mpg 10e lecture_ch06
TRANSCRIPT
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
2
© 2011 Pearson Education, Inc.
Atmospheric Moisture
• Adiabatic Processes
• Clouds
• The Buoyancy of Air
• Precipitation
• Atmospheric Lifting and Precipitation
• Global Distribution of Precipitation
• Acid Rain
3
© 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
4
© 2011 Pearson Education, Inc.
The Hydrologic Cycle
5
Figure 6-1
© 2011 Pearson Education, Inc.
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
6
Figure 6-2
© 2011 Pearson Education, Inc.
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
7
© 2011 Pearson Education, Inc.
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
8
Figure 6-4
© 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
9
Figure 6-5
© 2011 Pearson Education, Inc.
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
10
Figure 6-6
© 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
11
Figure 6-7
© 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
12
Figure 6-8
© 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%
13
Figure 6-8
Saturation specific humidity at temperature of 20°C
© 2011 Pearson Education, Inc.
Measures of Humidity
• Temperature and relative humidity are inversely related
• Dewpoint temperature
• Sensible temperature
14
Figure 6-9
© 2011 Pearson Education, Inc.
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
15
Figure 6-10
© 2011 Pearson Education, Inc.
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
16
Figure 6-13
© 2011 Pearson Education, Inc.
Clouds
• Definition of clouds
• Influence on radiant energy
• Classification (3 primary cloud forms)– Cirrus clouds
17
Figure 6-15a
© 2011 Pearson Education, Inc.
Clouds
– Stratus clouds
– Cumulus clouds
18
Figure 6-15b
Figure 6-15c
© 2011 Pearson Education, Inc.
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
19
Figure 6-16
© 2011 Pearson Education, Inc.
Fog
20
Figure 6-18
© 2011 Pearson Education, Inc.
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
21
Figure 6-20
© 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
22
Figure 6-21
© 2011 Pearson Education, Inc.
The Buoyancy of Air
• Determination of stability via temperature and lapse rate
• Stable
• Unstable
23
Figure 6-23
Figure 6-24
© 2011 Pearson Education, Inc.
The Buoyancy of Air
• Conditional instability
• Visual determination of instability
24
Figure 6-26
Figure 6-25
© 2011 Pearson Education, Inc.
Precipitation
• Originates from clouds
• Condensation insufficient to form raindrops
• Other processes important
• Collision/coalescence—tiny cloud drops collide and merge to form larger drops
25
Figure 6-27
© 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
26
Figure 6-28
© 2011 Pearson Education, Inc.
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
27
Figure 6-30
© 2011 Pearson Education, Inc.
Atmospheric Lifting
• Four types of atmospheric lifting
28
Figure 6-32
© 2011 Pearson Education, Inc.
Global Distribution of Precipitation
• High precipitation regions, tropics• Low precipitation regions, deserts and poles
29
Figure 6-34
© 2011 Pearson Education, Inc.
Global Distribution of Precipitation
30
Figure 6-35
© 2011 Pearson Education, Inc.
Global Distribution of Precipitation
31
Figure 6-37
© 2011 Pearson Education, Inc.
Acid Rain
• Definition of acid rain• Sources of acid rain• Principal acids—
sulfuric and nitric• Number of hydrogen
ions—pH
32
Figure 6-38
© 2011 Pearson Education, Inc.
Acid Rain
• Distribution of acid rain in the United States
33
Figure 6-39
© 2011 Pearson Education, Inc.
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
© 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
35
© 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
36