1. Humidity and Stability
AOS 101 Section 301
March 23, 2009

2. A Brief Aside

3. Humidity Humidity is a measure of the water vapor content of the air
Humidity is important because the latent heat involved in the evaporation and condensation of water vapor plays a large role in our day-to-day weather

4. Measurements of Humidity
You can measure the humidity of the atmosphere in a number of ways:

5. Measurements of Humidity
You can measure the humidity of the atmosphere in a number of ways:
Vapor Pressure
The portion of atmospheric pressure exerted by water vapor alone

6. Measurements of Humidity
You can measure the humidity of the atmosphere in a number of ways:
Vapor Pressure
The portion of atmospheric pressure exerted by water vapor alone
The maximum (saturation) vapor pressure increases as the temperature increases

7. Measurements of Humidity
You can measure the humidity of the atmosphere in a number of ways:
Dewpoint
The temperature the air has to be cooled to in order to reach saturation

8. Measurements of Humidity
You can measure the humidity of the atmosphere in a number of ways:
Dewpoint
The temperature the air has to be cooled to in order to reach saturation
The air becomes saturated because the cooled air reaches a saturation vapor pressure equal to its actual vapor pressure

9. Measurements of Humidity
You can measure the humidity of the atmosphere in a number of ways:
Relative Humidity
The ratio of the actual vapor pressure to the saturation vapor pressure

10. Measurements of Humidity
You can measure the humidity of the atmosphere in a number of ways:
Relative Humidity
The ratio of the actual vapor pressure to the saturation vapor pressure
This measure doesn’t actually say anything about how humid the air is, since the water vapor content of the air is limited by its temperature

11. Vapor Pressure
TIME

12. Vapor Pressure
What is going on here?
TIME

13. Vapor Pressure
Relative Hum.
TIME

14. Vapor Pressure
Relative Hum.
What is the relative humidity here?
TIME

15. Vapor Pressure
Relative Hum.
Sat. Vapor Pres.
TIME

16. Vapor Pressure
Relative Hum.
Actual vapor pressure less than
saturation vapor pressure = unsaturated
Sat. Vapor Pres.
Actual vapor pressure equal to
saturation vapor pressure = saturated
TIME

17. Vapor Pressure
Relative Hum.
Sat. Vapor Pres.
Temperature
TIME

18. Vapor Pressure
Relative Hum.
Steep drop in temperature while
air is unsaturated
Sat. Vapor Pres.
Temperature
Air cools more slowly after
reaching saturation: Why?
TIME

19. Vapor Pressure
Relative Hum.
Sat. Vapor Pres.
Temperature
Dewpoint
TIME

20. Temperature
This “bend” in the temperature line after the air reaches saturation is incredibly important when discussing atmospheric stability…
TIME

21. Atmospheric Stability
Stability is a measure of the tendency of a system to return to its original state once it has been disturbed
In the atmosphere, we are concerned with whether a “parcel” of air, when forced to rise, will return to its original altitude (stable) or continue to rise (unstable)

22. Stability: A Cartoon
If the red ball is displaced slightly from its location…

23. Stability: A Cartoon
If the red ball is displaced slightly from its location…

24. Stability: A Cartoon
If the red ball is displaced slightly from its location…
The ball returns to its original location because of the effect of gravity. Here, gravity is behaving as a restoring force – a force that restores the system back to its original state.
This is a stable system.

25. Stability: A Cartoon
Let’s change the system:

26. Stability: A Cartoon
If the red ball is displaced slightly from its location...

27. Stability: A Cartoon
If the red ball is displaced slightly from its location...

28. Stability: A Cartoon
If the red ball is displaced slightly from its location...

29. Stability: A Cartoon
If the red ball is displaced slightly from its location...
Gravity now accelerates the ball away from its original position.
This is an unstable system.

30. Atmospheric Stability
When the atmosphere is stable, a small vertical displacement of air will be acted on to return to its original altitude:

31. Atmospheric Stability
When the atmosphere is stable, a small vertical displacement of air will be acted on to return to its original altitude:

32. Atmospheric Stability
When the atmosphere is stable, a small vertical displacement of air will be acted on to return to its original altitude:

33. Atmospheric Stability
When the atmosphere is stable, a small vertical displacement of air will be acted on to return to its original altitude:
Stable atmospheres are typified by clear skies

34. Atmospheric Stability
When the atmosphere is unstable, a small vertical displacement of air will be acted on to accelerate upward:

35. Atmospheric Stability
When the atmosphere is unstable, a small vertical displacement of air will be acted on to accelerate upward:

36. Atmospheric Stability
When the atmosphere is unstable, a small vertical displacement of air will be acted on to accelerate upward:

