1. Lesson 15 Adiabatic Processes
Hess, McKnight’s Physical Geography, 10 ed.
138 – 139 pp.

2. Adiabatic Processes
Recall prior to spring break that as the temperature of an air parcel decreases, its relative humidity increases.
Once a parcel has cooled to the dew point, it becomes saturated and condensation occurs.
Adiabatic cooling is one of the most common ways clouds form.

3. Adiabatic Cooling Clouds are the result of condensation
This condensation occurs when an air parcel’s temperature reaches the dew point
Air parcels are free to flow horizontally or vertically through the atmosphere
Most commonly, clouds are formed with vertical movement, namely rising air

4. Adiabatic Cooling, cont.
As an air parcel rises, it becomes less- pressurized
With less pressure exerted on the parcel, it therefore expands
This expansion causes
cooling and increases RH
Adiabatic cooling

5. Adiabatic Heating
Conversely, as a parcel of air descends it compresses due to higher pressure
This is a warming process, therefore the air warms

6. Dry Adiabatic Rate Consider an air parcel that is unsaturated
RH < 100%
If this air parcel rises, it will cool at what is known as the dry adiabatic lapse rate (DALR)
10°C per 1000 meters
5°F per 1000 feet

7. Dry Adiabatic Rate, cont.
As this air parcel rises its relative humidity increases (because it’s expanding)
Eventually the air temperature cools and equals the dew point temperature
At this point the RH=100%
The elevation that this occurs at is known as the lifted condensation level (LCL)
At this point, condensation occurs and cloud forms

8. Moist Adiabatic Rate
At this point, the air parcel is saturated and the RH=100%
If the parcel continues to rise, it will cool even further
However, because it is saturated (i.e. 100% RH) it will cool slower (MALR)
6°C per 1000 meters
3.3°F per 1000 feet
This is the result of latent heat which is released once condensation begins

9. Heat Transfer Theory
Evaporation is a cooling process because liquid water is converted into a gas, which takes heat energy from the environment.

10. Final Thoughts… Dry Adiabatic Lapse Rate (DALR):
10°C per 1000 meters
10°F per 1000 feet
Moist Adiabatic Lapse Rate (MALR):
6°C per 1000 meters
3.3°F per 1000 feet

11. Example 1

13. Stability Hess, McKnight’s Physical Geography, 10 ed. 143– 147 pp.
Lesson 16
Stability
Hess, McKnight’s Physical Geography, 10 ed.
143– 147 pp.

14. Stable vs. Unstable
The atmosphere and individual air parcels can be characterized as either stable or unstable.
Stability occurs when vertical motion is suppressed.
No clouds form (unless they are forced, i.e. orographic lift)
Instability (unstable air) occurs when air tends to rise on its own
The temperature of an air parcel relative to the air which surrounds it determines stability

15. Lapse Rates We know there are two types of adiabatic lapse rates
Dry adiabatic lapse rate (DALR)
Moist adiabatic lapse rate (MALR)
There is a third lapse rate, known as the environmental lapse rate (ELR)

16. Environmental Lapse Rate (ELR)
The ELR is basically the actual air temperature at any height in the atmosphere
It is also known as a temperature profile, vertical temperature gradient, or normal lapse rate
The average value:
6.5°C per 1000 meters
3.6°F per 1000 feet

17. Environmental Lapse Rate (ELR)

18. Types of Stability/Instability
There are three types:
Absolute stability
Absolute instability
Conditional instability
Let’s look at examples of each of these…

19. Absolute Stability
If we (somehow) lift the parcel: It will cool at the dry adiabatic lapse rate. The parcel will find itself cooler than the environmental (sounding) temperature. At the same pressure, a cooler parcel will be more dense than the environment. Being denser, the parcel will descend back to where it came from. STABLE!

20. Absolute Instability
If we (somehow) lift the parcel:  It will cool at the dry adiabatic lapse rate. The parcel will find itself warmer than the environmental (sounding) temperature. At the same pressure, a warmer parcel will be less dense than the environment. Being less dense, the parcel will ascend and move farther from where it came from. UNSTABLE!

21. Conditional Instability
If we (somehow) lift the parcel:  It will cool at the dry adiabatic lapse rate. The parcel will find itself at the same temperature than the environmental (sounding) temperature. Being the same density, the parcel will not be accelerated in any direction and will remain where it is. NEUTRAL STABILITY! -- Dry Neutral, or Conditional Instability

22. Final Thoughts http://www.youtube.com/watch?v=Ox33T FnfWbE
Please omit pages 85, 86, 91, and 92 from your homework.