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Chapter 12 Lecture Outline
Foundations of
Earth Science
Seventh Edition
Moisture, Clouds,
and Precipitation
Natalie Bursztyn
Utah State University
© 2014 Pearson Education, Inc.
Water’s Changes of State
• Heat energy
• One calorie is the heat necessary to raise the
temperature of one gram of water one degree Celsius
• Latent heat
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Stored or hidden heat
Not derived from temperature change
Heat exchanged between water and surroundings
Important in atmospheric processes
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Water’s Changes of State
• Three states of matter
1. Solid — ice
2. Liquid — water
3. Gas — water vapor
• To change state, heat must be either
absorbed or released
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Water’s Changes of State
• Processes
• Evaporation
– Liquid is changed to gas
– 600 calories per gram added
– Latent heat of vaporization
• Condensation
– Water vapor (gas) is changed to a liquid
– Heat energy is released
– Latent heat of condensation
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Water’s Changes of State
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Water’s Changes of State
• Processes
• Melting
– Solid is changed to a liquid
– 80 calories per gram added
– Latent heat of melting
• Freezing
– Liquid is changed to a solid
– Heat is released
– Latent heat of fusion
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Water’s Changes of State
• Processes
• Sublimation
– Solid is changed directly to a gas
– 680 calories per gram of water are added
• Deposition
– Water vapor (gas) changed to a solid
– Heat is released
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Water’s Changes of State
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Water’s Changes of State
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Focus Question 12.2
• If the temperature remains unchanged and
the amount of water vapor in the air
decreases, how does relative humidity
change?
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Humidity: Water Vapor in the Air
• Amount of water vapor in the air
• Saturated air
– Air filled to capacity with water vapor
– Capacity is temperature dependent
– Warm air has a much greater capacity
• Water vapor adds pressure
– Vapor pressure
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Humidity: Water Vapor in the Air
• Measuring humidity
• Mixing ratio
– Mass of water vapor in a unit of air compared to the
remaining mass of dry air
– Measured in g/kg
• Relative humidity
– Ratio of the air’s actual water vapor content compared with
the amount of water vapor required for saturation
• (at that temperature and pressure)
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Humidity: Water Vapor in the Air
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Humidity: Water Vapor in the Air
• Measuring humidity
•
Relative humidity
– Expressed as a percent
– Saturated air
• Content equals capacity
• Has 100% relative humidity
•
Relative humidity can be changed in two ways
1. Changing the air temperature
• Lowering the temperature raises the relative humidity
2. Dew point temperature
• Temperature to which a parcel of air would need to be
cooled to reach saturation
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Humidity: Water Vapor in the Air
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Humidity: Water Vapor in the Air
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Humidity: Water Vapor in the Air
• Two types of hygrometers are used to
measure humidity:
1. Psychrometer
• Compares temperatures of wet-bulb
thermometer and dry-bulb thermometer
• Greater difference = lower relative humidity
• If air is saturated, both thermometers read the
same temperature
2. Hair hygrometer
• Reads the humidity directly
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Humidity: Water Vapor in the Air
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Focus Question 12.2
• If the temperature remains unchanged and
the amount of water vapor in the air
decreases, how does relative humidity
change?
• Relative humidity decreases
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The Basis of Cloud Formation: Adiabatic
Cooling
• Adiabatic temperature changes
• Air is compressed
– Motion of air molecules increases
– Air warms
– Descending air is compressed
• Air expands
– Air parcel does work on the surrounding air
– Air cools
– Rising air expands
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The Basis of Cloud Formation: Adiabatic
Cooling
• Adiabatic rates
• Dry adiabatic rate
– Unsaturated air
– Rising air expands & cools at 1°C/100 m
– Descending air compresses and warms at 1°C/100 m
• Wet adiabatic rate
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Begins at condensation level
Air has reached the dew point
Condensation is occurring and latent heat is being liberated
Heat released by condensing water reduces cooling rate
Rate varies from 0.5°C to 0.9°C/100 m
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The Basis of Cloud Formation: Adiabatic
Cooling
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Focus Question 12.4
• How do orographic lifting and frontal wedging
force air to rise?
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Processes That Lift Air
• Orographic lifting
• Elevated terrains act as barriers
• Result can be a rainshadow desert
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Processes That Lift Air
• Frontal wedging
• Cool air acts as a barrier to warm air
• Fronts are part of middle-latitude cyclones
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Processes That Lift Air
• Convergence
• Air flows together and rises
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Processes That Lift Air
• Localized convective lifting
• Unequal surface heating causes pockets of air to rise
because of their buoyancy
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Focus Question 12.4
• How do orographic lifting and frontal wedging
force air to rise?
• Elevated terrain and cool air act as barriers that force
air masses to move up to get over them.
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The Weathermaker: Atmospheric Stability
• Stability of air determines:
• Type of clouds that develop
• Intensity of the precipitation
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The Weathermaker: Atmospheric Stability
• Types of stability
• Stable air
– Resists vertical displacement
• Cooler and denser than surrounding air
• Wants to sink
– No adiabatic cooling
– Widespread clouds with little vertical thickness
– Precipitation is light to moderate
– Absolute stability
• Environmental lapse rate less than wet adiabatic rate
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The Weathermaker: Atmospheric Stability
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The Weathermaker: Atmospheric Stability
• Absolute instability
• Acts like a hot air balloon
• Rising air
– Warmer and less dense than surrounding air
– Rises until it reaches altitude with same temperature
• Adiabatic cooling
• Environmental lapse rate greater than dry adiabatic rate
• Clouds are often towering
• Conditional instability
– Atmosphere is stable for an unsaturated parcel of air but
unstable for a saturated parcel
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The Weathermaker: Atmospheric Stability
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The Weathermaker: Atmospheric Stability
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Focus Question 12.6
• What is the function of condensation nuclei in
cloud formation?
