Why do clouds form?

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Why do clouds form?
The physics behind the phenomenon
Pictures and presentation by Andrew J. Orgonik

Plainedge Middle School
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We have all looked up at clouds
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and maybe even gazed down on them from above!
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They add a dynamic view to the landscape
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can make an ordinary sunset amazing!
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and are omens of bad weather
ahead.
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What are the ingredients for cloud formation?
And how do all of these components come together
to create the clouds we see almost everyday?!
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Ingredients Required for Clouds
Water vapor (water as a gas)
Conditions favoring the change of state (from gas
to liquid or ice)
A surface for water vapor to condense on
(condensation nuclei)
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Why do we have clouds composed of water when it
makes up less than 5 of our atmosphere, and is
some areas is less than 1?
As we already know, nitrogen (78 by volume) and
oxygen (21) are the dominant gases in our
troposphere.
why dont we have nitrogen and oxygen clouds???
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Water can exist at all states, and change state
at temperatures experienced on the Earths
surface and lower atmosphere
Also a liquid!
Water vapor gas (invisible)
ice
liquid
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Conversely, nitrogen and oxygen are not liquid at
Earth temperatures!
Must be cooled to 196o C to condense! (liquefy)
Must be cooled to 183o C to condense! (liquefy)
As clouds are composed of tiny liquid droplets,
nitrogen and oxygen clouds are impossible in the
Earths troposphere.
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How does water vapor get into the air in the
first place?
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T
E
T
E
E
T
By evaporation and transpiration
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Evaporation requires energy
Some liquid water molecules gain more energy than
the surrounding liquid molecules
These molecules can now change into the gas phase
(evaporate) because they can break free of the
attractive forces from the other liquid molecules
The liquid molecules left behind have less energy
Therefore evaporation is a cooling process!
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The GREATER the temperature, the greater the
evaporation rate (more molecules of liquid have
the energy needed to escape)
This basic idea is really important for
understanding how clouds form!
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Water vapor exerts a pressure called vapor
pressure. This is a fundamental for
understanding cloud formation!
L
H
Pressure gauge
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In a closed container of water, molecules of
liquid evaporate and change into a gas.
L
H
Pressure gauge
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In a closed container of water, molecules of
liquid evaporate and change into a gas.
L
H
Pressure gauge
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In a closed container of water, molecules of
liquid evaporate and change into a gas.
L
H
Pressure gauge
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In a closed container of water, molecules of
liquid evaporate and change into a gas.
A small increase in pressure will be detected due
to the addition of motion of water molecules in
the air above.
H
Pressure gauge
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In the atmosphere, this is called vapor pressure
The part of the total atmospheric pressure
attributable to its water vapor content.
L
H
Pressure gauge
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At first (in a sealed container), more molecules
of water will leave the waters surface
(evaporate) than will return (condensation)
L
H
Pressure gauge
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At first (in a sealed container), more molecules
of water will leave the waters surface
(evaporate) than will return (condensation)
L
H
Pressure gauge
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At first (in a sealed container), more molecules
of water will leave the waters surface
(evaporate) than will return (condensation)
L
H
Pressure gauge
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At first (in a sealed container), more molecules
of water will leave the waters surface
(evaporate) than will return (condensation)
L
H
Pressure gauge
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At first (in a sealed container), more molecules
of water will leave the waters surface
(evaporate) than will return (condensation)
L
H
Pressure gauge
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At first (in a sealed container), more molecules
of water will leave the waters surface
(evaporate) than will return (condensation)
L
H
Pressure gauge
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However, as more and more molecules evaporate
from the waters surface, the steadily increasing
vapor pressure in the air above forces more and
more vapor molecules to return to the liquid.
L
H
Pressure gauge
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However, as more and more molecules evaporate
from the waters surface, the steadily increasing
vapor pressure in the air above forces more and
more vapor molecules to return to the liquid.
L
H
Pressure gauge
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However, as more and more molecules evaporate
from the waters surface, the steadily increasing
vapor pressure in the air above forces more and
more vapor molecules to return to the liquid.
L
H
Pressure gauge
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However, as more and more molecules evaporate
from the waters surface, the steadily increasing
vapor pressure in the air above forces more and
more vapor molecules to return to the liquid.
L
H
Pressure gauge
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However, as more and more molecules evaporate
from the waters surface, the steadily increasing
vapor pressure in the air above forces more and
more vapor molecules to return to the liquid.
L
H
Pressure gauge
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Eventually, a balance is reached in which the
number of vapor molecules returning to the
surface balances the number of liquid molecules
leaving the surface.
L
H
Pressure gauge
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Eventually, a balance is reached in which the
number of vapor molecules returning to the
surface balances the number of liquid molecules
leaving the surface.
L
H
Pressure gauge
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Eventually, a balance is reached in which the
number of vapor molecules returning to the
surface balances the number of liquid molecules
leaving the surface.
L
H
Pressure gauge
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Eventually, a balance is reached in which the
number of vapor molecules returning to the
surface balances the number of liquid molecules
leaving the surface.
Evaporation Condensation!
L
H
Pressure gauge
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Eventually, a balance is reached in which the
number of vapor molecules returning to the
