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Water in the Atmosphere
Earth Science
Chapter 23
States of Water
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Water in the atmosphere can exist in three
states: solid (ice), liquid (rain), or gas (water
vapor).
Types of changes:
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Solid to liquid: melting
Liquid to gas: evaporation
Gas to liquid: condensation
Water may change from one state to another by
adding or taking away energy.
Most water vapor comes from lakes, oceans,
marshes and glaciers.
Water vapor is spread throughout the
troposphere by convection currents and wind.
Evaporation
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The molecules of liquid water are
always in motion
Molecules with sufficient energy
to escape the water’s surface are
said to evaporate.
As temperature increases, the
molecules gain in energy and
evaporation increases.
Water and other liquids absorb
heat energy from their
surroundings when they
evaporate.
Then the molecules left behind
have less energy so that makes
evaporation a cooling process.
Specific Humidity and Capacity
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The amount of water vapor actually present in
the air is called the specific humidity.
The capacity of air for holding water vapor
depends on the temperature of the air.
The warmer the air, the more water vapor it can
hold.
Saturated air occurs when the specific humidity
equals the air’s capacity.
The air’s capacity for holding water vapor
roughly doubles for every rise of 11°C.
Relative Humidity
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Relative humidity compares the
actual amount of water vapor in
the air to the maximum amount
of water vapor the air can hold at
that temperature.
Relative humidity is calculated
using the following equation:
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(Specific humidity/ Capacity) x 100% =
Relative humidity
Saturated air has a humidity of
100%.
Finding Relative Humidity
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Instruments used to
measure relative humidity
are called hygrometers.
Hygrometers measure the
affect of the water vapor
on different instruments.
Experimentation is
combined to form a data
table that is then used to
calculate relative humidity.
Condensation
and Dew Point
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As temperature decreases, air cannot hold as much
water vapor and it begins to condense on surfaces.
The water vapor that condenses on surfaces is called
dew.
Water vapor that condenses in the air can form clouds or
fog.
The temperature at which saturation occurs is called the
dew point.
Condensing water vapor molecules release heat energy
to their surroundings and slow down the rate at which
the air cools.
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For water vapor to
condense, air must be
cooled below its dew
point.
Air can lose heat in the
following ways:
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Contacting a cooler
surface
Radiating heat
Mixing with colder air
Expanding when it rises
Condensation
Requires Cooling
Condensation
Requires  Condensation can not occur
unless water vapor has
Nuclei
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something to condense upon.
The tiny particles in the air on
which water vapor condense
are called condensation nuclei.
Condensation nuclei are usually
substances such as salt, sulfate
particles or nitrate particles.
Dew and Frost
from Contact
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Condensation usually happens when air is cooled
below its dew point.
If the temperature is above 0°C, dew usually
forms on surfaces that cool quickly.
Clear nights tend to have greater cooling and
heavier dew.
If the temperature is below 0°C, water vapor
condenses into a solid called frost.
Fogs from
Radiation and
Advection
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When a surface layer of air
a few hundred meters
thick is cooled below the
dew point, water vapor
condenses in the entire
layer and form fog.
Radiation or ground fogs
form under conditions
similar to those that form
dew.
Advection fogs result when
warm, moist air blows over
cool surfaces.
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A cloud is a large collection of very tiny droplets
of water or ice crystals that are so small and
light they float in the air.
Clouds form when air above the surface cools
below its dew point.
The shape of the cloud depends upon the air
movement that forms it.
If air movement is horizontal clouds form in
layers and are called stratiform clouds.
If air movement is
vertical, cumuliform
clouds grow
upward in
great piles.
Origin
of
Clouds
Temperature of Clouds
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Clouds that form at temperatures above
freezing are made entirely of water drops.
Below freezing, clouds are mixtures of
snow crystals and super cooled water.
Super cooled water is water that has
cooled below 0°C without freezing.
Below -18°C, clouds are entirely snow and
ice crystals.
Cloud Names and Meanings
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Three names represent the three main
cloud types.
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Cirrus – thin, feathery or tufted clouds that
form so high they are made entirely of ice
crystals
Stratus – low sheets or layers of clouds
Cumulus – formed by vertically rising air
currents, they are piled high in thick, puffy
masses
More Cloud Names
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All other cloud names are combinations of
the original three types.
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Cirrostratus – high, thin, smooth or fibrous
sheets of ice-crystal clouds, indicate rain or
snow
Stratocumulus – layers of round puffs, often
cover the entire sky in the winter
Cirrocumulus – small globular patches of
cloud made of ice crystals
Even More Cloud Names and
Meanings
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The prefix “alto” (high) and the word nimbus
(rain cloud) are also used to describe clouds.
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Altocumulus – look like stratocumulus clouds, but the
puffs appear smaller because they are higher up
Altostratus – stratus clouds that appear at a higher
level, they appear gray or bluish and produce no halo
around the sun or moon.
