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Water in the Atmosphere
Vocabulary
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Precipitation
Condensation
Latent heat
Evaporation
Sublimation
Deposition
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Humidity
Saturated
Relative humidity
Dew point
Hygrometer
Psychrometer
Water is the MOST important gas
in the atmosphere
• It is the source of condensation (when
water vapor or gaseous water changes to
the liquid state), such as clouds, fog, and
dew.
• It is the source of all precipitation (any
form of water that falls from a cloud,
such as rain, sleet, snow, hail, or glaze)
Remember that air is made up of
several different gases
Nitrogen %
78
Oxygen %
21
Argon %
1
Water vapor % 0 to 4
CO2 %
0.01 to 0.1
Water is Unique
• Water exists on Earth in THREE states of
matter—solid, liquid, and gas.
• Water can freely change from one of these
states to another at the temperatures and
pressures experienced on Earth. It evaporates
from the oceans as a gas, and then it returns
again as a liquid, producing the hydrologic or
water cycle.
• All water in this cycle passes through the
atmosphere as water vapor, which at any one
time would only be a liquid layer 2 mm deep!
Changing States of Matter
SOLID TO LIQUID
The process of changing states
requires that energy is
transferred in the form of HEAT.
When ice MELTS, or changes from
a solid to a liquid, heat is
absorbed but the temperature
doesn’t go up until all of the ice is
melted. Where does the heat
go?
This heat energy breaks the
molecular bonds between ice
crystals allowing water to
become a noncrystalline liquid.
Latent heat
The heat absorbed or released during a
change of state is called LATENT HEAT.
This heat does not produce a temperature
change. Latent means ‘hidden’.
This “hidden heat” needed to melt solid ice is
stored in the liquid water. When liquid water
becomes ice again, an EQUAL AMOUNT of
latent heat is released as was absorbed to
melt the ice.
Latent heat is also stored when liquid water
changes into water vapor at 100 °C.
Latent heat plays a crucial role in many
atmospheric processes. It is the major
source of energy for thunderstorms,
tornadoes or hurricanes.
LIQUID TO GAS
The process of changing a liquid to a gas is
called EVAPORATION.
The energy absorbed by the liquid water
molecules during evaporation allows them to
gain enough kinetic energy to escape the
surface of the liquid and become gas
molecules known as water vapor.
This energy is the LATENT HEAT OF
VAPORIZATION.
GAS TO LIQUID
• The opposite process, where water vapor
changes to the liquid state, is called
CONDENSATION.
• When water vapor condenses to become
liquid water again (e.g., forming a cloud or
fog), the stored latent heat is released.
This is called the LATENT HEAT OF
CONDENSATION and is equal to what
was absorbed during evaporation.
ICE TO WATER VAPOR
• SUBLIMATION is the
conversion of a solid
directly into a gas
without becoming a
liquid in between.
• You may have
noticed ice on your
windshield
disappearing without
melting.
Dry ice (frozen carbon
dioxide) becomes a
gas without melting in
between.
DEPOSITION
• DEPOSITION is the
reverse process to
sublimation, when a
vapor converts
directly into a solid.
• If you have ever
observed “Jack
Frost” on a window
or a windshield, you
have witnessed
deposition.
HUMIDITY
• Humidity is the general term for the
amount of water vapor in the air.
• Meteorologists use two main ways to
express the water vapor content of air:
relative humidity and dew point
temperature.
Saturation
Imagine a closed jar half full of water and the other
half with dry air.
Water molecules begin to evaporate and enter the
air. This increases the air pressure (due to the
particles’ motion). As more and more particles
evaporate and enter the air, the pressure
steadily increases and forces some of those
molecules to condense again.
When the number of water molecules evaporating
balances the number condensing, the air is said
to be SATURATED.
When saturated, warm air holds more water
vapor than cold air does.
Relative Humidity
The most common measure of moisture content is
RELATIVE HUMIDITY, a ratio that compares
how much water vapor the air holds at a
certain temperature and pressure to how much
it could hold before becoming saturated.
Relative humidity tells us how close to saturation
the air is, rather than the actual amount of water
vapor in the air (the humidity).
Changes in relative humidity
• One way to change relative humidity is to
change the amount of water vapor in the air.
Adding water vapor raises the relative humidity,
while removing water vapor lowers it.
• The other way to change relative humidity is to
raise or lower the temperature of the air
because the amount of moisture needed for
saturation depends on temperature.
• When air above Earth’s surface is cooled below
its saturation level, some of the water vapor
condenses to form clouds or fog.
Changes in Relative Humidity
For a constant amount of water vapor,
 cooling the air temperature results in a rise
in relative humidity,
 while raising the air temperature results
in a decrease in relative humidity.
Dew Point
The DEW POINT is the
temperature to which a
parcel of air would need to
be cooled to reach
saturation. Cooling below
that temperature would
yield dew, fog, or clouds.
For every 10 degree Celsius rise in air
temperature, the amount of water needed
for saturation doubles.
High dew point temperatures mean air that is
moist, while low dew point temps mean dry air.
