Water _ Atmospheric Moisture Lect 5
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Transcript of Water _ Atmospheric Moisture Lect 5
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Chapter 7 Lecture
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Water and Atmospheric Moisture
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Learning Objectives
•
Describe the heat properties of water, and identify thetraits of its three phases: solid, liquid, and gas.
•
Define humidity and relative humidity.
•
Explain dew-point temperature and saturated conditionsin the atmosphere.
•
Define atmospheric stability, and relate it to a parcel of air
that is ascending or descending.
•
Illustrate three atmospheric conditions—unstable,
conditionally unstable, and stable—with a simple graph
that relates the environmental lapse rate (ELR) to the dry
adiabatic rate (DAR) and moist adiabatic rate (MAR).
•
Identify the requirements for cloud formation.
•
Explain the major cloud classes and types, including fog.
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Something about Water
• Pure water is “3-less”: colorless,
odorless, and tasteless.
• Pure water rarely occurs in nature
because it is a good solvent (dissolves
solids).
• Liquid water density is 1g/cm3.
• Human body contains 70% water.
• A human can survive 50 to 60 days
without food but only 2 or 3 dayswithout water.
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Water on Earth
Water covers 71% of Earth by area.
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Where did Earth’s Water Come
From?
•
Earth’s water originated from icy comets
and hydrogen-oxygen-laden debris.
• Outgassing is a process by which water
and water vapor emerge from layers
deep within and below the crust (about
25 km deep).
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Water and Ice
• Water density reaches maximum at 4!C.
• From 4!C to !29!C, water expands and ice
forms. 9% increase in volume.
• Density = mass/volume
• 9% increase in volume, 9% decrease in
density
•
Pure ice density = 0.91 " water density
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Damages Due to Volume Expansion
During Ice Formation
• Breaking roads
• Breaking pipes
• Damaging vehicle’s engine
• Sinking ships
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Humidity
• Humidity refers to water vapor in the air.
• Humidity is primarily a function of the air
temperature and the water vapor
temperature.
• Relative humidity:
Relativehumidity =
Actual water vapor in the air
Maximum water vapor possible
in the air at that temperature
!100%
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Maximum Specific Humidity
•
Maximum specifichumidity is themaximum possible
water vapor per unitmass of air.
• Maximum specifichumidity increases asthe air temperature
increases.• Maximum specific
humidity decreases as
the air temperature
decreases.
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Saturation and Dew-Point Temperature
• As the relative humidity is at 100%, air is
at saturation.
• At saturation, evaporation rate is equal to
condensation rate (equilibrium).
• The dew-point temperature is the
temperature at which a given mass of air
becomes saturated and net condensation
begins to form water droplets.
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Humidity Instrument (1) Hair Hygrometer
Principle: Human hair changes in lengthwith respect to changes in relative humidity.
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Humidity Instrument (2) Sling
Psychrometer
Td: dry-bulb temperatureTw: wet-bulb temperatureRelative humidity depends on Td and Tw.
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Stability: Balance Between Buoyancy
and Gravity
Air parcel is a body of air has specific temperature and humidity.
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Atmospheric Stability
• Stability refers to the tendency of an air
parcel either to remain in place or to change
vertical position by ascending or descending.
• An air parcel is stable if it resists
displacement upward, or when disturbed,
tends to return to its starting place.
• An air parcel is unstable if it continues to rise
until it reaches an altitude where the
surrounding air has a density and
temperature similar to its own.
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Determining Stability
•
The degree of stability depends ontemperature difference between inside
an air parcel and the air surrounding
the parcel.
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Adiabatic and Diabatic
• Adiabatic means occurring without a loss
or gain of heat.
• Diabatic means occurring with an
exchange of heat.
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Adiabatic Ascending Process
•
In an adiabaticascending
process, if an
air parcel rises,
it will expand
and thetemperature
inside the air
parcel will
decrease.
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Adiabatic Descending Process
• In an adiabaticdescending
process, the
air parcel
will be
compressedand the
temperature
inside the airparcel will
increase.
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Dry Adiabatic Rate (DAR)
•
The dry adiabatic rate (DAR) is the rate atwhich “dry” air cools by expansion (if
ascending) or heats by compression (if
descending).
