1. Atmospheric Moisture and Stability
Lecture 8

2. Water is responsible for many of Earth’s natural processes

3. Water can exist in all three phases in our atmosphere
What atmospheric variable do we use to quantify the amount of water in any given volume of air at one time?
Answer: Moisture

4. Moisture (Variables)
Relative Humidity (RH) is defined as the ratio of the amount of water vapor in the air to the amount of water vapor the air can hold (given as a percentage)
Dewpoint is defined as the temperature the air would have to be cooled to reach saturation (RH=100%)
Warmer air can hold more water vapor, so warmer air will, by definition, have a higher dewpoint
Mixing Ratio is the ratio of the mass of water to the mass of dry air

5. 1. Add more water vapor to the air
There are only TWO ways to saturate the air (or increase the relative humidity)
1. Add more water vapor to the air
2. Cool the air until its temperature is closer to the dew point temperature
Remember the water vapor molecules are moving faster in warm air and less likely to stick together and condense. If air cools to the dew point temperature, there is saturation.

6. Moisture
An air parcel with a large moisture content has the potential for that parcel to produce a great amount of precipitation.
- Air with a mixing ratio of 13 g/kg will likely rain a greater amount of water than air with a mixing ratio of 6 g/kg.

7. Two parcels of air:
PARCEL 1: Temperature = 31 oF, Dew point = 28 oF
PARCEL 2: Temperature = 89 oF, Dew point = 43 oF
Parcel 2 contains more water vapor than Parcel 1, because its dew point is higher.
Parcel 1 has a higher relative humidity, because it wouldn’t take much cooling for the temperature to equal the dew point! Thus, Parcel 1 is more likely to become saturated. But if it happened that both parcels became saturated then Parcel 2 would have the potential for more precipitation.
RH is not simply equal to the dew point divided by the temperature but is a good representation.

8. Types of Heat
Sensible Heat is the sort of heat you can measure with a thermometer
It’s also the type of heat you feel when you step on a hot surface with bare feet
Latent Heat is the heat required to change a substance from one phase to another
This is most commonly important with water, which is the only substance that exists on the Earth is three different phases
Gases are more energetic than liquids, which are more energetic than solids, so to move up in energetic states, energy is taken from the environment, and vice versa

9. Latent Heat

10. Moisture and the Diurnal Temperature Cycle
Review: Water has a high heat capacity (it takes lots of energy to change its temperature)
As a result, a city with a dry climate (like Sacramento, CA) will have a very large diurnal (daily) temperature cycle
A city with high water vapor concentration (like Key West, FL) will have a small diurnal cycle
Late July averages:
Sacramento: ~94/60
Key West: ~90/79

11. The other key component to the hydrologic cycle- Stability
What is stability?
Stability refers to a condition of equilibrium
If we apply some perturbation to a system, how will that system be affected?
Stable: System returns to original state
Unstable: System continues to move away from original state
Neutral: System remains steady after perturbed

12. Stable: Marble returns to its original position
Stability Example
Stable: Marble returns to its original position
Unstable: Marble rapidly moves away from initial position

13. How does a bowl and marble relate to the atmosphere?
Stability
How does a bowl and marble relate to the atmosphere?
When the atmosphere is stable, a parcel of air that is lifted will want to return back to its original position:

14. Stability Cont.
When the atmosphere is unstable (with respect to a lifted parcel of air), a parcel will want to continue to rise if lifted:

15. What do we mean by an air parcel?
Imaginary small body of air a few meters wide
Can expand and contract freely
Does not break apart
Only considered with adiabatic processes - External air and heat cannot mix with the air inside the parcel
Space occupied by air molecules inside parcel defines the air density
Average speed of molecules directly related to air temperature
Molecules colliding against parcel walls define the air pressure inside

16. Buoyancy and Stability
Imagine a parcel at some pressure level that is held constant, density remains the same so the only other variable that is changing is temperature. (REMEMBER: the Ideal Gas Law)
So if ρparcel < ρenv. Parcel is positively buoyant
In terms of temperature that would mean:
T of parcel > T of environment – buoyant! (unstable)
T of parcel < T of environment – sink! (stable)
T of parcel = T of environment – stays put (neutral)

17. Atmospheric Stability
This is all well and good but what about day to day applications?

18. Review: Atmospheric Soundings
Vertical “profiles” of the atmosphere are taken at 0000 UTC and 1200 UTC at ~95 stations across the country and many more around the world. Sometimes also launched at other times when there is weather of interest in the area.
Weather balloons rise to over 50,000 feet and take measurements of several meteorological variables using a “radiosonde.”
Temperature
Dew point temperature
Wind
- Direction and Speed
Pressure