surface balances the number of liquid molecules
leaving the surface.
Evaporation Condensation!
L
H
Pressure gauge
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Eventually, a balance is reached in which the
number of vapor molecules returning to the
surface balances the number of liquid molecules
leaving the surface.
Evaporation Condensation!
L
H
Pressure gauge
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This condition is often referred to as
saturation vapor pressure. However,
equilibrium vapor pressure is a more
scientifically accurate term because air does not
hold water vapor like a sponge holds water.
L
H
Pressure gauge
air molecules only co-exist with water vapor
molecules!
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L
H
Pressure gauge
If the water is now heated, what will happen to
the evaporation rate?
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Thats right! The added energy will increase the
evaporation rate because heat is absorbed by the
liquid molecules (remember evaporation is
dependent on temperature!)
L
H
Pressure gauge
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Thats right! The added energy will increase the
evaporation rate because heat is absorbed by the
liquid molecules (remember evaporation is
dependent on temperature!)
L
H
Pressure gauge
And what will happen to the vapor pressure?
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The vapor pressure will increaseuntil
L
H
Pressure gauge
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a new, higher equilibrium vapor pressure is
reached.
L
H
Pressure gauge
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a new, higher equilibrium vapor pressure is
reached.
L
H
Pressure gauge
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Until a new, higher equilibrium vapor pressure is
reached.
L
H
Pressure gauge
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In summary
The higher the temperature, the more moisture is
required for the equilibrium vapor pressure to be
reached. More water molecules are needed in the
air (and returning to the liquid phase) to
balance the evaporating molecules
Saturation is a term that might make it easier
to explain this phenomenon, but it is inaccurate
It is not the case that warm air holds more
water vapor, it is just that the evaporation rate
is higher at this temperature, and therefore the
equilibrium level is higher.
If warm air is very dry (low in water vapor), it
wont reach the equilibrium level!
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Cold air doesnt hold less water vapor, it is
just that the evaporation rate decreases at lower
temperatures, and therefore the equilibrium vapor
pressure is lower.
Colder air does not require as much moisture to
reach the equilibrium level. Less water
molecules are needed in the air (and returning to
the liquid phase) to balance the evaporating ones
Therefore, cooling air (with a fixed amount of
water vapor) makes it easier for the
equilibrium vapor pressure to be reached. this
is why cooling air favors condensation (but more
about this later!)
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IMPORTANT TO KNOW BEFORE WE MOVE ON
Equilibrium vapor pressure is the pressure (due
to temperature) at which evaporation
condensation
The amount of water vapor required for
equilibrium varies according to temperature
For every 10o C increase in temperature, the
amount of water vapor required for equilibrium
(saturation) almost doubles!
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Equilibrium mass of water vapor (g) per kilogram
of dry air 0.3 0.75 2 3.5 7 14 26.5 47
TEMPERATURE (oC) -30 -20 -10 0 10 20 30 40
Data taken from Lutgens and Tarbuck
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IMPORTANT TO KNOW BEFORE WE MOVE ON
The evaporation rate is dependent on TEMPERATURE
The condensation rate is dependent on the HUMIDITY
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The atmosphere acts much like our closed
container in the model described before
Gravity, rather than a lid, prevents water vapor
from escaping into space
Water vapor is constantly evaporating from liquid
surfaces (lakes, oceans, etc.) just like it was
in our closed container
However, in nature, a balance between evaporation
and condensation is not always achieved
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More water molecules may leave the surface of a
drop of water than return to it (NET
EVAPORATION or drying)
Liquid water molecules
Water vapor molecule
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More water molecules may leave the surface of a
drop of water than return to it (NET
EVAPORATION or drying)
Liquid water molecules
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More water molecules may leave the surface of a
drop of water than return to it (NET
EVAPORATION or drying)
Water vapor molecule
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More water molecules may leave the surface of a
drop of water than return to it (NET
EVAPORATION or drying)
EVAPORATION (E) gt CONDENSATION (C)
The liquid drop gets smaller and will completely
evaporate!
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ORMore water molecules may condense on the
surface of a drop of water than leave it (NET
CONDENSATION or wetting)
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More water molecules may condense on the surface
of a drop of water than leave it (NET
CONDENSATION or wetting)
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More water molecules may condense on the surface
of a drop of water than leave it (NET
CONDENSATION or wetting)
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More water molecules may condense on the surface
of a drop of water than leave it (NET
CONDENSATION or wetting)
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More water molecules may condense on the surface
of a drop of water than leave it (NET
CONDENSATION or wetting)
CONDENSATION (C) gt EVAPORATION (E)
The liquid drop gets bigger!
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In order for clouds to form, we need CgtE!
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Well then, what conditions will favor
condensation over evaporation?
Decreasing temperatures (remember that amount of
water vapor required for equilibrium is lower at
lower temperatures.)
High water vapor pressure (which translates to
high humidity large quantities of water vapor
in the air)
Condensation nuclei (a critical component we will
discuss soon!)
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We can think of the equilibrium vapor pressure as
the turning point because this is where E C
(a balance point.) The scale can be tipped in
either direction here
E
C
EQUILIBRIUM
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We can think of the equilibrium vapor pressure as
the turning point because this is where E C
(a balance point.) The scale can be tipped in
either direction here
C
E
CLOUDS DRY UP!
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We can think of the equilibrium vapor pressure as
the turning point because this is where E C
(a balance point.) The scale can be tipped in
either direction here
E
Cloud droplets can grow!
C
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How can we track the evaporation rate versus the
condensation rate in order to determine if clouds
will form???
BY USING THE FORMULA FOR RELATIVE HUMIDITY
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RELATIVE HUMIDITY
Airs water vapor content

