Download Moisture, Clouds, and Precipitation Earth Science, 13e Chapter 17

1/19/2015
Moisture, Clouds, and
Precipitation
Earth Science, 13e
Chapter 17
Stanley C. Hatfield
Southwestern Illinois College
© 2012 Pearson Education, Inc.
Changes of state of water, H2O
Water is the only substance in
atmosphere that exists as 3 states of
matter: a solid (ice), liquid (water) and
gas (water vapor)
To change state, heat must be
• Absorbed, or
• Released
• When water changes state, it only changes
the distances and interactions among the
water molecules.
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Changes of state of water
Heat energy
• Measured in calories – one calorie is the
heat necessary to raise the temperature of
one gram of water one degree Celsius
• How many calories of heat are absorbed by 1 gram of
water, when a 10oC temperature rise occurs?
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A random question?
Suppose you had a glass of
ice water and warmed it, the
temperature of the ice-water
mixture stays at 0oC until all
the ice has melted
• Where does the energy go?
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Changes of state of water
Heat energy
• Latent heat
• Stored or hidden heat (added energy goes into
breaking molecular attractions between the
water molecules in the ice)
• Not derived from temperature change (just like
our example)
• Important in atmospheric processes
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Changes of state of water
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Changes of state of water
Processes
• Evaporation
• Liquid is changed to gas
• 600 calories per gram of water are added –
called latent heat of vaporization
• Heat energy is absorbed
• Condensation
• Water vapor (gas) is changed to a liquid
• Heat energy is released – called latent heat
of condensation
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Changes of state of water
Processes
• Melting
• Solid is changed to a liquid
• 80 calories per gram of water are added –
called latent heat of melting
• Freezing
• Liquid is changed to a solid
• Heat is released – called latent heat of
fusion
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Changes of state of water
Processes (least familiar with)
• Sublimation
• Solid is changed directly to a gas (e.g., ice
cubes shrinking in a freezer)
• 680 calories per gram of water are added
• Deposition
• Water vapor (gas) changed to a solid (e.g.,
frost in a freezer compartment)
• Heat is released
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Humidity
Amount of water vapor in the air
• Saturated air is air that is filled with
water vapor to capacity
• Capacity is temperature dependent –
warm air has a much greater capacity
• Water vapor adds pressure (called
vapor pressure) to the air
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Humidity
Measuring humidity (ex: p. 493)
• Mixing ratio – measure how humid a parcel of air is
• Mass of water vapor in a unit of air compared
to the remaining mass of dry air
• Often measured in grams per kilogram
• 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)
• How near the air is to saturation
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Humidity
Measuring humidity
• Relative humidity
• Relative humidity can be changed in two ways
• 1. Add or subtract moisture to the air
• Adding moisture raises the relative humidity
• Removing moisture lowers the relative
humidity
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Humidity
Measuring humidity
• Relative humidity
• Relative humidity can be changed in two ways
• 2. Changing the air temperature
• Lowering the temperature raises the relative
humidity
• Dew point temperature
• Temperature to which a parcel of air would need
to be cooled to reach saturation
• Refer to p. 494 for example
• What temperature would the dew point be
for 10 g water vapor, in 1kg of air, at 15oC?
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Relative humidity changes at
constant temperature
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Relative humidity changes at
constant water-vapor content
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Humidity
Measuring humidity
• More on Dew point temperature
• Cooling the air below the dew point
causes condensation
• e.g., dew, fog, or cloud formation
• Water vapor requires a surface to
condense on
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Adiabatic heating/cooling in
cloud formation
Like air in a bike pump when in use…
• Adiabatic temperature changes occur when
• Air is compressed
• Motion of air molecules increases
• Air will warm
• Descending air is compressed due to increasing
air pressure
• Air expands
• Air will cool
• Rising air will expand due to decreasing air
pressure
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FYI: Adiabatic heating/cooling
• Adiabatic rates
• Dry adiabatic rate
• Unsaturated air
• Rising air expands and cools at 1°C per
100 meters (5.5°F per 1,000 feet)
• Descending air is compressed and warms
at 1°C per 100 meters
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FYI: Adiabatic heating/cooling
• Adiabatic rates
• Wet adiabatic rate
• Commences at condensation level
• Air has reached the dew point
• Condensation is occurring and latent heat is
being liberated
• Heat released by the condensing water reduces
the rate of cooling
• Rate varies from 0.5°C to 0.9°C per 100 meters
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Adiabatic cooling
of rising air
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4 Processes that lift air
1. Orographic lifting
• Air is forced to rise over a mountainous
barrier
• Result can be a rainshadow desert
• Watch video 7.2a/7.2b –orographic uplift and rainshadow desert
2. Frontal wedging
• Warmer, less dense air is forced over
cooler, denser air.
• Fronts are part of the storm systems
called middle-latitude cyclones (more later)
• Watch video 7.2c – 7.2f – cold fronts and warm front battles
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Orographic lifting
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Frontal wedging
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Processes that lift air
3. Convergence where the air is flowing
together and rising (low pressure)
• can’t go down, so goes up (p. 501)
4. Localized convective lifting
• Localized convective lifting occurs where
unequal heating on land causes pockets of
air to rise because of their buoyancy
• Like heating air over a parking lot, compared to the air in a
nearby wooded park.
