Download Text PPT Ch 18

Prentice Hall
EARTH SCIENCE
Tarbuck

Lutgens
Chapter
18
Moisture, Clouds,
and Precipitation
18.1 Water in the Atmosphere
Water’s Changes of State
 Precipitation is any form of water that falls
from a cloud.
 When it comes to understanding
atmospheric processes, water vapor is the
most important gas in the atmosphere.
Water’s Changes of State
 Solid to Liquid
• The process of changing state, such as melting
ice, requires that energy be transferred in the form
of heat. Energy is absorbed.
• Latent heat is the energy absorbed or released
during a change in state. Temperature can NOT
change during a change of state!
Water’s Changes of State
•  Liquid to Gas
• Evaporation is the process of changing a
liquid to a gas. Energy is absorbed, and
the surroundings cool. So when you
step out of the shower you feel cool
because the water on your skin is
evaporating.
• Condensation is the process where a
gas, like water vapor, releases heat and
changes to a liquid, like water.
Dew on a Spider Web
18.1 Water in the Atmosphere
Water’s Changes of State
 Solid to Gas
• Sublimation is the conversion of a solid directly
to a gas without passing through the liquid state.
Dry ice = solid carbon dioxide sublimes without
becoming a liquid first.
• Deposition is the conversion of a vapor directly
to a solid.
Dry ice machine on stage
Changes of State
18.1 Water in the Atmosphere
Humidity
 Humidity is a general term for the amount
of water vapor in air.
 Saturation
• Air is saturated when it contains the maximum
quantity of water vapor that it can hold at any
given temperature and pressure.
• When saturated, warm air contains more water
vapor than cold saturated air.
18.1 Water in the Atmosphere
Humidity
 Relative Humidity
• Relative humidity is a ratio of the air’s actual
water-vapor content compared with the amount
of water vapor air can hold at that temperature
and pressure.
• To summarize, when the water-vapor content of
air remains constant, lowering air temperature
causes an increase in relative humidity, and
raising air temperature causes a decrease in
relative humidity. Warm air holds more water
vapor than cold air – so winter air is dry.
Check your understanding
So, on a cool summer morning, the relative
humidity is _______ than in the warmer
afternoon (if the amount of water vapor
stays constant).
Relative Humidity Varies
with Temperature
18.1 Water in the Atmosphere
Humidity
 Dew Point
• Dew point is the temperature to which a parcel of air
would need to be cooled to reach saturation.
High dew point = moist air
Low dew point = dry air
** Dew point can NEVER be greater than the
temperature! Why not?
(only slightly in the atmosphere in the case of the
updraft of a thunderstorm when hail forms)
 Measuring Humidity
• A hygrometer is an instrument to
measure relative humidity.
• A psychrometer is a hygrometer with dryand wet-bulb thermometers. Evaporation
of water from the wet bulb makes air
temperature appear lower than the dry
bulb’s measurement. The two
temperatures are compared to determine
the relative humidity.
http://www.youtube.com/watch?v=QbcaCxuA1LI
Digital psychrometer
Sling Psychrometer
Air must continually pass over bulbs of each
thermometer– one wet & one dry. Water
evaporates from wet bulb and it cools. Dryer air
will let wet bulb cool more. Compare
temperatures to a table to get the relative
humidity.
Relative Humidity Table – pg 525
Why is Winter Air Dry?
• Water vapor content of air stays constant
• Warm air holds more water vapor than cold air
• In winter outside air is cold so it doesn’t hold a lot of
moisture
• When you pump that air into your house and warm it up
the relative humidity drops to less than 10%
• Causes static electricity, dry skin, sinus headaches, etc.
18.1 Review
1. What is the most important gas for
understanding atmospheric processes?
2. What happens to heat during a change of
state? Temperature?
3. How does the temperature of air influence
its ability to hold water?
4. List two ways relative humidity can be
changed?
5. What does relative humidity describe about
air?
6. If the dew point is low, is the air moist or
dry?
18.2 Cloud Formation
- Clouds form when air is cooled to its dew point - Air must
be saturated to be at the dew point
But these clouds form during warmest part of day – so how
is that possible?