X 100
Amount of water vapor required for equilibrium
vapor pressure
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Since the condensation rate is proportional to
the airs water vapor content we can substitute
into the formula below
RELATIVE HUMIDITY
Condensation Rate
Airs water vapor content

X 100
Evaporation Rate
Amount of water vapor required for equilibrium
vapor pressure
Since the amount of water vapor required for
equilibrium is proportional to the evaporation
rate, we can also substitute this into the formula
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Relative Humidity indicates how near the air is
to the equilibrium (when CE)
Condensation Rate
RELATIVE HUMIDITY

X 100
Evaporation Rate
When the condensation rate and the evaporation
rate are equal (equilibrium), the relative
humidity is 100.
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Relative Humidity indicates how near the air is
to the equilibrium (when CE)
Condensation Rate
RELATIVE HUMIDITY

X 100
Evaporation Rate
When the condensation rate is 1/2 the evaporation
rate, the relative humidity is 50.
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Relative Humidity indicates how near the air is
to the equilibrium (when CE)
Condensation Rate
RELATIVE HUMIDITY

X 100
Evaporation Rate
When the condensation rate is 1/4 the evaporation
rate, the relative humidity is 25.
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How do we determine how close we are to a
relative humidity of 100? (cloud-building levels)
By determining the difference between the air
temperature and the DEW POINT TEMPERATURE
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DEW POINT TEMPERATURE is the temperature at which
air would need to be cooled to reach the
equilibrium vapor pressure
This is the temperature at which CE!
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Above this temperature, the evaporation rate will
outpace the condensation rate
C
E
CLOUDS DRY UP!
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Above this temperature, the evaporation rate will
outpace the condensation rate
C
E
CLOUDS DRY UP!
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Above this temperature, the evaporation rate will
outpace the condensation rate
C
E
CLOUDS DRY UP!
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Above this temperature, the evaporation rate will
outpace the condensation rate
C
E
CLOUDS DRY UP!
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Below the dew point temperature, the condensation
rate will outpace the evaporation rate
E
Cloud droplets can grow!
C
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This means there will be more water vapor in the
air than the equilibrium level.
E
Cloud droplets can grow!
C
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Balance will be obtained again as this extra
water vapor condenses out to form clouds.
E
Cloud droplets can grow!
C
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Balance will be obtained again as this extra
water vapor condenses out to form clouds.
Will decrease as condensation increases
Airs water vapor content
RELATIVE HUMIDITY

