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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
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?
The heat breaks the molecular
bonds between ice crystals
allowing water to become a
noncrystalline liquid.
Latent heat
This “hidden heat” or LATENT HEAT is the
heat needed to melt solid ice that doesn’t
produce a temperature change. It is stored
in the liquid water. When liquid water
becomes ice again, the SAME AMOUNT
of latent heat is released.
Latent heat is also stored when liquid water
changes into water vapor at 100 °C.
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 is called the
LATENT HEAT OF VAPORIZATION.
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
LATENT HEAT OF CONDENSATION.
Latent heat plays a crucial role in many
atmospheric processes. Without latent
heat release, we would not have towering
summer thunderstorms, tornadoes or
hurricanes.
ICE TO WATER VAPOR
and vice versa
When solid water (ice) becomes water
vapor WITHOUT becoming a liquid
in between, this process is called
SUBLIMATION. You may have
noticed ice on your windshield
disappearing without melting; dry ice
(frozen carbon dioxide) becomes a
fog without melting.
DEPOSITION is the
reverse process, when
water vapor becomes
ice directly. If you have
ever observed “Jack
Frost” on a window, you
have witnessed
deposition.
HUMIDITY
• Humidity is 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 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.
Warm air can hold more water vapor than cold
air can before it becomes saturated.
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.
Changes in relative humidity
• One way is to change the amount of water
vapor in the air.
• The other is to raise or lower the
temperature of the air. This happens in
the atmosphere when a parcel of air is
cooled below the saturation level and
clouds or fog form.
Cooling air temperature results in a rise in
relative humidity, while raising air temperature
results in a decrease in relative humidity.
Dew Point
The DEW POINT is the
temperature to which a parcel of
air would have 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.
Low dew point temperatures mean air that is
moist, while high dew point temps mean
dry air.
Measuring Humidity
• Hygrometers continuously
measure 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 of which
measures the dry air
temperature, while the other is
covered with a wet fabric and
measures the temperature
when evaporation cools it.
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 therefore EXPANDS.
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.
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.
On the leeward side of the mountains, the
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.
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.
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 results in
winds toward the land from
both the Gulf of Mexico
and the Atlantic Ocean.
This convergence and
lifting of air over the
Florida panhandle results
in the greatest number of
mid-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 them
(without expending
energy) to great
heights 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 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.
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!!
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 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.
• If the surface temperature is above 4°C,
snow will melt before it reaches the
ground. 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.
• At colder temperatures, snow 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 forms 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
and turns to ice when it impacts objects.
Hail
Hail is produced in
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 as it
falls again, and this
process can be repeated
several times, giving it
many layers of ice.