Download Document

Atmospheric Stability





Adiabatic Processes
The concept of a parcel
Parcel and environmental lapse rates
Atmospheric dry stability
Determining stability
Air parcels



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?)
Lapse Rates

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
The Adiabatic Lapse
Rate



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)
Adiabatic Processes
(Chalkboard)
The Adiabatic Lapse
Rate
The adiabatic lapse rate for DRY air on Earth is
Γd = g/cp
Γd = 9.81 m s-2 / 1004 J kg-1 K-1
Γd = 0.00977 K m-1
Γd = 9.77 K km-1
The Adiabatic Lapse
Rate
This means that a rising(sinking) air parcel will
cool(warm) at a rate of about 10 oC 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
The Adiabatic Lapse
Rate
What is the dry adiabatic lapse rate (Γd = g/cp) in
these atmospheres?
Atmosphere
g (m s-2)
cp (J kg-1 K-1)
Venus
8.87
844
Mars
3.71
844
Titan
1.352
1039
The Adiabatic Lapse
Rate
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
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.
Determining Stability
(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
Atmospheric Moisture
Twice now, we’ve mentioned moist adiabatic
lapse rates.
Maybe we should talk about atmospheric
moisture before we go down that road any
further…
Humidity,
Condensation and
Clouds






Circulation of water in the atmosphere
Evaporation, condensation and saturation
Humidity
Dew and frost
Fog
Clouds
Circulation of Water
in the Atmosphere
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
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.
Humidity
Mixing Ratio
The ratio of the mass of water vapor in air to the mass of
dry air:
w = mv / md
 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 = mv / (md + mv)
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)
Vapor Pressure
Saturation vapor pressure depends only on temperature…
Formula:
e s : Saturation vapor pressure
e s0 : Saturation vapor pressure at
273 K = 6.11 mb
 L  1 1 
Latent heat of vaporization =
L
:
es  es0 exp    
2.5x10 J kg
Rv T0 T  Rv : Gas constant for water vapor =
461 J kg K
T0 : 273 K
T : Temperature
6
-1
-1
-1
Vapor Pressure
Saturation vapor pressure depends only on temperature…
Formula:

 1
1 
es  6.11 exp 5423
 
273 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 / es
Since es 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 and Frost



dew
frost
frost point and deposition
• Frost is one of the few examples of deposition in nature.
Fog
Fog




radiation fog
advection fog
upslope fog
evaporation (mixing) fog
• Fog is an extreme hazard to
aircraft.
Clouds
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
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.
Precipitation
Processes
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
Precipitation Types
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
Measuring
Precipitation
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