X 100
Amount of water vapor required for equilibrium
vapor pressure
E
C
This is why relative humidity doesnt usually
exceed 100!
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Dew Point Temperature, unlike Relative Humidity,
measures the actual moisture content of the air.
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If the Dew Point Temperature is high (ex. 85o F),
it means that the equilibrium vapor pressure is
also high.
This means that there is so much water vapor in
the air, the air doesnt need to be cooled much
for relative humidity to equal 100 (equilibrium.)
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Even though the evaporation rate is high (high
temperature), the condensation rate is also high
because there is so much water vapor in the air
VERY HIGH!
Condensation Rate
RELATIVE HUMIDITY

X 100
Evaporation Rate
Does not need to be lowered much for CE!
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If the Dew Point Temperature is low (ex. 33o F),
it means that the equilibrium vapor pressure is
also low.
This means that the air is so dry (C rate so
low), the air must be cooled a lot for relative
humidity to equal 100 (equilibrium.)
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The amount of moisture in the air is so low that
the evaporation rate must be lowered A LOT (by
lowering temperature) for the condensation rate
to equal the evaporation rate.
VERY LOW
Condensation Rate
RELATIVE HUMIDITY

X 100
Evaporation Rate
Must be lowered a lot for CE!
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For every 10o C (18o F) increase in dew point
temperature, the air contains twice as much water
vapor.
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Dry air but high R.H.
Air Temp.
Dew Pt. Temp.
R.H. ?
100
Moister air, but lower R.H.!
Moist air AND high R.H.!
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How does Relative Humidity Change?
1 By the addition or removal of water vapor
If water vapor is added to air this rate will
increaseand R.H. will too!
Condensation Rate
RELATIVE HUMIDITY
X 100

Evaporation Rate
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How does Relative Humidity Change?
1 By the addition or removal of water vapor
If water vapor is removed from air this rate will
decreaseand R.H. will too!
Condensation Rate
RELATIVE HUMIDITY
X 100

Evaporation Rate
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INITIAL CONDITION
1 kg air
20o C
7 g water vapor
Vapor required for equilibrium ?
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Equilibrium mass of water vapor (g) per kilogram
of dry air 0.3 0.75 2 3.5 7 14 26.5 47
TEMPERATURE (oC) -30 -20 -10 0 10 20 30 40
Data taken from Lutgens and Tarbuck
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INITIAL CONDITION
1 kg air
R.H. ?
20o C
7 g water vapor
Vapor required for equilibrium 14 g
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7 g
RELATIVE HUMIDITY
Airs water vapor content

X 100
Amount of water vapor required for equilibrium
vapor pressure
14 g
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INITIAL CONDITION
1 kg air
R.H. 50
20o C
7 g water vapor
Vapor required for equilibrium 14 g
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Addition of 5 grams of water vapor by
evaporation, NO temperature change!
1 kg air
20o C
7 g water vapor 5 g 12 g
Vapor required for equilibrium 14 g
E
98
Addition of 5 grams of water vapor by
evaporation, NO temperature change!
1 kg air
R.H. 12
100
20o C
12 g water vapor
14
Vapor required for equilibrium 14 g
E
99
Addition of 5 grams of water vapor by
evaporation, NO temperature change!
1 kg air
R.H. 86
20o C
12 g water vapor
in a sealed container, we will eventually reach
100 even without heating!
Vapor required for equilibrium 14 g
E
100
When moist air mixes with drier air, it may raise
the relative humidity of the drier air to 100.
This can happen without cooling the air any
further. (humidity increases and the condensation
rate now exceeds the evaporation rate!)
Evaporation from the pond increases humidity in
the air over the pond. This creates a cloud
(fog) over the water when there is none over the
land!
Fog over St. Johns Pond Cold Spring Harbor, NY
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St. Johns Pond
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Foggy Morning at Judys PondNewcomb, NY
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How does Relative Humidity Change?
2 Temperature increases or decreases
Condensation Rate
RELATIVE HUMIDITY
X 100