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Condensation and Cloud Formation
• Condensation
• Water vapor changes to a liquid and forms dew, fog,
or clouds
• Water vapor requires a condensation surface
– On the ground
• Grass, a car window, etc.
– In the air are tiny bits of particulate matter called
condensation nuclei
• Dust, smoke, ocean salt crystals, etc.
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Condensation and Cloud Formation
• Clouds
• Made of millions and millions of
– Minute water droplets, or
– Tiny crystals of ice
• Classification based on form
– Cirrus
• High, white, thin
– Cumulus
• Globular cloud masses
– Stratus
• Sheets or layers that cover much of the sky
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Condensation and Cloud Formation
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Condensation and Cloud Formation
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Condensation and Cloud Formation
• Clouds classified based on height
• High clouds
– Above 6000 m
• Cirrus, cirrostratus, cirrocumulus
• Middle clouds
– 2000 to 6000 m
• Altostratus and altocumulus
• Low clouds
– Below 2000 m
• Stratus, stratocumulus, and nimbostratus (nimbus
means “rainy”)
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Condensation and Cloud Formation
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Condensation and Cloud Formation
• Clouds of vertical development
– From low to high altitudes
– Called cumulonimbus
– Often produce rain showers and thunderstorms
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Focus Question 12.6
• What is the function of condensation nuclei in
cloud formation?
• The condensation nuclei are particles that allow
microscopic cloud droplets to form in suspension in
the atmosphere. Once condensation takes place,
initial cloud droplet growth rate is rapid.
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Focus Question 12.7
• Define fog.
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Fog
• Fog is a cloud with its base at or near the
ground
• Considered an atmospheric hazard
• Most fogs form because of
– Radiation cooling, or
– Movement of air over a cold surface
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Fog
• Fogs caused by cooling
• Advection fog
– Warm, moist air moves over a cool surface
• Radiation fog
– Earth’s surface cools rapidly
– Forms during cool, clear, calm nights
• Upslope fog
– Humid air moves up a slope
– Adiabatic cooling occurs
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Fog
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Fog
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Fog
• Evaporation fogs
• Steam fog
– Cool air moves over warm water
– Water has a steaming appearance
• Frontal fog, or precipitation fog
– Forms during frontal wedging when warm air
lifted over colder air
– Rain evaporates to form fog
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Fog
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Fog
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Focus Question 12.7
• Define fog.
• Cloud with its base at or near the ground.
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How Precipitation Forms
• Cloud droplets
• < 20 micrometers (0.02 millimeter) in diameter
• Fall incredibly slowly
• Formation of precipitation
• Bergeron process
– Temperature in the cloud is supercooled
– Ice crystals collect water vapor
– Large snowflakes form and fall to the ground or melt and turn
to rain
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How Precipitation Forms
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How Precipitation Forms
• Formation of precipitation
• Collision-coalescence process
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Warm clouds
Large hygroscopic condensation nuclei
Large droplets form
Droplets collide with other droplets during their descent
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How Precipitation Forms
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Focus Question 12.9
• Explain why snow can sometimes reach the
ground as rain, but the reverse does not
occur.
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Forms of Precipitation
• Rain and drizzle
– Rain
• Droplets have at least a 0.5 mm diameter
– Drizzle
• Droplets have less than a 0.5 mm diameter
• Snow
– Ice crystals, or aggregates of ice crystals
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Forms of Precipitation
• Sleet and glaze
– Sleet
• Small particles of ice in winter
• Occurs when warmer air overlies colder air
• Rain freezes as it falls
– Glaze, or freezing rain
• Impact with a solid causes freezing
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Forms of Precipitation
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Forms of Precipitation
• Hail
– Hard rounded pellets
• Concentric shells
• Most diameters range from 1 to 5 cm
– Formation
• In large cumulonimbus clouds
• Layers of freezing rain are caught in violent upand down-drafts
• Pellets fall when they become too heavy
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Forms of Precipitation
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Forms of Precipitation
• Rime
– Forms on cold surfaces
• Freezing of supercooled fog
• Freezing of cloud droplets
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Focus Question 12.9
• Explain why snow can sometimes reach the
ground as rain, but the reverse does not occur.
– As precipitation falls it can increase in temperature,
becoming rain, but in the reverse when rain freezes, it
becomes ice as sleet or glaze – not snow.
– Snow forms under conditions where there is little
moisture in the air, which allows it to be light and
fluffy. While it is possible to have limited moisture at
or near ground level, this condition is not possible
when precipitation is already falling.
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Measuring Precipitation
• Rain
– Easiest form to measure
– Measuring instruments
• Standard rain gauge
– Uses a funnel to collect rain
– Cylindrical tube measures in cm or inches
• Radar is also used to measure the rate of
rainfall
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Measuring Precipitation
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Measuring Precipitation
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Measuring Precipitation
• Snow has two measurements:
1. Depth
2. Water equivalent
• General ratio is 10 snow units to 1 water unit
• Varies widely
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