Water _ Atmospheric Moisture Lect 5

7/18/2019 Water _ Atmospheric Moisture Lect 5

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4/12/2

Chapter 7 Lecture

© 2015 Pearson Education, Inc.

Water and Atmospheric Moisture


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.


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).


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


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.


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.


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.


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.


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.


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.


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.


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.


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.


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

•

Cloud droplet’s diameter is about 0.001 mmto 0.2 mm.

• Raindrop’s diameter is about 0.4 mm to

4 mm.


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

•

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)


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.


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.

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Water _ Atmospheric Moisture Lect 5