37. Atmospheric Stability
When the atmosphere is unstable, a small vertical displacement of air will be acted on to accelerate upward:
Unstable atmospheres are typified by clouds and precipitation

38. Humidity and Stability
Humidity plays a role in the stability of the atmosphere, because the stability of the air is related to the temperature of a lifted air parcel compared to the temperature of its surroundings
The latent heat released in condensation will change how the air cools as it rises (the “bend”), which changes the stability for a saturated air parcel compared to an unsaturated one

39. Stability: Buoyancy Force
In the atmosphere, a parcel of air that is warmer than its surroundings will experience a buoyancy force that lifts it upward

40. Stability: Buoyancy Force
In the atmosphere, a parcel of air that is warmer than its surroundings will experience a buoyancy force that lifts it upward
As the parcel rises, it encounters reduced pressure. The parcel expands to match the lower pressure, and in the process it cools. If the parcel is still warmer than its (new) environment, it continues to rise due to a positive buoyancy force.

41. Stability: Buoyancy Force
In the atmosphere, a parcel of air that is warmer than its surroundings will experience a buoyancy force that lifts it upward
As the parcel rises, it encounters reduced pressure. The parcel expands to match the lower pressure, and in the process it cools. If the parcel is still warmer than its (new) environment, it continues to rise due to a positive buoyancy force.
Once the parcel rises and cools to a level where its temperature matches the temperature of its environment, it no longer experiences a buoyancy force, and stops rising.

42. Thermodynamic Diagram
1000
Height
100 50 10
-20
-10
Temperature 10 20

43. Thermodynamic Diagram
1000
Environmental Temperature
Height
100 50 10
-20
-10
Temperature 10 20

44. Thermodynamic Diagram
In the atmosphere, a parcel of air that is warmer than its surroundings will experience a buoyancy force that lifts it upward
1000
Height
100 50 10
-20
-10
Temperature 10 20

45. Thermodynamic Diagram
As the parcel rises, it encounters reduced pressure. The parcel expands to match the lower pressure, and in the process it cools. If the parcel is still warmer than its (new) environment, it continues to rise due to a positive buoyancy force.
1000
Height
100 50 10
-20
-10
Temperature 10 20

46. Thermodynamic Diagram
Once the parcel rises and cools to a level where its temperature matches the temperature of its environment, it no longer experiences a buoyancy force, and stops rising.
1000
Height
100 50 10
-20
-10
Temperature 10 20

47. Thermodynamic Diagram
So far, we haven’t considered the effect of latent heat release. Let’s see what happens when we add humidity to the air and allow it to saturate by cooling as it rises:
1000
Height
100 50 10
-20
-10
Temperature 10 20

48. Thermodynamic Diagram
Start with a humid (but not saturated) parcel of air at the same temperature as before:
1000
Height
100 50 10
-20
-10
Temperature 10 20

49. Thermodynamic Diagram
The air parcel continues on the same path as before, rising and cooling at the same rate, until it saturates.
1000
Height
100 50 10
-20
-10
Temperature 10 20

50. Thermodynamic Diagram
The saturated parcel will continue to cool as it rises, but will do so at a slower rate, because the latent heat of condensation works to keep the air parcel warmer.
Here is the “bend” from earlier.
1000
Height
100 50 10
-20
-10
Temperature 10 20

51. Thermodynamic Diagram
The saturated parcel will continue to cool as it rises, but will do so at a slower rate, because the latent heat of condensation works to keep the air parcel warmer.
Here is the “bend” from earlier.
1000
Height
100 50 10
-20
-10
Temperature 10 20

52. Lapse Rates
The rate at which the temperature decreases with height is known as the lapse rate:

53. Lapse Rates
The rate at which the temperature decreases with height is known as the lapse rate:
Dry Adiabatic Lapse Rate – The rate at which a dry parcel of air cools as it rises = 9.8 C/km
Moist Adiabatic Lapse Rate – The rate at which a saturated parcel of air cools as it rises = 6 C/km

54. Lapse Rates
An environmental lapse rate that is less than the moist adiabatic lapse rate is considered absolutely stable. This is because even a saturated parcel of air cannot rise and find itself to be warmer than the environmental temperature.
LR < 6 C/km

55. Lapse Rates
An environmental lapse rate that is more than the dry adiabatic lapse rate is considered to be absolutely unstable. This is because even a dry air parcel will find itself to be warmer than its environment when it is lifted slightly.
LR > 9.8 C/km

56. Lapse Rates
An environmental lapse rate that is between the dry and moist adiabatic lapse rates is considered to be conditionally unstable; a dry air parcel will be stable in that environment, but a saturated air parcel will not.
9.8 C/km > LR > 6 C/km