Evaporation Rate
If temperature decreases, this rate will too, and
R.H. will increase!
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How does Relative Humidity Change?
2 Temperature increases or decreases
Condensation Rate
RELATIVE HUMIDITY
X 100

Evaporation Rate
In addition, the vapor pressure will decrease.
This means less water vapor is needed for
equilibrium.
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How does Relative Humidity Change?
2 Temperature increases or decreases
Condensation Rate
RELATIVE HUMIDITY
X 100

Evaporation Rate
As a result, the amount of vapor present in the
air may become enough for clouds to form, even if
it wasnt before (at higher temperatures.)
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INITIAL CONDITION
1 kg air
R.H. 50
20o C
7 g water vapor
Vapor required for equilibrium 14 g
107
COOLING AIR TO 10o C
1 kg air
10o C
7 g water vapor
Vapor required for equilibrium ? g
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Equilibrium mass of water vapor (g) per kilogram
of dry air 0.3 0.75 2 3.5 7 14 26.5 47
TEMPERATURE (oC) -30 -20 -10 0 10 20 30 40
Data taken from Lutgens and Tarbuck
109
COOLING AIR TO 10o C
1 kg air
R.H. ?
10o C
7 g water vapor
Vapor required for equilibrium 7 g
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COOLING AIR TO 10o C
1 kg air
R.H. 7 g
100
10o C
7 g
7 g water vapor
Vapor required for equilibrium 7 g
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COOLING AIR TO 10o C
1 kg air
R.H.
100
10o C
7 g water vapor
A change in R.H. without the addition of any
water vapor!
Vapor required for equilibrium 7 g
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A real-life example of this is when fog and dew
form early in the morning (especially on humid
days)when air temperatures are at their lowest.
This especially happens on and around plants
because they enrich air with humidity with the
transpiration from their leaves.
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Early morning view from the top of Mount Goodnow
(Newcomb, NY)
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(No Transcript)
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How does Relative Humidity Change?
2 Temperature increases or decreases
Condensation Rate
RELATIVE HUMIDITY
X 100

Evaporation Rate
If temperature increases, this rate will too, and
R.H. will decrease!
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How does Relative Humidity Change?
2 Temperature increases or decreases
Condensation Rate
RELATIVE HUMIDITY
X 100

Evaporation Rate
Now the vapor pressure will increase, and it will
require more vapor for equilibrium.
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How does Relative Humidity Change?
2 Temperature increases or decreases
Condensation Rate
RELATIVE HUMIDITY
X 100

