The document discusses atmospheric moisture and its impacts. It describes the hydrologic cycle which governs the movement of water across Earth. Water exists in three phases - solid, liquid, and gas - and undergoes phase changes through processes like evaporation, condensation, and sublimation. Various measures of humidity quantify the amount of water vapor in air, including relative humidity. Clouds form through condensation and influence climate. Precipitation forms through processes like collision-coalescence and the Bergeron process. Global precipitation patterns are uneven, with wet tropical regions and dry deserts.

1. Chapter 6: Atmospheric
Moisture
McKnight’s Physical Geography:
A Landscape Appreciation,
Tenth Edition, Hess

2. © 2011 Pearson Education, Inc.
Atmospheric Moisture
• The Impact of Moisture on the Landscape
• The Hydrologic Cycle
• The Nature of Water: Commonplace but
Unique
• Phase Changes of Water
• Water Vapor and Evaporation
• Measures of Humidity
• Condensation
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Atmospheric Moisture
• Adiabatic Processes
• Clouds
• The Buoyancy of Air
• Precipitation
• Atmospheric Lifting and Precipitation
• Global Distribution of Precipitation
• Acid Rain
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4. © 2011 Pearson Education, Inc.
The Impact of Moisture on the
Landscape
• Formation of fog, haze, clouds, and precipitation
• Short term impacts of precipitation—floods
• Longer term impacts (i.e., caves) on Earth’s
surface
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The Hydrologic Cycle
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Figure 6-1

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The Nature of Water:
Commonplace but Unique
• Chemistry of water
– Atoms and molecules
– Two hydrogen and one
oxygen molecule (H2O)
– Covalent bonds
– Electrical polarity of
water molecule
– Hydrogen bonds
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Figure 6-2

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The Nature of Water:
Commonplace but Unique
• Important properties of water
– Exists as a liquid at most points on Earth’s surface
– Expands when it freezes; less dense than liquid
water; ice floats in water
– Hydrogen bonding creates surface tension, a “skin” of
molecules giving water a stickiness quality
– Capillarity
– Good solvent
– High specific heat
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Phase Changes of Water
• Water typically exists in
three states
– Solid: ice
– Liquid: liquid water
– Gas: water vapor
• Latent heat is required
to convert water to its
different phases
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Figure 6-4

9. © 2011 Pearson Education, Inc.
Phase Changes of Water
• Phase change processes
– Condensation: gas to
liquid
– Evaporation: liquid to gas
– Freezing: liquid to solid
– Melting: solid to liquid
– Sublimation: solid to gas
and gas to solid
• Latent heat required for
each process
• Latent heat as a source
of atmospheric energy
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Figure 6-5

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Water Vapor and Evaporation
• Properties of water vapor
– Colorless, odorless,
invisible
– Air feels sticky
• Evaporation
– Warmer temperatures
evaporate more water
– Vapor pressure
– Windiness reduces
evaporation
– Evapotranspiration
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Figure 6-6

11. © 2011 Pearson Education, Inc.
Measures of Humidity
• Humidity—amount of water
vapor in the air
• Absolute humidity—mass of
vapor for a given volume of
air
• Specific humidity—mass of
water vapor for a given mass
of air
• Vapor pressure—contribution
of water vapor to total
atmospheric pressure
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Figure 6-7

12. © 2011 Pearson Education, Inc.
Measures of Humidity
• Relative humidity—how close
the air is to saturation
• Saturation represents the
maximum amount of water
vapor the air can hold
• Saturation depends on
temperature
• Saturation vapor pressure
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Figure 6-8

13. © 2011 Pearson Education, Inc.
Measures of Humidity
• Relative humidity—example
calculation
– Assume air at 20°C has 10 g of
water vapor per kg of dry air
– To calculate relative humidity,
use the curve to get saturation
conditions at 20°C (15 g/kg)
– RH = (10g/15g) X 100% = 66.7%
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Figure 6-8
Saturation specific humidity at
temperature of 20°C

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Measures of Humidity
• Temperature and relative
humidity are inversely related
• Dewpoint temperature
• Sensible temperature
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Figure 6-9

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Condensation
• Conversion of vapor to liquid water
• Surface tension makes it nearly
impossible to grow pure water
droplets
• Supersaturated air
• Need particle to grow droplet
around, a cloud condensation
nuclei
• Liquid water can persist at
temperatures colder than 0°C
without a nuclei—supercooled
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Figure 6-10