- Air Compression and Expansion
Think of pumping up bicycle tire - When you put air in
the hose gets warm because compressing air increases motion
of gas molecules & this causes temperature to rise
Tire losing air - When air is released it expands and
cools
These are Adiabatic Temperature Changes =
Changes that happen even though heat isn’t added or
subtracted
18.2 Cloud Formation
Air Compression and Expansion
 Adiabatic Temperature Changes
(adiabatic = without loss or gain of heat)
• When air is allowed to expand, it cools, and
when it is compressed, it warms.
 Expansion and Cooling
• Dry adiabatic rate is the rate of cooling or
heating that applies only to unsaturated air.
• Wet adiabatic rate is the rate of adiabatic
temperature change in saturated air.
Cloud Formation by Adiabatic Cooling
18.2 Cloud Formation
Processes That Lift Air
 Four mechanisms that can cause air to rise
are orographic lifting, frontal wedging,
convergence, and localized convective
lifting.
 Orographic Lifting (remember orogeny?)
• Orographic lifting occurs when mountains act
as barriers to the flow of air, forcing the air to
ascend.
• The air cools adiabatically; clouds and
precipitation may result.
Orographic Lifting
•
•
•
•
Elevated terrain acts as barrier
Causes air to rise and reach dew point
Clouds form and it rains on windward side
Other side of barrier is dry because all of
precipitation fell out
• Air descends and warms
up making it even drier
• Rain Shadow Desert forms
18.2 Cloud Formation
Processes That Lift Air
 Frontal Wedging
• A front is the boundary between two adjoining
air masses having contrasting characteristics
(warm/cold, etc.)
Orographic Lifting and Frontal Wedging
Frontal wedging - Warm air
meets cold air and a front
forms
Warm air is less dense so it rises
over cold air
18.2 Cloud Formation
Processes That Lift Air
 Convergence
• Convergence is when air flows together and
rises.
• Leads to adiabatic cooling and possible
cloud formation
• This is why it rains many afternoons in
Florida
 Localized Convective Lifting
• Localized convective lifting occurs where
unequal surface heating causes pockets of air to
rise because of their buoyancy.
• Rising parcels of air are called thermals
• Birds use this to soar high in sky
• People use it when they want to go hang gliding
• If parcel rises above condensation level then
clouds form and may produce rain
Convergence and Localized
Convective Lifting
18.2 Cloud Formation
Stability
 Density Differences
• Stable air tends to remain in its original position,
while unstable air tends to rise.
 Stability Measurements
• Air stability is determined by measuring the
temperature of the atmosphere at various
heights.
• The rate of change of air temperature with height
is called the environmental lapse rate.
18.2 Cloud Formation
Stability
 Degrees of Stability
• A temperature inversion occurs in a layer of
limited depth in the atmosphere where the
temperature increases rather than decreases with
height. (warmer air over cooler air)
 Stability and Daily Weather
When stable air is forced above the
Earth’s surface, the clouds that form
are widespread and have little vertical
thickness compared to their horizontal
dimension. (spread out wide but thin
from top to bottom)
18.2 Cloud Formation
Condensation
 For any form of condensation to occur, the
air must be saturated.
 Types of Surfaces
• Generally, there must be a surface for water
vapor to condense on.
• Condensation nuclei are tiny bits of particulate
matter that serve as surfaces on which water
vapor condenses when condensation occurs in
the air. (Like when they ‘seed’ clouds’)

18.2 Review
1. Describe what happens to air temperature when work is done on the
air to compress it.
2. What does stability mean in terms of air movement?
3. What are the four mechanisms that cause air to rise?
4. Describe conditions that cause condensation of liquid water in air.
5. What is a temperature inversion?
6. Hypothesize about other regions on Earth, other than the Florida
peninsula, where convergence might cause cloud development and
precipitation.
18.3 Cloud Types and Precipitation
Types of Clouds
 Clouds are classified on the basis of their
form and height.
• Cirrus (cirrus = curl of hair) are clouds that are
high, white, and thin.
• Cumulus (cumulus = a pile) are clouds that
consist of rounded individual cloud masses.
• Stratus (stratus = a layer) are clouds best
described as sheets or layers that cover much
or all of the sky.
Cirrus Clouds
Cloud Form #2 - Cumulus
• Rounded individual cloud mass
• Usually have flat base and rise as domes or towers
• Fluffy tops described as cauliflower
head
Cloud Form #3 - Stratus
• Sheets or layers that cover the sky
• No distinct units
• Usually bring precipitation
18.3 Cloud Types and Precipitation
Types of Clouds
 High Clouds – Base above 6000 meters
• Cirrus clouds are high, white, and thin.
