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Chapter 4: Moisture and Atmospheric Stability
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Movement of Water
Through The Atmosphere

Hydrologic cycle:
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Concept Check 4.1
2. The quantity of water lost to evaporation over the oceans is no equaled
by precipitation. Why, then, does the seal level not drop?
The deficit is made up by runoff from the
continents.
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Water: A Unique Substance
Water:
Water is the only liquid on the surface of the Earth in large quantities.
It exists in all forms on Earth.
Ice (solid state) is less dense than liquid.
Water has a high heat capacity.
It has a unique ability to form hydrogen bonds (H2O).
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Water has Hydrogen Bonds
Hydrogen Atoms (-)
(-)
Oxygen Atom (+)
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(+)
(-)
(-)
(+)
Water’s Changes of State
Ice, liquid water, and water vapor:
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Latent Heat
Latent heat of melting
Adding heat to melt ice does not
result in a temperature change.
Melting 1 gram of ice requires
80 calories.
The heat is used in breaking
the hydrogen bonds.
Latent heat of fusion
Freezing 1 gram of water releases
80 calories.
The heat goes into the environment
and warms the environment.
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Latent Heat
Latent heat of vaporization
600 calories/gram is the energy
absorbed by water during evaporation.
Evaporation is a cooling process.
Latent heat of condensation
600 calories/gram is the energy
released by water during condensation.
Important part of weather
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Energy is released, which warms the
surrounding air.
Latent Heat
Latent heat of sublimation
process that turns a solid to a gas.
(680 calories released)
Disappearing ice cubes in freezer are
a result of sublimation.
Latent heat of deposition
process of changing a vapor to a solid.
(680 calories absorbed)
Frost accumulating in a freezer is
a result of deposition.
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Concept Check 4.2
2. Explain what happens as ice melts to become liquid water.
Melting causes some, but not all, of the hydrogen
bonds to break.
As a result, the water molecules in liquid water
display a more compact arrangement than in ice,
So water in its liquid phase is denser than it is
in the solid phase.
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Concept Check 4.3
*
3. Explain why evaporation is a cooling process.
When water evaporates,
it absorbs heat energy from its surroundings
in order to convert from liquid to vapor.
This causes the surrounding temperature to drop.
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Water Vapor in the Atmosphere
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Humidity: Water Vapor in the Air
Humidity is amount of water vapor in the air.
Absolute humidity is the mass of water vapor in a given volume of air.
The mixing ratio is the mass of water vapor in a unit of air compared to the
remaining mass of dry air.
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Humidity: Water Vapor in the Air
Vapor pressure is defined as the part of the total atmospheric pressure
attributable to its water-vapor content.
Pvapor
pressure from water vapor
= ---------------------------------------total atmospheric pressure
air pressure
Total atmospheric pressure is sum of the pressures from all the gases
Relative humidity indicates how close air is to saturation rather than the
actual quantity of water vapor in the air.
Dew point is the temperature to which air needs to be cooled to reach saturation.
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Vapor Pressure and Saturation
Saturation:
Saturation is the equilibrium point
between evaporation and condensation.
Where the number of water molecules
leaving the surface is equal to the
number of molecules returning.
It increases with temperature.
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Amount of water vapor
required to saturate 1 kg
of dry air.
*
Increases with temperature
-10 0C
2.5 gm/kg
100C
7.5 gm/kg
250C
20 gm/kg
350C
34.5 gm/kg
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Give It Some Thought
*
5. Refer to Table 4-1 to answer this question. How much
more water is contained in saturated air at a tropical
location with a temperature of 400C compared to a polar
Location with a temperature of -100C?
47 g/kg at 40°C compared to
2 g/kg at –10°C,
47 g/kg
-------- =
2 g/kg
23.5
40°C air holds
23.5 times more water vapor
than –10°C air.
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Relative Humidity
Relative humidity is the ratio of the air’s
actual water vapor content
and amount of water vapor required
for saturation
at a specific temperature.
actual water vapor content
RH = -----------------------------water vapor required for saturation
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Relative Humidity
How relative humidity changes:
If water vapor is added, relative humidity goes up.
If water vapor is removed, relative humidity goes down.
100% relative humidity equals saturation.
