Ch06

Chapter 6: Atmospheric Moisture

McKnight’s Physical Geography: A Landscape Appreciation,

Tenth Edition, Hess

© 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


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


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


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


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


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



• 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



• 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



• Temperature and relative humidity are inversely related

• Dewpoint temperature• Sensible temperature

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Figure 6-9


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


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


Clouds

• Definition of clouds• Influence on radiant

energy• Classification (3 primary

cloud forms)– Cirrus clouds

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Figure 6-15a


Clouds

– Stratus clouds

– Cumulus clouds

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Figure 6-15b

Figure 6-15c


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


Fog

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Figure 6-18


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

21Figure 6-20


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


The Buoyancy of Air

• Determination of stability via temperature and lapse rate

• Stable• Unstable

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Figure 6-23

Figure 6-24


The Buoyancy of Air

• Conditional instability

• Visual determination of instability

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Figure 6-26

Figure 6-25


Precipitation

• Originates from clouds• Condensation insufficient to

form raindrops• Other processes important• Collision/coalescence—tiny

cloud drops collide and merge to form larger drops

25Figure 6-27


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


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


Atmospheric Lifting

• Four types of atmospheric lifting

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Figure 6-32


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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Figure 6-35



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Figure 6-37


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


Acid Rain

• Distribution of acid rain in the United States

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Figure 6-39


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

atmosphere34


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 stability35


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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Ch06

Technology

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