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Chapter 3 – Moisture, Latent Heat, Fog and Stability
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Overview
 Composition of the atmosphere
 Dry and with water vapor.
 Vertical structure of the atmosphere.
 Troposphere, stratosphere, mesosphere, thermosphere.
 Definition of lapse rate.
 The adiabatic process and the related lapse rates.
 Dry adiabatic lapse rate.
 Wet adiabatic lapse rate.
 Properties of water.
 Phases of water and the change among them.
 Latent heat of phase changes.
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Overview (continued)
 How moisture in the atmosphere is reported.
 Absolute humidity.
 Relative humidity.
 Dew point.
 Fog types—formation and dissipation.
 Atmospheric stability.
 Absolute and conditional stability.
 Absolute instability.
 An Example: The Chinook or Foehn wind.
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Composition of the Atmosphere
 Major constituents.
 Nitrogen.
 Oxygen.
 The rest is mostly Argon,
with small amounts of:
 Carbon dioxide.
 Neon.
 Helium.
 Methane.
 Variable amount of water
vapor.
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Structure of the Atmosphere
 The atmosphere has four major
layers and three boundaries.
 Primary distinguishing feature
is the temperature change with
height.
 Most of the water and most of
the weather occurs in the
troposphere, the lowest layer.
Courtesy of NOAA
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Lapse Rates
 The way the temperature decreases or increases with altitude
in the atmosphere is called the lapse rate.
 The environmental lapse rate is what is seen at any given time
and place and changes with altitude. Those shown on the
previous slide are averages—in mid-latitudes it averages
3.5°F/1000 ft. in the lower troposphere.
 The dry adiabatic lapse rate is how fast unsaturated air cools as it
is pushed up. Its nominal value is -5.5°F/1000 ft.
 The wet adiabatic lapse rate is how fast saturated air cools as it is
pushed up. Its nominal value is -3.2°F/1000 ft.
 The reason for the differences between dry and wet rates is
shown later in this chapter.
 The definition of adiabatic is on the next slide.
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More About Lapse Rates
 The average environmental lapse
rate is shown in the top figure. The
air is NOT rising.
 Adiabatic lapse rates are named for
the adiabatic process.
©1997, USA Today. Reprinted with permission
 Adiabatic processes occur without the
addition or loss of any energy from/to
the outside.
 Air rising cools adiabatically.
 Air sinking warms adiabatically.
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The Properties of Water
 The properties of water are uniquely suited to life.
 It expands when frozen, making ice float.
 It changes phase at temperatures found on the Earth.
 It is an excellent solvent.
 Its properties are also well suited to weather processes.
 It releases a lot of energy when it condenses or freezes.
 It is an efficient absorber of infrared energy.
 It can transport energy from one location to another.
 There is an abundant supply.
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Properties of Water (continued)
 The amount of water vapor in air (by weight) varies:
 From less than 1% (very dry air).
 To about 4% (very moist air).
 Moist air is lighter than dry air.
 Nitrogen (79%) has a molecular weight of 28.
 Oxygen (21%) has a molecular weight of 32.
 Argon (<1%) has a molecular weight of 14.
 Average: 28.7.
 Water (H2O) has a molecular weight of 18.
 Every molecule of air replaced by one of water lowers the
molecular weight of the air by 10.7 for every 18 grams of water.
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Phase Changes and Latent Heat
 The phases of water are shown below, along with the names
of the phase-change processes.
 While water melts or evaporates, it stores large amounts of
energy, called latent heat.
540 + 80 = 620 calories per gram
540 + 80 = 620 calories per gram
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The Importance of Latent Heat
 The latent heat tied up in water (80 cal/g), and the even larger
amounts added in water vapor (540 cal/g), is carried with the
water or vapor as it is transported in the atmosphere.
 This transport of latent heat is added to conduction,
convection, and radiation as a means of energy transport.
 The condensation of water is a key driver of many kinds of
weather phenomena, such as thunderstorms and hurricanes,
because the released latent heat upon condensation provides
energy to the systems.
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Measures of Humidity
 The concentration of water vapor in the atmosphere is
reported in three primary ways:
 Absolute humidity is the mass of water vapor in a given mass
of air, normally given as grams per kilogram (g/Kg) of dry air.

The mass the air can hold increases with increasing temperature.
 Relative humidity (RH) is a ratio of the mass of water vapor in
the air to the maximum it can hold at that temperature (as a
percent).