Evaporation Rate
The amount of water vapor in the air will not be
enough for the condensation rate to balance
evaporation.
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INITIAL CONDITION
1 kg air
R.H. 50
20o C
7 g water vapor
Vapor required for equilibrium 14 g
Assume no liquid water in flask
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HEATING AIR TO 30o C
1 kg air
30o C
7 g water vapor
Vapor required for equilibrium ? g
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Equilibrium mass of water vapor (g) per kilogram
of dry air 0.3 0.75 2 3.5 7 14 26.5 47
TEMPERATURE (oC) -30 -20 -10 0 10 20 30 40
Data taken from Lutgens and Tarbuck
121
HEATING AIR TO 30o C
1 kg air
30o C
7 g water vapor
Vapor required for equilibrium 26.5 g
122
HEATING AIR TO 30o C
1 kg air
R.H. ?
30o C
7 g water vapor
Vapor required for equilibrium 26.5 g
123
HEATING AIR TO 30o C
1 kg air
R.H. 7 g
100
30o C
26.5 g
7 g water vapor
Vapor required for equilibrium 26.5 g
124
HEATING AIR TO 30o C
1 kg air
R.H. 26
30o C
7 g water vapor
A decrease in R.H. without a change in water vapor
Vapor required for equilibrium 26.5 g
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Fog and dew dissipate later in the morning, once
temperatures have risen above the dew point
temperature
126
View from Mt. Goodnow, several hours after the
previous picture
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There is a physical problem to making clouds that
must be discussed, as we have not yet discussed
an important ingredient of cloud formation.
128
Clouds are composed of microscopic particles of
liquid water (averaging under 0.02 mm!)
They must be small or they would not remain
suspended in air!
129
The rate of evaporation from a water droplet
increases as the size of the molecule decreases.
130
Liquid Water Droplet Size and Evaporation Rate
EVAPORATION RATE INCREASING
Cloud droplets dont grow instantaneously, they
do it gradually. This presents a problem,
because for a tiny droplet to grow, it must be in
an environment such that the rate of condensation
is greater than that of evaporation.
131
Liquid Water Droplet Size and Evaporation Rate
EVAPORATION RATE INCREASING
If a typical baby cloud droplet of radius 0.001
millimeter is to grow, the relative humidity of
its environment must be greater than 300 because
the rate of evaporation from such a small drop is
so great.
132
Relative Humidities of 300 are not observed in
our atmosphere, yet clouds do exist! So
something else must be needed to save these
budding clouds.
133
Cloud droplets can survive by latching onto
microscopic solid particles, or condensation
nuclei in our atmosphere. These solid particles
can be dust, smoke, and salt particles.
From volcanoes
From Forest Fires
The Ocean Salt water droplets from the ocean are
carried by updrafts into the atmosphere. When
the water evaporates, the salt is left behind.
Pollution
(First three pictures are not by the author)
134
The best condensation nuclei are hygroscopic, or
water absorbent
We can think of them as water-droplet magnets
Water vapor molecules
Condensation Nucleus
135
The best condensation nuclei are hygroscopic, or
water absorbent
We can think of them as water-droplet magnets
136
The best condensation nuclei are hygroscopic, or
water absorbent
We can think of them as water-droplet magnets
137
The best condensation nuclei are hygroscopic, or
water absorbent
We can think of them as water-droplet magnets
138
The best condensation nuclei are hygroscopic, or
water absorbent
We can think of them as water-droplet magnets
139
The best condensation nuclei are hygroscopic, or
water absorbent
We can think of them as water-droplet magnets
140
The best condensation nuclei are hygroscopic, or
water absorbent
We can think of them as water-droplet magnets
Liquid water (drops coalesced together)
Condensation nuclei allow a water droplet to grow
to a size large enough that can now avoid being
dried out by evaporation.
141
Condensation nuclei hold the liquid droplets long
enough so another vapor molecule can condense on
it.
142
Condensation nuclei hold the liquid droplets long
enough so another vapor molecule can condense on
it.
They increase the probability that more water
molecules will hit the growing drop rather than
leave it!
143
Condensation nuclei hold the liquid droplets long
enough so another vapor molecule can condense on
it.
They increase the probability that more water
molecules will hit the growing drop rather than
leave it!
144
Condensation nuclei hold the liquid droplets long
enough so another vapor molecule can condense on
it.
They increase the probability that more water
molecules will hit the growing drop rather than
leave it!
Due to condensation nuclei, clouds can form even
at relative humidities that are below 100! (Even
as low as 75!)
145
Condensation nuclei hold the liquid droplets long
enough so another vapor molecule can condense on
it.
They increase the probability that more water
molecules will hit the growing drop rather than
leave it!
Due to condensation nuclei, clouds can form even
at relative humidities that are below 100! (Even
as low as 75!)
146
If the condensation nuclei is soluble (such as
salt), they are even more effective at keeping
the growing liquid droplet together. The reason
for this is that dissolving anything in water
