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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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

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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

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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

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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

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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

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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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