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Atmospheric Moisture
and Stability
Lecture 8
Water is responsible for many of
Earth’s natural processes
http://www.srh.weather.gov/jetstream/atmos/hydro.htm
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
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
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.
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.
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.
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
Latent Heat
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
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
Stability Example
Stable: Marble returns to its original position
Unstable: Marble rapidly moves away from initial position
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:
http://www.chitambo.com/clouds/cloudshtml/humilis.html
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:
http://www.physicalgeography.net/fundamentals/images/cumulonimbus.jpg
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
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)
Atmospheric Stability
This is all well and good but what about day to day
applications?
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
http://www2.ljworld.com/photos/2006/may/24/98598/
Adiabatic Lapse Rate Mixing Ratio
Moist Adiabatic
Lapse Rate
Temperature
Dewpoint
Temperature
Vertical Profile of Atmospheric
Temperature allows us to assess
stability of the atmosphere
We must compare
the parcel's
temperature Tp with
the temperature of
the surrounding
environment Te.
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
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
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
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
DALR vs. MALR
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
Absolute Stability
• Inversion layers are
always absolutely
stable
– Temperature
increases with
height
– Warm air above
cold air = very
stable
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
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
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)
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
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
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
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
Lifting by Convection
• A deep stable layer
restricts continued
vertical growth
• A deep unstable layer
will likely lead to
development of rainproducing clouds
• These clouds are
more vertically
developed than
clouds developed by
convergence lifting
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
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
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
Lifting due to Topography
Lifting Along Frontal Boundaries
• Front – The transition zone between two
air masses of different densities
• Lifting occurs along two different types of
fronts
– Cold Front
– Warm Front
Lifting Along Cold Fronts
• A colder,denser air mass lifts the warm,
moist air ahead of it
• As the air rises, it cools and condenses,
producing clouds and precipitation
• The steep slope of the cold front leads to
more vigorous rising motion
• Hence, cold fronts are often associated
with thunderstorms
Lifting Along Cold Fronts
Lifting Along Warm Fronts
• A warmer, less dense air mass rises up
and over the cold air ahead of the warm
front
• Air rises, cools and condenses
• Warm fronts have gentler slopes and
move slower than cold fronts
• Generally, precipitation is more steady and
widespread
Lifting Along Warm Fronts
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!