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Adiabatic Processes
The concept of a parcel
Parcel and environmental lapse rates
Atmospheric dry stability
Determining stability

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A parcel is a “blob” of air
Small enough to have only one value of T, p, ρ, etc.
Large enough to contain a significant number of molecules. (Are there enough particles to talk about temperature as average kinetic energy, for example?)

 Parcel lapse rate – the rate at which temperature changes as the parcel is lifted to a higher altitude
 Environmental lapse rate – the rate at which the air surrounding the parcel changes as altitude increases

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An adiabatic process is one during which no heat is exchanged between the substance in question and its surroundings
Many atmospheric motions occur rapidly enough that parcels do not exchange a significant amount of heat with the environment
Examples:
• rising air in a thunderstorm
• Air rising over a topographic barrier (like a mountain)

(Chalkboard)

The adiabatic lapse rate for DRY air on Earth is
Γ d
= g/c p
Γ d
= 9.81 m s -2 / 1004 J kg -1 K -1
Γ d
= 0.00977 K m -1
Γ d
= 9.77 K km -1

This means that a rising(sinking) air parcel will cool(warm) at a rate of about 10 o C per km of ascent(descent) unless:
• It exhanges significant mass or heat with the environment
•
• It becomes saturated with respect to water vapor
It rises(sinks) so slowly that radiation heat transfer is possible

What is the dry adiabatic lapse rate ( Γ d these atmospheres?
= g/c p
) in
Atmosphere
Venus
Mars
Titan g (m s -2 )
8.87
3.71
1.352
c p
(J kg -1 K -1 )
844
844
1039

We have thus far only discussed the
DRY ADIABATIC LAPSE RATE
Water vapor condensation releases 2.5 MJ of energy for each kg of water condensed – this latent heat changes the adiabatic lapse rate for condensing air parcels to the
MOIST ADIABATIC LAPSE RATE

Atmospheric Stability
 stable and unstable equilibria
 air parcels
 adiabatic process
 adiabatic lapse rates
• Stability does not control whether air will rise or sink.
Rather, it controls whether rising air will continue to rise or whether sinking air will continue to sink.

(Chalkboard)

A Stable Atmosphere
 environmental lapse rate
 absolute stability
 stabilizing processes
• Stable air provides excellent conditions for high pollution levels.

An Unstable Atmosphere
 absolute instability
 warming of surface air
 destabilizing processes
 superadiabatic lapse rates
• Unstable air tends to be well-mixed.

Conditionally Unstable Air
 conditional instability
 dry and moist adiabatic lapse rates are different
 Environmental lapse rate is between the two

Twice now, we’ve mentioned moist adiabatic lapse rates.
Maybe we should talk about atmospheric moisture before we go down that road any further…

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Circulation of water in the atmosphere
Evaporation, condensation and saturation
Humidity
Dew and frost
Fog
Clouds

Circulation of Water in the
Atmosphere
 evaporation
 condensation
 precipitation
 hydrologic cycle
• The total amount of water vapor stored in the atmosphere amounts to only one week’s supply of precipitation for the planet.

Fig. 4-1, p. 80

Stepped Art
Fig. 4-1, p. 80

Evaporation, Condensation and Saturation
 saturation
 condensation nuclei
• In very clean air, about 10,000 condensation nuclei are typically found in one cubic centimeter of air, a volume approximately the size of your fingertip.

Mixing Ratio
The ratio of the mass of water vapor in air to the mass of dry air: w = m v
/ m d
 Usually expressed in g kg -1
 Some typical values:
• Tropical marine boundary layer air: w ≈ 18 g kg -1
• Polar air: w ≈ 1 g kg -1
• Stratospheric air: w ≈ 0.1 g kg -1

Specific Humidity
The ratio of the mass of water vapor in air to the total mass of the air (dry air plus water vapor):
SH = m v
/ (m d
+ m v
) w = SH / (1 – SH)
SH = w / (1 + w)

Vapor Pressure
 actual vapor pressure
 saturation vapor pressure
• “Saturation” describes a condition of equilibrium: liquid water is evaporating at exactly the same rate that water vapor is condensing.

Vapor Pressure
Actual vs. Saturation (or equilibrium) vapor pressure…
( Chalkboard )

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Vapor Pressure
Saturation vapor pressure depends only on temperature…
Formula: e s
 e s 0 exp


L
 R v


1
T
0

1
T




 e s
: Saturation vapor pressure e s 0
:
Saturation vapor pressure at
273 K = 6.11 mb
L :
R v
Latent heat of vaporization =
2.5x10
6 J kg -1
: Gas constant for water vapor =
461 J kg -1 K -1
T
0
: 273 K
T :
Temperature



Vapor Pressure
Saturation vapor pressure depends only on temperature…
Formula: e s

6.11 exp 5423

1
273

1
T



Vapor Pressure
Saturation vapor pressure depends only on temperature…
Graph:

Relative Humidity
 definition of relative humidity
 saturation and supersaturation
 condensation
 relative humidity and temperature
• When the general public uses the term “humidity”, they mean “relative humidity.”