• Soaring birds use these pockets of air to rise to great heights.
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Stability of air (affect size of clouds
and amount of precipitation)
Types of stability
• A. Stable air – Absolute Stability
• Resists vertical movement
• Cooler than surrounding air
• Denser than surrounding air
• Wants to sink
• If the parcel of air wants to rise = unstable air
• Absolute stability occurs when the
environmental lapse rate is less than the wet
adiabatic rate
• Often results in widespread clouds with little
vertical thickness and precipitation
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Absolute stability
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Stability of air
Types of stability
• B. Absolute instability
• Acts like a hot air balloon
• Rising air
• Warmer than surrounding air
• Less dense than surrounding air
• Continues to rise until it reaches an altitude
with the same temperature
• Clouds are often towering
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Absolute instability
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Stability of air
Types of stability
• C. Conditional instability occurs when
the atmosphere is stable for an
unsaturated parcel of air but unstable
for a saturated parcel
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Conditional instability
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Stability of air
Determines to a large degree
• Type of clouds that develop
• Intensity of the precipitation
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Condensation and cloud formation
Condensation
• Water vapor in the air changes to a liquid and
forms dew, fog, or clouds
• Watch video 7.2g – convective cloud formation
• Water vapor requires a surface to condense
on
• Possible condensation surfaces on the ground can
be the grass, a car window, etc.
• Possible condensation surfaces in the atmosphere
are called condensation nuclei
• Dust, smoke, etc
• Ocean salt crystals which serve as hygroscopic
(“water-seeking”) nuclei
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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
• A. Form (three basic forms)
• 1. Cirrus – high, white, thin
• 2. Cumulus – globular cloud masses often
associated with fair weather
• 3. Stratus – sheets or layers that cover
much of the sky
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Cirrus clouds
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Altocumulus clouds
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Cumulus clouds
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Condensation and
cloud formation
Clouds
• Classification based on
• B. Height
• 1. High clouds – above 6,000 meters
• Types include cirrus, cirrostratus,
cirrocumulus
• 2. Middle clouds – 2,000 to 6,000 meters
• Types include altostratus and altocumulus
• 3. Low clouds – below 2,000 meters
• Types include stratus, stratocumulus, and
nimbostratus (nimbus means “rainy”)
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Condensation and
cloud formation
Clouds
• Classification based on
• B. Height
• 4. Clouds of vertical development
• From low to high altitudes
• Called cumulonimbus
• Often produce rain showers and
thunderstorms
• Watch video 7.2h – cumulonimbus cloud speed
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Classification of clouds
according to height and form
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More examples of cloud images in
folder
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Fog
Considered an atmospheric hazard
Cloud with its base at or near the
ground
Most fogs form because of
• Radiation cooling, or
• Movement of air over a cold surface
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Fog
Types of fog
• A. Fogs caused by cooling
• 1. Advection fog – warm, moist air moves
over a cool surface
• 2. Radiation fog
• Earth’s surface cools rapidly
• Forms during cool, clear, calm nights
• 3. Upslope fog
• Humid air moves up a slope
• Adiabatic cooling occurs
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Fog
Types of fog
• B. Evaporation fogs
• 1. Steam fog
• Cool air moves over warm water and moisture
is added to the air
• Water has a steaming appearance
• 2. Frontal fog, or precipitation fog
• Forms during frontal wedging when warm air
is lifted over colder air
• Rain evaporates to form fog
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Precipitation
Cloud droplets
• Less than 20 micrometers (0.02 millimeter)
in diameter
• Fall incredibly slow
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Particle sizes involved in
condensation and precipitation
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Precipitation
Formation of precipitation
• Bergeron process
• Temperature in the cloud is below freezing
• Ice crystals collect water vapor
• Large snowflakes form and fall to the ground or
melt during descent and fall as rain
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The Bergeron process
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Precipitation
Formation of precipitation
• Collision-coalescence process
• Warm clouds
• Large hygroscopic condensation nuclei
• Large droplets form and fall more rapidly
than smaller droplets
• Large droplet collide with other droplets
during their descent and become bigger
• Become so big they break apart into
several small drops, which then grow by
the same process
• Common in the tropics
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The collisioncoalescence
process
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Precipitation
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
• Sleet and glaze
• Sleet
• Wintertime phenomenon
• Small particles of ice
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Precipitation
Forms of precipitation
• Sleet and glaze
• Sleet
• 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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Precipitation
Forms of precipitation
• Hail
• Hard rounded pellets
• Concentric shells
• Most diameters range from 1 to 5 cm
• Formation
• Occurs in large cumulonimbus clouds with
violent up- and downdrafts
• Layers of freezing rain are caught in up- and
downdrafts in the cloud
• Pellets fall to the ground when they become
too heavy
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Precipitation
Forms of precipitation
• Rime
• Forms on cold surfaces
• Freezing of
• Supercooled fog, or
• Cloud droplets
• Looks really cool
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End of Chapter 17
HW:
P. 492 #1-4
p.498 #1,4,6
p. 499 #1-2
p. 502 #1-2
p.506 #2-5
p.509 #1-3,5
p. 515 #1-2
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