Measuring Humidity
• Hygrometers are instruments
that continuously measure
relative humidity. You can make
a simple one using a human
hair, because it stretches when
the air is moist.
• A PSYCHROMETER uses two
thermometers, one to measure
the dry air temperature, while
the other is covered with a wet
fabric and measures the
temperature when evaporation
cools it. The amount of cooling
is directly proportional to the
dryness of the air.
Condensation:
Dew, fog, or clouds?
• Dew and fog – Usually form near the
ground at night when the ground cools by
radiating its heat upwards, and the air at
the surface cools below its saturation point
• Clouds – Can form day or night and much
higher in the atmosphere.
So what causes clouds to condense?
Air Compression
When you use a bicycle
pump, you are doing
work to COMPRESS
the air into a smaller
volume.
The energy you used
results in a
TEMPERATURE RISE
of the air.
Air Expansion
The opposite
occurs when air
is released from
a bicycle tire.
The air EXPANDS
and COOLS.
ADIABATIC TEMPERATURE
CHANGES
When air is compressed or allowed to
expand, its temperature changes
WITHOUT adding or removing heat.
These temperature changes are called
ADIABATIC temperature changes.
Air expands = COOLS
Air is compressed = WARMS
Expansion and Cooling in the
Atmosphere
We have already learned that as you
go higher in the atmosphere, air
pressure decreases because there
are fewer and fewer air molecules
above.
Any time a parcel of air (think of a
transparent balloon) moves upwards,
it is surrounded by lower pressure
and it therefore EXPANDS.
Dry Adiabatic Lapse Rate
UNSATURATED air
• cools at the rate of 10°C for every 1000 m
it moves upwards
• warms at the rate of 10°C for every
1000 m it moves downwards
This is called the DRY ADIABATIC RATE.
But what happens when it cools to its
dew point temperature?
If this parcel of air rises high enough, it WILL
cool to its dew point, and the air becomes
saturated. The process of CONDENSATION
begins.
As you have learned, condensation releases
LATENT HEAT, and this somewhat offsets
the cooling by expansion.
So above the condensation level, the
adiabatic rate of cooling SLOWS.
Wet Adiabatic Lapse Rate
Above the condensation level, the air cools
and warms at the WET ADIABATIC RATE.
This is between 5 and 9 °C for every
1000 m the air moves upwards (cools)
or downwards (warms).
The exact rate depends on how much
moisture is in the air and therefore how
much latent heat is released at
condensation.
CLOUD FORMATION BY
ADIABATIC COOLING
Processes That Lift Air
In general, air resists moving up or down.
However, there are four processes that can
cause air to rise above its usual level in
the atmosphere.
1) OROGRAPHIC LIFTING
The first is OROGRAPHIC LIFTING, when
air flow forces a parcel of air to go up
the slope of a mountain or other
elevated terrain.
On the windward side,
the air rises at the dry
adiabatic rate until
clouds form and
possibly precipitation
results. This process
usually removes whatever
water the air holds.
On the leeward side of the mountains, the
now much drier air descends, and it
warms adiabatically. Because it is
warming, cloud formation and precipitation
is NOT likely.
This can result in a RAIN SHADOW DESERT
on the leeward side of mountains. Eastern
Washington and Oregon are very dry
compared to the western (windward) slopes
of the Cascade Mountains because most
moisture in the air was left on the other
side of the mountains as precipitation.
2) FRONTAL WEDGING
Air masses (with different temperatures,
densities, and moisture contents) move over the
Earth’s surface. When they collide, the
boundary is called a FRONT. When a warm air
mass moves over a
cold air mass, the
front looks like a large
WEDGE. Warm air
rises over the cooler
air until it reaches its
condensation level,
resulting in overcast
skies and rain.
3) CONVERGENCE
Another way that air masses can collide
is CONVERGENCE. This occurs when
air flows together from more than one
direction, forcing the air upward, where it
cools and clouds form.
Florida experiences this
type of lifting on
summer days
because it is a
peninsula.
As we know, air over
land heats more
rapidly than air over
water. This causes
winds to blow
This convergence and lifting
toward the land
of air over the Florida
from both the Gulf
panhandle results in the
of Mexico and the
greatest number of midAtlantic Ocean.
afternoon thunderstorms
in the U.S.
4) LOCALIZED CONVECTIVE
LIFTING
Unequal heating of Earth’s surface on
summer days can heat the air directly
above it more than the surrounding air,
causing localized updrafts.
These rising
parcels of
warmer air are
called
THERMALS.
Paved parking lot
Thermals and clouds
Birds such as eagles
and hawks use
thermals to lift
themselves to great
heights without
expending energy for
hunting.
Sailplanes use thermals
to stay aloft for longer
flights.
These parcels of heated air can rise above the
condensation level, resulting in localized clouds
and mid-afternoon showers.
STABILITY
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Consider a parcel of air that has been forced
to rise. We know it is cooler than it was
because it has expanded.
But is it warmer or cooler than the
surrounding air?
If it is cooler, then it is denser than the
surrounding air, and it will sink if it can,
back to its original position. This is
STABLE AIR, and it resists vertical
movement.