• Cirrostratus clouds are flat layers of clouds.
• Cirrocumulus clouds consist of fluffy masses.
 Middle Clouds 2000 – 6000 meters
• Altocumulus clouds are composed of rounded
masses that differ from cirrocumulus clouds in
that altocumulus clouds are larger and denser.
• Altostratus clouds create a uniform white to gray
sheet covering the sky with the sun or moon
visible as a bright spot.
18.3 Cloud Types and Precipitation
Types of Clouds
 Low Clouds - Form below 2000 meters
• Stratus clouds are best described as sheets or
layers that cover much or all of the sky.
• Stratocumulus clouds have a scalloped bottom
that appears as long parallel rolls or broken
rounded patches.
• Nimbostratus clouds are the main precipitation
makers.
Cloud Classification
18.3 Cloud Types and Precipitation
 Clouds of Vertical Development
• Some clouds do not fit into any of the other
categories. They may start
in one layer and extend into
another.
• Form in unstable air
• Cumulus are usually fair
weather but grow out of control
under right circumstances
– become cumulonimbus
• Produce rain or
thunderstorms
18.3 Cloud Types and Precipitation
Fog
 Fog is defined as a cloud with its base at or
very near the ground.
 Fog Caused by Cooling
• As the air cools, it becomes denser and drains
into low areas such as river valleys, where thick
fog accumulations may occur.
 Fog Caused by Evaporation
• When cool air moves over warm water, enough
moisture may evaporate from the water surface
to produce saturation.
Fog caused by cooling
• Warm moist air over Pacific moves over cold California
Current then carried onshore by prevailing winds
• Cool, clear, calm nights air near surface of Earth cools
when Earth loses heat gained during day and reaches its
dew point
– As the air cools
it becomes dense
and sits in low areas
‘Steam fog’ caused by
evaporation
18.3 Cloud Types and Precipitation
How Precipitation Forms
 For precipitation to form, cloud droplets
must grow in volume by roughly one million
times.
 Cold Cloud Precipitation
• The Bergeron process is a theory that relates
the formation of precipitation to supercooled
clouds, freezing nuclei, and the different
saturation levels of ice and liquid water.
The Bergeron Process
18.3 Cloud Types and Precipitation
How Precipitation Forms
 Cold Cloud Precipitation
• Supercooled water is the condition of water
droplets that remain in the liquid state at
temperatures well below 0oC.
• Supersaturated air is the condition of air that is
more concentrated than is normally possible
under given temperature and pressure
conditions.
18.3 Cloud Types and Precipitation
How Precipitation Forms
 Warm Cloud Precipitation
• The collision-coalescence process is a theory
of raindrop formation in warm clouds (above 0oC)
in which large cloud droplets collide and join
together with smaller droplets to form a raindrop.
18.3 Cloud Types and Precipitation
Forms of Precipitation
 The type of precipitation that reaches
Earth’s surface depends on the temperature
profile in the lower few kilometers of the
atmosphere.
 Rain and Snow
• In meteorology, the term rain means drops of
water that fall from a cloud and have a diameter
of at least 0.5 mm.
• At very low temperatures (when the moisture
content of air is low) light fluffy snow made up
of individual six-sided ice crystals forms.
18.3 Cloud Types and Precipitation
Forms of Precipitation
 Rain and Snow
• Sleet is the fall of clear-to-translucent ice.
• Hail is produced in cumulonimbus clouds.
• Hailstones begin as small ice pellets that grow
by collecting supercooled water droplets as they
fall through a cloud.
Liquid Forms of Precipitation
• Rain
• Size: 0.5-5mm diameter
• Drizzle
• Size: <0.5mm diameter
• Falls very slow
Solid Forms of Precipitation
• Snow
– Ice particles
– Usually falls as flakes
• small flakes at low temps
• larger flakes closer to
freezing
• depends on moisture
content in air
Solid Forms of Precipitation
• Sleet
– ice pellets
– rain falls through
layer of freezing air
Solid Forms of Precipitation
• Freezing Rain
– Glaze Ice
– Rain doesn’t freeze
until it strikes surface
near ground
Largest Recorded Hailstone