Nimbostratus – dark, gray layers of cloud that
produce steady rain
Temperature
Changes in
Clouds
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Air moves upward in clouds because it is
buoyant.
Air is buoyant because it is warmer than
the surrounding air.
Rising dry air cools at a rate of 1°C per 100
m this is called the dry-adiabatic lapse rate.
The cooling is caused because the air is
expanding due to less pressure.
Moist-Adiabatic Lapse Rate
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Air rising in a cloud does not cools as fast as
rising dry air does.
In a cloud, air cools at 0.6°C per 100 m.
The condensing water in the cloud releases heat
to the air which makes it cool more slowly.
The rate of temperature change of a rising or
sinking saturated parcel of air (a cloud) is called
the moist-adiabatic lapse rate.
Clouds with
Vertical
Development
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Clouds with vertical development form when
rising air currents are buoyant or lighter than
the surrounding air.
The rising air in the cloud is warmer than the
surrounding air because saturated air cools
slower than dry air even though it does gets
cooler as it rises.
If a shallow layer of air is unstable, cumulus
clouds form.
If a deep layer of air is unstable, cumulonimbus
clouds may form which produce severe
thunderstorms.
Cumulus
Clouds
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Rising buoyant air forms cumulus clouds.
The clouds have flat bases and billowy tops.
They form over heated ground that cause the
raising air to remain buoyant.
The flat cloud base is the area where the water
vapor begins to condense.
The height of the cloud is called the
condensation level where the temperature is
equal to the dew point.
Lifting Condensation Level
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The lifting condensation
level is the level the air
must reach for
condensation to begin.
It is calculated by dividing
the difference in ground
air temperature and the
dew point by 0.8°C and
multiplying by 100.
Meteorologists use this to
calculate the highest
possible cloud tops.
Layer Clouds
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Layer clouds form in stable air where motions
are mainly horizontal.
Stable air has a lapse rate that is smaller than
the moist-adiabatic lapse rate.
Clouds can form in stable air in two ways:
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The air can be forced slowly upward to its
condensation level or,
Layer clouds form if radiation or mixing cools a layer
of air to its dew point
Precipitation
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Precipitation is the falling of any form of
water from the air to Earth’s surface.
Precipitation occurs when cloud droplets
grow heavy enough to fall to Earth.
Raindrops can form in two ways:
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Warm-cloud processes
Ice processes
Warm-cloud Process
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In the warm cloud
process:
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Tiny droplets form by
condensation
The drops grow by
bumping into other
droplets
The bigger droplets fall
faster and capture smaller
droplets.
Mixing of air from different
parts of the cloud results
in droplets of different
sizes
Ice Processes
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Droplets can also form by ice processes.
Temperatures in the upper layers of clouds are
below freezing.
Super-cooled water evaporates faster than ice
and the water vapor condense on the ice
crystals.
When ice crystals get heavy enough they begin
to fall and capture smaller crystals on the way
down.
Forms of Precipitation
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Precipitation comes in many forms including:
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Drizzle – small drops that are close together and fall
slowly
Raindrops – larger drops that are farther apart and
fall faster
Snow – clumps of six-sided ice crystals that may melt
as they fall
Sleet – raindrops that fall through freezing air and
form into pellets of ice
Hail – a frozen raindrop that forms in cumulonimbus
clouds and grows larger the longer it falls.
Measuring Precipitation
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Rainfall is measured by an instrument called a
rain gauge.
Snowfall is measured in inches and tenths of an
inch and a measuring stick is typically used.
The rain equivalent is determined by melting a
definite depth of snow.
On average 10 inches of snow = 1 inch of rain
and varies depending on the type of snow.
Location of
Precipitation
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Precipitation occurs in every part of the world.
The differences in rainfall between two
locations depends upon how warm air is and
how far it rises, and proximity to large bodies
of water or large forest areas.
Rainy areas on Earth occur where air rises in
large quantities.
Rainy Areas
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Areas that typically get a lot of rain include:
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Windward side of mountain ranges.
Storm areas of all kinds
Areas favored by global wind belts
Dry Areas of Earth
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In areas of sinking air, the air is warmed by
compression and thus precipitation does not occur.
Sinking air occurs:
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On the leeward sides of mountains
High-pressure areas
Areas where global wind belts diverge or blow away from
an area.
Rainmaking
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Two methods of
rainmaking:
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Method #1: Cold, solid
C02 pellets are dropped
into a super-cooled cloud.
The pellets form ice
crystals until they are
heavy enough to fall.
Method #2: Artificial ice
nuclei are put into the
cloud. Precipitation
again forms through the
ice processes method.
Acid Clouds and Acid Rain
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Acid rain is low pH rain that
forms from sulfate & nitrate
particles released into the air.
Cloud droplets that form on
these nuclei are very acidic and
can cause damage to soils and
plant life.
Acid rain also destroys
microorganisms & plankton in
lakes & streams because the
water becomes to acidic,
destroying the food web.
Acid rain also damages rock,
concrete, metals, paints, and
plastics used for buildings.