But what if the parcel of air is warmer than
the surrounding air?
It is less dense than
the surrounding air,
and so it will continue
to rise until it is the
same temperature as
the air around it. This
is UNSTABLE air,
and it tends to rise.
Hot air balloons are an example of unstable air.
• UNSTABLE AIR tends to rise
• STABLE AIR tends to stay at its original
position.
Stability Measurements
Meteorologists measure the temperature of the
atmosphere at various heights to determine air
stability. They usually release balloons which
carry aloft an instrument called a RADIOSONDE.
Aircraft also take observations.
The change of air temperature with height is called
the ENVIRONMENTAL LAPSE RATE.
Degrees of Stability
Air is stable when the temperature
decreases gradually with increasing
altitude.
However, the MOST
stable conditions
happen when air
temperature actually
increases with
height, leading to a
TEMPERATURE
INVERSION.
Temperature inversions frequently happen
on clear nights when the air near the
ground cools rapidly by radiation, and the
air above it remains warmer. There is very
little vertical air movement.
Temperature inversions can lead to very
polluted air because the pollutants are
trapped and not diluted with fresher air or
blown away by winds.
In Fairbanks, temperature inversions happen during the
winter when bitterly cold air moves in and, because it is
denser than the air it replaces, it sinks below it. There is
no heating during the day because the sun is so low, so
the cold air stays near the ground. This inversion traps
auto exhaust and smoke, leading to ice fog sometimes for
several weeks at a time. The air quality becomes very
poor. It is often 20 degrees warmer in the hills nearby.
Stability and Daily Weather
Do clouds form when the air is stable?
Yes, because there is still orographic lifting,
frontal wedging, and convergence.
What do these clouds look like?
The clouds that do form are WIDESPREAD
and have little vertical thickness compared
to their horizontal dimension.
What kind of precipitation could occur?
Any precipitation that occurs is light to
moderate.
What kind of clouds form when the air is
UNSTABLE?
These clouds are towering, with strong
vertical dimensions. They generate
thunderstorms and even tornadoes.
Precipitation
from these
clouds can be
extremely heavy
and include hail.
Condensation Surfaces
Recall that water vapor condenses when the air
is saturated.
Generally, water vapor needs a SURFACE to
condense upon.
For dew, car windows and grass serve this
purpose.
For clouds, tiny bits of particulate matter called
CONDENSATION NUCLEI are the surfaces
upon which water vapor condenses. These
can be dust, smoke, soot, volcanic ash, air
pollution, or salt crystals.
Cloud Type Tutorial
How Precipitation Forms
• Cloud droplets are VERY tiny, less than 20
micrometers in diameter.
• They fall VERY slowly within a cloud and
would evaporate before falling very far (a
few meters) BELOW a cloud into the
unsaturated air below.
• To become PRECIPITATION, droplets
must grow in volume ONE MILLION
TIMES!! (That’s about 100 times in
diameter.)
Cold Cloud Precipitation
The Bergeron Process governs
cold cloud precipitation.
Cloud droplets can become supercooled at
temperatures below freezing. But, any ice
crystals nearby cannot co-exist with the
water droplets because the air “appears”
supersaturated to the ice crystals. The
growth of ice crystals is favored (AKA
‘vampire’ snowflakes), and the water
droplets evaporate to provide water
vapor for the ice crystals. Large enough
crystals fall as snowflakes.
Warm Cloud Precipitation
Raindrops in warm clouds are formed
by the COLLISION-COALESCENCE
PROCESS.
Within a cloud, some larger drops form
and move through the cloud, colliding
with and coalescing (join together)
with smaller, slower droplets. This
means bigger and bigger droplets!
Forms of Precipitation
What we see reaching the ground may NOT
be what fell out of the clouds!
The type of precipitation that reaches Earth’s
surface depends on the TEMPERATURE
PROFILE of the lowest few kilometers of
the atmosphere.
For instance, starting as snow…
• If the surface temperature is above
4°C, snow will melt before it reaches
the ground and reach the ground as
rain. Even on a hot summer day, a
heavy downpour may have started as a
snowstorm high in the clouds!
• At temperatures above -5 °C, ice
crystals clump together and the result
is heavy, wet snow.
• Below -5 °C, snowfall is light and
fluffy.
Rain and other forms of precip
• Rain is drops of water bigger than 0.5 mm
in diameter.
• Sleet is small particles of mostly clear ice
that form when rain falls through a layer of
air near the ground that is below freezing.
• Glaze or freezing rain results when rain
becomes supercooled when it falls through a
subfreezing layer of air near the ground BUT
DOESN’T FREEZE and turn into ice UNTIL
it impacts objects on the ground.
Glaze or Ice Storms
Hail
Hail is produced in
thunderstorm or
cumulonimbus clouds.
It starts as small ice
pellets that grow by
collecting supercooled
water droplets as they
fall through the cloud. If
hail is carried aloft by a
strong updraft, it gets
bigger by being coated with
more water, then it falls
again, and this process can
be repeated several times,
giving it many layers of ice.