Relative Humidity
The ratio of the actual vapor pressure to the saturation vapor pressure.
rh = e / e s
Since e s depends on temperature, the relative humidity measures closeness to saturation, not actual water vapor content.

Fig. 4-5, p. 83

Fig. 4-7, p. 85

Relative Humidity and Dew
Point
 dew point temperature: the temperature to which air must be lowered to reach 100% relative humidity
 dew point depression and relative humidity
• The dew point temperature is useful for forecasting heat index, precipitation probabilities, and the chance of frost.

Measuring Humidity
 psychrometers
 hygrometers

Dew and Frost
 dew
 frost
 frost point and deposition
• Frost is one of the few examples of deposition in nature.

Fog
 radiation fog
 advection fog
 upslope fog
 evaporation (mixing) fog
• Fog is an extreme hazard to aircraft.

Classification of Clouds
 major cloud types
 cloud appearance
 cloud base
• It’s easy to identify clouds, but it takes practice.
The ability to identify clouds allows you to forecast many aspects of the weather using nothing but your eyes.

Table 4-2, p. 98

Cloud Identification
 high clouds
 middle clouds
 low clouds
 clouds with vertical development

High Clouds
 cirrus
 cirrocumulus
 cirrostratus
• Cirrostratus clouds can sometimes be quite thick.

Middle Clouds
 altocumulus
 altostratus
• Altocumulus clouds are very pretty, especially just after sunrise or just before sunset.

Low Clouds
 nimbostratus
 stratocumulus
 stratus
• Marine stratocumulus is the most common cloud type in the world.

Clouds with Vertical
Development
 cumulus
 cumulus congestus
 cumulonimbus
• Not all cumulus clouds grow to be thunderstorms, but all thunderstorms start out as cumulus clouds.

Some Unusual Clouds
 lenticular clouds
 pileus
 mammatus clouds
 contrails
• Several alleged ‘flying saucer’ reports have turned out to be lenticular clouds.

Cloud Development and
Stability
 surface heating and free convection
 uplift along topography
 widespread ascent
 lifting along weather fronts

Convection and Clouds
 thermals
 fair weather cumulus
• Fair weather cumulus provide a visual marker of thermals.
• Bases of fair-weather cumulus clouds marks the lifting condensation level , the level at which rising air first becomes saturated.

Topography and Clouds
 orographic uplift
 rain shadow
• The rain shadow works for snow too. Due to frequent westerly winds, the western slope of the Rocky Mountains receives much more precipitation than the eastern slope.

Conditional Stability
• Environmental lapse rate between wet and dry adiabatic lapse rates
• Lifting Condensation Level (LCL)
• Level of Free Convection (LFC)
(Chalkboard)

Summary of Atmospheric
Stability
• Absolute Stability = environmental lapse rate greater than both wet and dry adiabatic lapse rates
• Absolute Instability = environmental lapse rate less than both wet and dry adiabatic lapse rates
• Conditional Stability = environmental lapse rate less than dry but greater than we adiabatic lapse rate. The environment is stable to moist but unstable to dry disturbances.

Collision and Coalescence
Process
 terminal velocity
 coalescence
 warm clouds
• A typical cloud droplet falls at a rate of 1 centimeter per second.
At this rate it would take
46 hours to fall one mile.

Stepped Art
Fig. 5-9, p. 116

Ice Crystal Process
 cold clouds
 supercooled water droplets
 saturation vapor pressures over liquid water and ice
 accretion
• The upper portions of summer thunderstorms are cold clouds!

Fig. 5-22, p. 124

Stepped Art
Fig. 5-22, p. 124

Cloud Seeding and
Precipitation
 cloud seeding
 silver iodide
• It is very difficult to determine whether a cloud seeding attempt is successful. How would you know whether the cloud would have resulted in precipitation if it hadn’t been seeded?

Precipitation in Clouds
 accretion
 ice crystal process

Rain
 rain
 drizzle
 virga
 shower
• Virga is much more commonly observed in the western
US, because the humid climate of the eastern US reduces the visibility.

Snow
 snow
 fallstreaks
 dendrite
 blizzard
• Snowflake shape depends on both temperature and relative humidity.

Sleet and Freezing Rain
 sleet
 freezing rain
 rime
• Sleet makes a ‘tap tap’ sound when falling on glass.

Snow Grains and Snow
Pellets
 snow grains
 snow pellets
 graupel

Hail
 updraft cycles
 accretion
• A hailstone can be sliced open to reveal accretion rings, one for each updraft cycle.

Stepped Art
Fig. 5-35, p. 134

Instruments
 standard rain gauge
 tipping bucket rain gauge
• It is difficult to capture rain in a bucket when the wind is blowing strongly.

Doppler Radar and
Precipitation
 radar
 Doppler radar

Stepped Art
Fig. 5-39, p. 135