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Adiabatic Processes
• Definition of adiabatic
process
• Dry adiabatic lapse rate
• Lifting condensation level
(LCL)
• Saturated adiabatic lapse
rate
• Parcel lapse rates versus
environmental lapse rate
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Figure 6-13

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Clouds
• Definition of clouds
• Influence on radiant
energy
• Classification (3 primary
cloud forms)
– Cirrus clouds
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Figure 6-15a

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Clouds
– Stratus clouds
– Cumulus clouds
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Figure 6-15b
Figure 6-15c

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Clouds
• Cloud types
– High clouds (over 6 km)
– Middle clouds (from 2 to
6 km)
– Low clouds (less than 2
km)
– Clouds of vertical
development
• Grow upward from low
bases to heights of over
15 km occasionally
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Figure 6-16

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Fog
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Figure 6-18

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Dew and Frost
• Dew
– Usually originates from terrestrial
radiation
– Moisture condensation on surfaces
that have been cooled to saturation
– Will appear as water droplets
• Frost
– Simply a cloud on the ground
– Occurs when air temperature lowers
to saturation point, when the
saturation point is below 0°C (32°F)
– Will appear as large numbers of
small white crystals
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Figure 6-20

22. © 2011 Pearson Education, Inc.
The Buoyancy of Air
• Definition of buoyancy
• Stable air—parcel is
negatively buoyant, will
not rise without an
external force
• Unstable air—parcel is
positively buoyant, will rise
without an external force
• Conditional instability
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Figure 6-21

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The Buoyancy of Air
• Determination of
stability via temperature
and lapse rate
• Stable
• Unstable
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Figure 6-23
Figure 6-24

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The Buoyancy of Air
• Conditional instability
• Visual determination of
instability
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Figure 6-26
Figure 6-25

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Precipitation
• Originates from clouds
• Condensation insufficient to
form raindrops
• Other processes important
• Collision/coalescence—tiny
cloud drops collide and
merge to form larger drops
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Figure 6-27

26. © 2011 Pearson Education, Inc.
Precipitation
• Ice crystal formation
– Bergeron process
– Ice crystals and supercooled
droplets coexist in cold
clouds
– Ice crystals attract vapor,
supercooled drops evaporate
to replenish the vapor
– Ice crystals fall as snow or
rain
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Figure 6-28

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Precipitation
• Types of precipitation
– Rain: liquid water
– Snow: cloud ice crystals
– Sleet: snow melted and
frozen again before hitting
land, ice pellets
– Glaze (Freezing Rain): water
falls as liquid, freezes to
surfaces
– Hail: strong updrafts are
required
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Figure 6-30

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Atmospheric Lifting
• Four types of atmospheric lifting
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Figure 6-32

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Global Distribution of
Precipitation
• High precipitation regions, tropics
• Low precipitation regions, deserts and poles
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Figure 6-34

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Global Distribution of
Precipitation
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Figure 6-35

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Global Distribution of
Precipitation
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Figure 6-37

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Acid Rain
• Definition of acid rain
• Sources of acid rain
• Principal acids—
sulfuric and nitric
• Number of hydrogen
ions—pH
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Figure 6-38

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Acid Rain
• Distribution of acid rain in the United States
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Figure 6-39

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Summary
• Moisture can impact the landscape in a variety of
ways, including fog, haze, and precipitation
• The hydrologic cycle shows the balance between
water removed from the oceans and water returned
by precipitation
• Water has a number of unique properties
• Water vapor is the gas form of water
• Evaporation rates change as surrounding
atmospheric conditions change
• There are several measures of vapor content in the
atmosphere 34

35. © 2011 Pearson Education, Inc.
Summary
• There are several measures of vapor content in the
atmosphere, called humidity measurements
• Condensation is the process by which vapor is
converted to liquid
• Adiabatic processes explain changes in parcel
temperature without the addition or subtraction of
heat to the parcel
• Clouds are a visual identification of saturation
• Air has buoyancy associated with it that describes
its stability
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36. © 2011 Pearson Education, Inc.
Summary
• Many processes are responsible for precipitation
• There are five primary types of precipitation
• Atmospheric lifting occurs through four primary
mechanisms
• The most highly variable rainfall worldwide occurs
over deserts
• Tropical regions are generally wet
• Acid rain affects the Northeast and results from
compounds released into the air by humans
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