A decrease in temperature equals an increase in relative humidity.
actual water vapor content
RH = -----------------------------water vapor required for saturation
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Relative Humidity
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Relative Humidity
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Relative Humidity
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A Review of Relative Humidity
As the temperature of air cools
Its ability to hold water vapor
decreases.
The result is that the
relative humidity
of the parcel increases.
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high
low
Variation in
temp
relative humidity
due to change in
temperature
high
R.H.
low
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Concept Check 4.6
4. List three ways that relative humidity changes in nature.
First would involve daily changes in temperatures (daylight versus
nighttime temperatures).
Second would be temperature changes that result as air moves horizontally
from one location to another.
A third way would be temperature changes
caused as air moves vertically in the atmosphere.
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Dew-Point Temperature
Dew point:
The dew point is the temperature air is cooled to reach saturation.
Dew-point temperature is a measure of actual moisture content.
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Concept Check 4.7
*
2. Which measure of humidity,
relative humidity or dew point
best describes the actual quantity of water vapor
in a mass of air?
Dew point.
High dew points imply very moist air and
low dew points correlate with dry air.
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How is Humidity Measured?
Hygrometer:
A hygrometer measures moisture the content of air.
A hair hygrometer operates on the principle that hair changes length in
proportion to changes in relative humidity.
A psychrometer consists of two identical thermometers;
one (dry thermometer) measures air temperature,
the other called the “wet bulb,” measures the dryness of air.
The larger the difference in temperatures between the wet and dry,
the lower the relative humidity (with a table).
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SLING psychrometer
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SLING psychrometer exercise
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Problems
Chapter 4 # 2 (pp 126)
Homework (Example)
Using the standard tables (appendix C Tables C-1 and C-2), Determine
the relative humidity and dew point temperature if the dry-bulb
thermometer reads 300C, and the wet-bulb thermometer reads 150C.
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RH = 16% Dew Point = 10C
Physical processes in the atmosphere
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Adiabatic Temperature Changes
An adiabatic process
is one in which no heat is transferred.
warms.
cools.
When air is compressed, it
When air expands, it
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Let Us Consider an “AIR PARCEL”
An imaginary volume to assist us in
simplifying the physical processes involved
the start point for model-building.
A volume of air enclosed in a thin balloon-like bubble.
A few hundred cubic meters in volume.
It acts independent of the surrounding air.
No heat is transferred into or out of the parcel.
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Adiabatic Temperature Changes
Adiabatic cooling
Cooling occurs when air
moves up and it expands
Unsaturated air
cools at a rate of
10° C/1000m
this is called the dry adiabatic rate.
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Adiabatic Temperature Changes
Condensation is triggered
when air rises high enough
to cool down enough
to reach its saturation point
and clouds form.
This is called the
lifting condensation level.
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Adiabatic Temperature Changes
above the lifting condensation level
When air ascends above
the lifting condensation level,
the rate at which it cools
is reduced.
The slower rate of cooling
is called the wet adiabatic rate
which varies from 5° C/1000m
to 9° C/1000m .
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Concept Check 4.9
*
4. Why does the adiabatic lapse rate of cooling
change when condensation occurs?
When a parcel of air ascends above the lifting condensation level,
the rate of cooling is reduced to the wet adiabatic rate
because the release of latent heat
partially offsets
the cooling due to expansion.
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Concept Check 4.9
5. Why is the wet adiabatic lapse rate not a constant?
The wet adiabatic rate is not a constant figure
because the amount of latent heat released
depends on the quantity of moisture
present in the air.
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Concept Check 4.9
*
6. The contents of an aerosol can are under very high pressure. When you push
the nozzle on such a can, the spray feels cold. Explain
As the gas propellant in the can leaves the nozzle,
it expands rapidly and thus
cools adiabatically.
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Air Wants to Stay Put
Air has a tendency to resist motion.
To move a mass requires that work is done.
To perform work energy must be available.
So, how does AIR get lifted so that it will
expand and cool,
reach saturation and form clouds,
then lift further so that precipitation occurs?
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Processes That Lift Air
Frontal Wedging
Orographic Lifting
Convergence
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Processes That Lift Air
Orographic lifting
occurs when elevated terrains,
such as mountains,
act as barriers to the flow of air.