 The dew point is the temperature the air would have to be
cooled to in order to reach saturation (100% RH).
 A fourth, seen occasionally, is the dew point depression—the
temperature minus the dew point.
 Humidity is measured with a Psychrometer.
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Relation of Moisture Variables
g/Kg
100% RH
100% RH
Air Parcel
30% RH
Dew Point
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Plot of Moisture Variables
RH
Temp
Relative Humidity (%)
Temperature and Dew Point (°F)
3-day plot of Temperature, Dew Point and Relative Humidity
DP
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Station Model—Moisture
 This station model adds dew
point, in °F.
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 The dew point is 59°F.
965
-23\
 The dew point depression is
therefore 8°F, so the relative
humidity is about 75%, as
estimated from the graph in
slide 14.
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Fog
 The principles in this chapter help us understand fog.
 Fog forms in many ways, can last for hours or days.
 Fog forms by either a decrease in temperature or the
addition of moisture (raising the dew point).
 The basic types to be discussed are:
 Radiation fog.
 Advection fog.
 Precipitation fog.
 Steam fog (sea smoke).
 Up-slope fog.
 Valley fog.
Temperature decrease.
Temperature decrease.
Moisture added.
Moisture added.
Temperature decrease.
Similar to radiation fog.
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Radiation Fog
Air near ground cools to the dew point, water condenses.
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Advection Fog
Warm air moving over a cool surface lowers temperature.
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Precipitation Fog
Evaporating water increases humidity to saturation.
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Steam Fog (Sea Smoke)
Water evaporates, raising the dew point temperature.
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Upslope Fog
Air is raised orographically, cooling to the dew point.
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1. Wind blows humid air up hills or mountains.
2. As the air rises, it cools to the dew point, fog drifts up the hill; widespread upslope fog is
common on the Great Plains, where the land slopes gently upward toward the Rockies.
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Valley Fog
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An especially persistent form of radiation
fog.
1
2
1. In valleys, especially in the west during the winter, radiation fog can become more than 1,500
feet thick.
2. Weak, winter sun isn’t strong enough to evaporate the fog completely, but might warm the
ground enough for a layer of fog up to around 500 feet above the ground to evaporate.
3. Such fogs can last for days, until a storm comes along with strong winds to push out the
cold air.
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Fog Summary
Fog Type
Radiation
Valley
Advection
Upslope
Precipitation
Typical
Occurrence
Clear night
with
radiational
cooling in low
wet areas.
During
Winter in
valleys with
radiational
cooling.
Warm, moist
air
transported
over colder
surface
Warm, moist
air lifted by
upslope air
flow, cooled
by expansion
to saturation.
Rain from
warm air
falling
through
colder air at
surface
saturates cold
air at surface.
Winds
3-5 knots for
gentle mixing
3-5 knots for
gentle mixing
4-15 knots
Sufficient for
Lifting
Not
Important
Dissipation
Daytime
warming of
ground warms
air through
conduction
and mixing.
Advent of
storm with
strong winds
replaces
saturated air
with dryer air.
Wind changes
direction so
warm air flow
is eliminated.
Sometimes
higher winds
lift off
ground.
Daytime
warming
evaporates
fog, or wind
may change
direction.
Frontal
Passage or
end of
precipitation.
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Sea Smoke
Cold air blows
over warmer
water, taking
on moisture
to saturation.
4-15 knots
Daytime
warming or
wind changes
direction
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Dew and Frost
Dew—Dew point >32° F
Frost—Dew point <=32° F
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Atmospheric Stability (1 of 3)
 Atmospheric stability can have one of three states. The first is
absolute stability. An air parcel lifted to any height will return
to its original position as it is heavier than the surrounding air.
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Atmospheric Stability (2 of 3)
 The second type is absolute instability. An air parcel will rise
on its own, since it is lighter than surrounding air, and only
stop when it reaches stable air, such as the tropopause.
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Atmospheric Stability (3 of 3)
 The third type is conditional stability. Air pushed up a little is
stable, but pushed up more becomes unstable.
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How Stability Can Change
 Atmospheric stability is
constantly changing.
 This example shows a
progression during a day.
 The top figure shows an earlymorning inversion, with stable
air.
 In the bottom figure, the
lower atmosphere has heated,
becoming unstable.
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How is Air “Pushed up”?
 In the stability discussion, we talked about air being pushed
up. How does that happen?