• Dry refers to air that it is less than
saturated (relative humidity is less
than 100%).
• The average DAR is 10°C/1000m.
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Normal Lapse Rate
• Normal lapse rate: average decrease in temperaturewith increasing altitude, a value of 6.4°C/1000m.
• Environmental lapse rate is the actual lapse rate at a
particular time and place. It can vary by several degrees
per thousand meters.
• Dry lapse rate: 10°C/1000m
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Moist Adiabatic Rate (MAR)
• The moist adiabatic rate (MAR) is the rate at which
an ascending air parcel that is moist and saturated
cools by expansion.
• The average MAR is about 6!C/1000m.
• Why MAR is much less than DAR? Because
condensation releases energy and heats
surrounding air.
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Unstable
Unstable as the environmental lapse rate is greater than the dry
adiabatic lapse rate, e.g., the environmental lapse rate = 12°C/1000m.
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Conditionally Unstable
Conditional unstable as the environmental lapse rate is greater than the
moist adiabatic lapse rate and less than the dry adiabatic lapse rate,
e.g., the environmental lapse rate = 7°C/1000m.
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Stable
Stable as the environmental lapse rate is less than the moist adiabatic
lapse rate, e.g., the environmental lapse rate = 5°C/1000m.
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Cloud Formation Processes
• Adiabatic cooling by vertical lift
• Air parcel cools to the dew-pointtemperature.
• Air parcel becomes saturated.
• Condensation occurs.
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Condensation
• In order for condensation to occur near the dew point, a
certain amount of foreign particles larger than 10!4 mm,
called cloud condensation nuclei (CCN) and to which watermolecules are attracted, must be present.
• Natural sources of CCN: meteoric dust, windblown clay and
silt, volcanic material, smoke from forest fires, and sea salt
• Anthropogenic CCN sources: combusting products (sulfur
and nitrogen compounds).
•
The concentration of Anthropogenic CCN is much higherthan that of natural sources of CCN.
• Cloud seeding: artificial introduction of nuclei
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Droplet Growth
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Cloud droplet’s diameter is about 0.001 mmto 0.2 mm.
• Raindrop’s diameter is about 0.4 mm to
4 mm.
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Droplet Growth
• Cloud droplet’s diameter is about 0.001 mm
to 0.2 mm.
• Raindrop’s diameter is about 0.4 mm to
4 mm.
• For precipitation to fall from clouds, cloud
droplets must grow to form raindrops.
•
Two possible processes: 1) drop collisionand 2) ice-crystal growth
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Cloud Types and Identification
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Four cloud types classified by altitude: low (up to 2000m), middle (2000–6000m)high (6000–13000m), and vertically developed (near surface to 13000m)
•
Three cloud types classified by form or shape: cirroform (hairlike, feathery),
stratiform (flat and layered), and cumuliform (dense, heavy)
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Fog
• Fog is a cloud layer on the ground.
• The air temperature and the dew-point
temperature at ground level are nearly
identical.
• Near ground, air is saturated.
• Fog directly affects visibility.
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Advection Fog
When warm and moist air overlays cooler ocean currents, or lake
surfaces, or snow masses, the layer of migrating air directly above thesurface becomes chilled to the dew point and advection fog develops.
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Summary of Chapter 7•
Water has three phases (or states): liquid (water), solid (ice), and gas(water vapor). A change from one state to another is a phase change. The
heat energy required for water to change phase is latent heat.
•
A change from liquid to solid is freezing; from solid to liquid is melting; from
liquid to vapor is evaporation (or vaporization); from vapor to liquid is
condensation; from vapor to solid is deposition; from solid to vapor is
sublimation.
•
The amount of water vapor in the atmosphere is humidity. Relative humidity
is a ratio of the amount of water vapor actually in the air to the maximum
amount possible at a given temperature. The temperature at which air
achieves saturation is the dew-point temperature.
•
The atmospheric stability refers to the tendency of an air parcel either to
remain in place or to change vertical position by ascending or descending. An air parcel is stable if it resists displacement upward or, when disturbed, ittends to return to its starting place. An air parcel is unstable if it continues to
rise until it reaches an altitude where the surrounding air has a density (air
temperature) similar to its own.
•
A cloud is an aggregation of tiny moisture droplets and ice crystals
suspended in the air.