When air reaches the leeward side, much of its moisture has been lost.
Adiabatic cooling can generate clouds and copious precipitation.
Many of the wettest places in the world are located on windward mountain slopes.
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Processes That Lift Air
Frontal wedging:
Masses of warm and cold air collide, producing fronts.
Cooler, denser air acts as a barrier over which the warmer, less dense air rises.
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Processes That Lift Air
Convergence:
Converging horizontal air flow
results in upward movement.
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Processes That Lift Air
Localized convective lifting:
Unequal surface heating causes localized pockets of air to rise.
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Concept Check 4.10
*
2. Explain why the Great Basin of the western United States is dry.
3.What term is applied to such a situation?
Concept Check 4.10
4. Describe localized lifting.
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*
Concept Check 4.10
2. Explain why the Great Basin of the western United States is dry.
The Great Basin lies on the leeward side of large mountains.
As air ascends the windward slopes of the mountains,
cloud formation and precipitation remove much of the water vapor.
Farther on, as the air descends the leeward slopes,
it is warmed by compression,
making condensation and precipitation unlikely.
What term is applied to such a situation?
Deserts such as the Great Basin are often termed
rain shadow deserts.
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Concept Check 4.10
4. Describe localized lifting.
Localized convective lifting occurs
because of unequal heating of the Earth’s surface.
This causes warm pockets of air to form,
which are less dense than the surrounding air,
making them buoyant and causing them to rise.
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So, How Do We Get Weather?
To get “Weather”
our air parcel needs to rise
so that it will expand
and cool
and reach the condensation level
and condensation
will occur.
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The Critical Weathermaker: Atmospheric Stability
Stability is the property of our air parcel
that describes its tendency to
remain in its original position
to rise (unstable)
to sink (stable)
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The Critical Weathermaker: Atmospheric Stability
environmental lapse rate
The stability of the atmosphere
is determined by
measuring the air temperature
at various heights.
This gives us the “actual” rate
at which temperature changes
as we move up in the
atmosphere.
The lapse rate of the air in
which our “parcel” exists
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The temperature in the atmosphere is never the same
The dry and wet adiabatic lapse rates are constant.
The environmental lapse rate is what “varies.”
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Concept Check 4.11
2. Explain the difference between the environmental lapse rate and adiabatic cooling.
The environmental lapse rate refers to the
temperature change of the atmosphere
as altitude increases.
It is variable and is determined by measurements (vertical
soundings).
Adiabatic cooling, on the other hand, applies
only to the cooling by expansion
that occurs in vertically ascending air.
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Our air parcel
can have three
kinds of motion
depending on
The stability of
the atmosphere
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Our air parcel
can have three
kinds of motion
depending on
The stability of
the atmosphere
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Our air parcel
can have three
kinds of motion
depending on
The stability of
the atmosphere
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Three types of atmospheric stability
By measuring the parameters
of the atmosphere
meteorologists can predict what
air parcels will do in the atmosphere
when lifting occurs.
Will the parcels
- rise
- sink
- stay put
It all depends on the stability of the atmosphere.
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Absolute Stability
Our rising air parcel is always cooler and heavier then the atmosphere it is in.
The air parcel will not rise by itself.
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Absolute Instability
Our rising air parcel is always warmer and
lighter then the atmosphere it is in.
The air parcel will rise by itself.
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Conditional Instability
Extra
Will now rise on its own.
Our rising air parcel is at first cooler than the environment
but after it is “forced up” it becomes warmer than the environment.
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The Critical Weathermaker: Atmospheric Stability
Conditional instability is the most common type of atmospheric instability.
This situation prevails when moist air has an environmental lapse rate
between the dry and wet adiabatic rates.
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Concept Check 4.12
3. List four ways instability can be enhanced.
a) intense solar heating warming the lowermost layer of the atmosphere;
b) the heating of an air mass from below as it passes over a warm surface;
c) general upward movement of air caused by processes such as orographic lifting,
frontal wedging, and convergence;
d) radiation cooling from cloud tops.
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Concept Check 4.12
4. List three ways stability can be enhanced.
a) radiation cooling of Earth’s surface after sunset
b) the cooling of an air mass from below as it traverses a cold surface;
c) general subsidence within an air column.
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End Chapter 4
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