lowers the vapor pressure of the water (lowers
the evaporation rate!)
147
What factors result in cloud formation?
148
Air rising and cooling to the dew point by
expansion (adiabatic cooling)
By forced liftingsuch as when air is forced over
a mountain
Pictures from the National Audubon Society Field
Guide to Weather
149
Air rising and cooling to the dew point by
expansion (adiabatic cooling)
By forced liftingsuch as when less dense warm
air is forced above more dense cold air (when two
air masses meet)
150
Air rising and cooling to the dew point by
expansion (adiabatic cooling)
By forced liftingsuch as when less dense warm
air is forced above more dense cold air (when two
air masses meet)
151
Air rising and cooling to the dew point by
expansion (adiabatic cooling)
By convection The Sun heating the ground (by
radiation), which then heats the air above (by
conduction), which then rises due to convection
(is less dense than the cooler air surrounding
it.)
Picture from the National Audubon Society Field
Guide to Weather
152
Clouds can be made of ice too, and we have not
introduced this idea into our discussion yet. It
is part of an important process of cloud
formation that occurs at temperatures around 10
to 20o C
Ice-crystal cirrus clouds
153
The equilibrium vapor pressure above ice crystals
is lower than above liquid droplets.
The reason for this is that ice crystals are
solid, and the individual water molecules are
held more tightly together than the water
molecules in a liquid (greater attractive forces
in the solid state.)
As a result, it is easier for water molecules to
escape from the liquid droplets (attractive
forces are less--higher vapor pressure)
154
Where liquid water and ice co-exist, water vapor
will move from where it is higher concentration
to lower concentration.
NOTE It is possible for ice and liquid water
to co-exist at below freezing temperatures.
Liquid water can be supercooled well below 0o C
and will not freeze unless it contacts a
freezing nuclei
Higher vapor pressure above liquid
Lower vapor pressure above ice
Liquid water
Ice
155
Where liquid water and ice co-exist, water vapor
will move from where it is higher concentration
to lower concentration.
Higher vapor pressure above liquid
Lower vapor pressure above ice
Liquid water
Ice
156
The ice crystals will collect more of these
liquid molecules than it will lose, because it
has such a low vapor pressure. The liquid turns
to ice on contact with the growing ice cloud.
Higher vapor pressure above liquid
Lower vapor pressure above ice
Liquid water
Ice
157
As a result, the ice crystal cloud will grow at
the expense of a liquid cloud nearby because of
the transfer of molecules from high to low
concentration.
Higher vapor pressure above liquid
Lower vapor pressure above ice
Liquid water
Ice
158
As a result, the ice crystal cloud will grow at
the expense of a liquid cloud nearby because of
the transfer of molecules from high to low
concentration.
Higher vapor pressure above liquid
Lower vapor pressure above ice
Liquid water
Ice
159
As a result, the ice crystal cloud will grow at
the expense of a liquid cloud nearby because of
the transfer of molecules from high to low
concentration.
Higher vapor pressure above liquid
Lower vapor pressure above ice
Turned to ice
Liquid water
Ice
160
As a result, the ice crystal cloud will grow at
the expense of a liquid cloud nearby because of
the transfer of molecules from high to low
concentration.
Higher vapor pressure above liquid
Lower vapor pressure above ice
Liquid water
Ice
161
As a result, the ice crystal cloud will grow at
the expense of a liquid cloud nearby because of
the transfer of molecules from high to low
concentration.
Higher vapor pressure above liquid
Lower vapor pressure above ice
Liquid water
Ice
162
As a result, the ice crystal cloud will grow at
the expense of a liquid cloud nearby because of
the transfer of molecules from high to low
concentration.
Liquid water
Ice
163
As a result, the ice crystal cloud will grow at
the expense of a liquid cloud nearby because of
the transfer of molecules from high to low
concentration.
Liquid water
Ice
164
As a result, the ice crystal cloud will grow at
the expense of a liquid cloud nearby because of
the transfer of molecules from high to low
concentration.
Liquid water
Ice
165
A REAL picture of the previous explanation!
(Hole-punch cloud)
Picture taken from the Astronomy Picture of the
Day website http//antwrp.gsfc.nasa.gov/apod/astr
opix.html
SEE APOD explanation (in speaker notes right
click and select speaker notes)
166
Required (and enjoyable!) reading
This book was essential for completing (and
understanding) this presentation
167
REFERENCES CONSULTED
Ahrens, Donald C. Meteorology Today 7th ed.
Thomson Brooks Cole, 2003.
Bohren, Craig F. Clouds in a Glass of Beer
Simple Experiments in Atmospheric Physics. New
York Dover Publications Inc., 2001.
Link, Robert A. Earth Science Teacher Emeritus,
Plainedge Middle School (personal communication)
My mentor and the one who made this all make
sense in the first place. Thanks forever Bob!
Lutgens, Frederick K., and Edward Tarbuck. The
Atmosphere An Introduction to Meteorology. New
Jersey Prentice Hall, 2001.
Wood, Elizabeth A. Science From Your Airplane
Window. New York Dover Publications Inc., 1975.