20. Adiabatic Lapse Rate
Mixing Ratio
Moist Adiabatic Lapse Rate
Temperature
Dewpoint
Temperature

21. Vertical Profile of Atmospheric Temperature allows us to assess stability of the atmosphere

22. Lapse Rates
Lapse Rate: The rate at which temperature decreases with height (Remember the inherent negative wording to it)
Environmental Lapse Rate: Lapse rates associated with an observed atmospheric sounding (negative for an inversion layer)
Parcel Lapse Rate: Lapse rate of a parcel of air as it rises or falls (either saturated or not)
Moist Adiabatic Lapse Rate (MALR): Saturated air parcel
Dry Adiabatic Lapse Rate (DALR): Dry air parcel

23. DALR Air in parcel must be unsaturated (Relative Humidity < 100%)
Rate of adiabatic heating or cooling = ~10°C for every 1000 meter (1 kilometer) change in elevation
Parcel temperature decreases by about 10° if parcel is raised by 1km, and increases about 10° if it is lowered by 1km

24. MALR
As rising air cools, its RH increases because the temperature approaches the dew point temperature, Td
If T = Td at some elevation, the air in the parcel will be saturated (RH = 100%)
If parcel is raised further, condensation will occur and the temperature of the parcel will cool at the rate of ~6.5°C per 1km in the mid-latitudes

25. DALR vs. MALR The MALR is less than the DALR because of latent heating
As water vapor condenses into liquid water for a saturated parcel, LH is released, lessening the adiabatic cooling
Remember no heat exchanged with environment

26. DALR vs. MALR

27. Absolute Stability
The atmosphere is absolutely stable when the environmental lapse rate (ELR) is less than the MALR
ELR < MALR <DALR
A saturated OR unsaturated parcel will be cooler than the surrounding environment and will sink, if raised

28. Absolute Stability Inversion layers are always absolutely stable
Temperature increases with height
Warm air above cold air = very stable

29. Absolute Instability
The atmosphere is absolutely unstable when the ELR is greater than the DALR
ELR > DALR > MALR
An unsaturated OR saturated parcel will always be warmer than the surrounding environment and will continue to ascend, if raised

30. Conditional Instability
The atmosphere is conditionally unstable when the ELR is greater than the MALR but less than the DALR
MALR < ELR < DALR
An unsaturated parcel will be cooler and will sink, if raised
A saturated parcel will be warmer and will continue to ascend, if raised

31. Conditional Instability
Example: parcel at surface
T(p) = 30°C, Td(p) = 14°C (unsaturated)
ELR = 8°C/km for first 8km
Parcel is forced upward, following DALR
Parcel saturated at 2km, begins to rise at MALR
At 4km, T(p) = T(e)…this is the level of free convection (LFC)

32. Conditional Instability
Example continued…
Now, parcel will rise on its own because T(p) > T(e) after 4km
The parcel will freely rise until T(p) = T(e), again
This is the equilibrium level (EL)
In this case, this point is reached at 9km
Thus, parcel is stable from 0 – 4km and unstable from 4 – 9km EL
LCL

33. Rising Air Consider an air parcel rising through the atmosphere
The parcel expands as it rises
The expansion, or work done on the parcel causes the temperature to decrease
As the parcel rises, humidity increases and reaches 100%, leading to the formation of cloud droplets by condensation

34. Rising Air
If the cloud is sufficiently deep or long lived, precipitation develops.
The upward motions generating clouds and precipitation can be produced by:
Convection in unstable air
Convergence of air near a cloud base
Lifting of air by fronts
Lifting over elevated topography

35. Lifting by Convection
As the earth is heated by the sun, thermals (bubbles of hot air) rise upward from the surface
The thermal cools as it rises, losing some of its buoyancy (its ability to rise)
The vertical extent of the cloud is largely determined by the stability of the environment

36. Lifting by Convection
A deep stable layer restricts continued vertical growth
A deep unstable layer will likely lead to development of rain-producing clouds
These clouds are more vertically developed than clouds developed by convergence lifting

37. Lifting by Convergence
Convergence exists when there is a horizontal net inflow into a region
When air converges along the surface, it is forced to rise

38. Lifting by Convergence
Large scale convergence can lift air hundreds of kilometers across
Vertical motions associated with convergence are generally much weaker than ones due to convection
Generally, clouds developed by convergence are less vertically developed

39. Lifting due to Topography
This type of lifting occurs when air is confronted by a sudden increase in the vertical topography of the Earth
When air comes across a mountain, it is lifted up and over, cooling as it is rising
The type of cloud formed is dependent upon the moisture content and stability of the air

40. Lifting due to Topography

41. Lifting Along Frontal Boundaries
Will discuss origin more in detail later in the semester as we begin to discuss cyclones and fronts
NEXT WEEK: Severe weather!