the environment since its lapse rate is greater.
Therefore, a displaced parcel will return to its original position: vertical motion is suppressed.
What conditions produce a stable atmosphere?:
Air aloft warms (by warm advection) and surface air cools (by radiative cooling at night or cold advection)‏
Subsiding air (frequently associated with a ridge of high pressure)‏
Inversions represent very stable air.
Tropopause is often very stable, as the stratosphere is warmed due to ozone.
Bullet 1: The PARCEL follows either the dry or moist adiabatic lapse rate (depending on if it is saturated or not).
Before bullet 3: A stable atmosphere means that the difference between the surface air and air aloft is SMALL.
Inversion is an example of very stable air.
If clouds WERE to form by forced vertical motion, the type of clouds that would be produced are thin layers of clouds with flat tops (stratus, cirrostratus, altostratus, nimbostratus).

10. Unstable Atmosphere Buoyant parcels are accelerated upward
As parcels rise and cool, they are still warmer than the environment since the environment is cooling faster than the adiabatic lapse rate
Larger instabilities lead
to larger updrafts
Large updrafts lead to
the formation of
cumulonimbus clouds
and thunderstorms

11. Causes of Instability Cooling of the air aloft:
Winds bringing in colder air (cold advection)‏
Clouds (or the air) emitting IR radiation to space (radiational cooling)‏
Warming of the surface air:
Daytime solar heating of the surface
Winds bringing in warm air (warm advection)‏
Air moving over a warm surface
Daytime solar heating: this is one of the reasons you see convection occur a lot in the later afternoon because the sun has reached its max of heating the surface of the earth

12. Conditionally Unstable
Environmental lapse rate is between moist and dry adiabatic lapse rates (common in atmosphere)‏
Ex: environmental rate of 7ºC/km
Conditional instability means that if unsaturated air (stable) could be lifted to a level where it becomes saturated, instability would result
Figure on next slide demonstrates conditional instability
If environmental lapse rate is 7ºC/km: FIGURE
If it is unsaturated, the parcel will follow the dry adiabatic lapse rate and it will be colder than the environment, so it is stable.
However, if it reaches a level where it is saturated now, it will cool at the moist adiabatic rate and therefore will be warmer than the environment and will be come unstable and rise.

13. Conditional Instability

14. Skew-T/Log-P Diagram
Reminder:

15. Stability on a Skew-T
Determine the stability of the atmosphere by looking at the PLOTTED temperature profile.
There will be varying layers of stability throughout the atmosphere.

16. Examples Layer between 700mb and 800mb is absolutely stable
ABSOLUTELY UNSTABLE
Layer between 700mb and
800mb is absolutely stable
Layer between 850mb and
950mb is absolutely unstable

17. Examples Layer between 600mb and 700mb is conditionally unstable

18. Lifting a Parcel
Initially, a parcel being lifted will cool at the Dry Adiabatic Lapse Rate
When the dry adiabat from the surface temperature meets the saturating mixing ratio line from the surface dew point, the parcel will have reached saturation and condensation can occur
This is called the Lifted Condensation Level (LCL)‏
Green: Mixing ratio lines…FOLLOW UP FROM SURFACE DEW POINT
Pink: Dry adiabat lines…FOLLOW UP FROM SURFACE TEMPERATURE
INTERSECTION: LCL

19. Lifting a Parcel
Once a parcel has reached the LCL, it will continue to rise, but instead cool at the Moist Adiabatic Lapse Rate
Often the temperature of the parcel at the LCL is still cooler than the temperature of the environment (negative area)‏
If the parcel is lifted further it will reach its
Level of Free Convection (LFC), the point at which the parcel becomes warmer than the environment and will be accelerated upward by buoyancy (positive area)‏
As it continues to rise it will eventually reach a point where it is cooler than the environment again.
This is the Equilibrium Level (EL)‏

20. Lifting a Parcel

21. Sources of Lift 4 ways to lift a parcel to the LCL Orographic
Frontal Boundary
Convergence
Convection

22. CAPE CAPE = Convective Available Potential Energy
CAPE is the energy available to a rising parcel to accelerate it
On a Skew-T, CAPE is proportional to the area between the parcel’s temperature and the environment’s when the parcel is warmer
CAPE gives an upper limit on how high updraft speeds can get in a severe storm
High values of CAPE are associated with the possibility of strong convection
Large hail requires very high CAPE values
Extreme
2,500+
Large
1,500-2,500
Positive
1 - 1,500

23. CAPE

24. CIN CIN = Convective Inhibition
This is the energy the must be overcome in order to lift a parcel to its LFC
On a Skew-T, CIN is proportional to the area between the parcel’s temperature and the environment’s when the parcel is colder
Large values of CIN will prevent the formation of storms, but often the presence of some CIN can add strength to a storm if this energy is overcome

25. CAPE and CIN

26. More Uses for Skew-T’s Finding cloud levels
Forecasting precipitation type
Forecasting max/min temperatures
Forecasting the possibility of microbursts

27. More Uses for Skew-T’s Finding cloud levels – useful for aviation
Clouds are most
likely present at 3
layers in this
skew-T. Can you
find them?
Right at surface, 680mb, and 480mb

28. More Uses for Skew-T’s Forecasting precipitation type
The 00C isotherm in this skew-T shows that the precipitation will fall through a layer which is above freezing, thus implying that freezing rain is possible
Rain is falling through above freezing layer but right at surface, temperature is just at 0 degrees C…therefore the rain may freeze.

29. More Uses for Skew-T’s Forecasting maximum/minimum temperature
12Z on left (Oct 3); 0Z on right (Oct 4)‏
How to do it:
From the morning sounding (12Z): if there is an inversion (like there is here): find the temperature at the warmest point of the inversion.
From that point, follow the dry adiabat down to the surface and that corresponding temperature is your Tmax

30. More Uses for Skew-T’s Forecasting the possibility of microbursts
The “inverted V” shape is a sign of possible dry microbursts (isolated pockets of strong winds associated with thunderstorms)‏

31. Parcels Movement on Skew-T
Just explain that if you are looking for the PARCELS temperature, the orange line is what you will follow. The temperature profile is the ENVIRONMENT.

32. Parcels Movement on Skew-T

33. A few skew-T reminders:
Plot the temperature (or dew point) ON the pressure line that is given.
i.e. 25C at 900mb
When plotting temperature, remember the temperature lines (isotherms) are slanted.
i.e. 25C at 300mb is NOT going to be directly above 25C at 1000mb
The parcel of air begins at the surface temperature but follows either the dry or moist adiabatic lapse as it rises in the atmosphere (NOT the plotted temperature profile = environmental lapse rate)‏

34. 900mb
1000mb
500mb
Just pointing out that your temperatures are going to be SLANTED, even if they are the same value as you go up in the atmosphere.