 Orographic lifting, such as the wind blowing up the side of a




mountain.
Frontal wedging, where the cold air pushes under the warm air
(cold front) or the warm air rides over the cool air (warm front).
Surface convergence, where surface winds from different
directions meet and the air must rise since it can’t go down.
Upper air divergence, where air below rises to replace that lost.
Convection, where the air is inherently unstable.
 We will see examples of these in future chapters, but one
example is shown on the next slide.
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Chinook Winds
 Chinook is a warm wind on
the lee side of mountains.
 Air is lifted orographically.
 It cools at the dry adiabatic
lapse rate.
 When saturated, it cools at
the wet adiabatic lapse rate.
 On the lee side, it warms at
the dry adiabatic lapse rate.
 Other places have other
©2004, US Power Squadrons. Reprinted with permission
names (Foehn in Europe).
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Summary (1 of 3)
 The Atmosphere is primarily Nitrogen and Oxygen.
 With small amounts of Argon and other gases.
 A variable concentration of water vapor (1 to 4 percent).
 Temperature profile varies with altitude in four layers:
 Decreasing in the troposphere (top is tropopause).
 Increasing in the stratosphere (top is stratopause).
 Decreasing in the mesosphere (top is mesopause).
 Increasing in the thermosphere (blends into space).
 Temperature change with height is called the lapse rate.
 Environmental lapse rate: What is currently in the atmosphere.
 Adiabatic lapse rate: How air cools when ascending or warms
when descending.
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Summary (2 of 3)
 Water is a very important substance in the atmosphere.
 It changes state at normal temperatures.
 It carries very large amounts of latent heat.
 It is an efficient absorber of infrared energy.
 Phases of water are solid (ice), liquid, and gas (vapor).
 Ice <-> Water: Melting or freezing. Latent heat 80 cal/g.
 Water <-> Vapor: Evaporation or condensation. 540 cal/g.
 Ice <-> Vapor: Sublimation or Deposition. 620 cal/g.
 Latent heat: Energy needed to melt ice or evaporate water.
 Release of this latent heat by condensation is a major driver
of atmospheric processes.
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Summary (3 of 3)
 Humidity is a measure of water vapor in the atmosphere.
 Absolute humidity is concentration in g of water per kg of dry air.
 Saturation absolute humidity is maximum air can hold at a temperature.
 Relative humidity is the ratio of absolute to saturation absolute humidity.
 Dew point is the temperature air must cool to for saturation to occur.
 Stability of the atmosphere is its tendency to rise.
 Absolute stability means air, if lifted, will return to the original height.
 Conditional stability means air lifted far enough will continue to rise.
 Absolute instability means air will rise on its own.
 Air can be lifted four ways.
 Orographic lifting is wind blowing up a mountain.
 Frontal wedging is cold air pushing up warm air.
 Surface convergence forces air between to rise.
 Convection.
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Chapter 3 Questions
QUESTION
ANSWER
In the lower atmosphere, the temperature
generally ___________ with altitude.
Decreases.
The troposphere is where most of the
weather occurs because:
It contains most of the moisture.
The two most abundant gasses in the
atmosphere are:
Nitrogen and Oxygen.
A temperature increase with altitude is
called what?
An inversion.
A warm, dry wind on the east side of the
Rockies is called what?
A Chinook wind.
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Chapter 3 Questions
QUESTION
ANSWER
When water vapor condenses heat energy
is:
Released into the atmosphere.
The energy that is released when water
freezes or vapor condenses is called:
Latent heat.
Saturation can be accomplished by what
two methods?
Lowering the temperature or adding
moisture.
The temperature at which water vapor
begins to condense is called:
The dew point.
When the relative humidity of air is 100%,
it is said to be:
Saturated.
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Chapter 3 Questions
QUESTION
ANSWER
A change in the temperature or density of
air without the addition or release of heat
energy is called:
An adiabatic process.
The term describing the tendency of lifted
air to return to its former level is:
Absolute Stability.
The term describing the tendency of air to
rise on its own is:
Absolute Instability.
The term describing the tendency of air to
continue to rise after being lifted to some
higher level is:
Fog that forms at night under clear skies
with little wind is called:
Conditional stability.
Radiation fog.
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End of Chapter 3
 Are there any questions?
 The next three chapters
make up Part II, Sensible
Weather (what we see and
feel).
 Chapter 4 covers Air Masses,
Fronts, and Cyclones
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