Why do clouds form?
The physics behind the phenomenon
Pictures and presentation by Andrew J. Orgonik
Plainedge Middle School
We have all
looked up at
clouds…
…and maybe even gazed
down on them from above!
They add a dynamic view to
the landscape…
…can make an
ordinary sunset
amazing!
…and are omens of bad weather ahead.
What are the “ingredients” for cloud formation? And how do all of these
components come together to create the clouds we see almost everyday?!
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)
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 don’t we have nitrogen and oxygen clouds???
Water can exist at all states, and change
state at temperatures experienced on the
Earth’s surface and lower atmosphere
Also a
liquid!
Water
vapor: gas
(invisible)
ice
liquid
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 Earth’s troposphere.
How does water vapor get into the
air in the first place?
T
E
E
T
T
By evaporation and transpiration
E
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!
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!
Water vapor exerts a pressure called “vapor
pressure.” This is a fundamental for
understanding cloud formation!
Pressure gauge
In a closed container of water, molecules of
liquid evaporate and change into a gas.
Pressure gauge
In a closed container of water, molecules of
liquid evaporate and change into a gas.
Pressure gauge
In a closed container of water, molecules of
liquid evaporate and change into a gas.
Pressure gauge
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.
Pressure gauge
In the atmosphere, this is called vapor pressure:
“The part of the total atmospheric pressure
attributable to its water vapor content.”
Pressure gauge
At first (in a sealed container), more molecules of
water will leave the water’s surface (evaporate)
than will return (condensation)
Pressure gauge
At first (in a sealed container), more molecules of
water will leave the water’s surface (evaporate)
than will return (condensation)
Pressure gauge
At first (in a sealed container), more molecules of
water will leave the water’s surface (evaporate)
than will return (condensation)
Pressure gauge
At first (in a sealed container), more molecules of
water will leave the water’s surface (evaporate)
than will return (condensation)
Pressure gauge
At first (in a sealed container), more molecules of
water will leave the water’s surface (evaporate)
than will return (condensation)
Pressure gauge
At first (in a sealed container), more molecules of
water will leave the water’s surface (evaporate)
than will return (condensation)
Pressure gauge
However, as more and more molecules evaporate from the
water’s surface, the steadily increasing vapor pressure in
the air above forces more and more vapor molecules to
return to the liquid.
Pressure gauge
However, as more and more molecules evaporate from the
water’s surface, the steadily increasing vapor pressure in
the air above forces more and more vapor molecules to
return to the liquid.
Pressure gauge
However, as more and more molecules evaporate from the
water’s surface, the steadily increasing vapor pressure in
the air above forces more and more vapor molecules to
return to the liquid.
Pressure gauge
However, as more and more molecules evaporate from the
water’s surface, the steadily increasing vapor pressure in
the air above forces more and more vapor molecules to
return to the liquid.
Pressure gauge
However, as more and more molecules evaporate from the
water’s surface, the steadily increasing vapor pressure in
the air above forces more and more vapor molecules to
return to the liquid.
Pressure gauge
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.
Pressure gauge
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.
Pressure gauge
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.
Pressure gauge
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!
Pressure gauge
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!
Pressure gauge
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!
Pressure gauge
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.***
Pressure gauge
***air molecules
only co-exist with
water vapor
molecules!
Pressure gauge
If the water is now
heated, what will
happen to the
evaporation rate?
That’s right! The added energy will increase the
evaporation rate because heat is absorbed by the
liquid molecules (remember evaporation is dependent
on temperature!)
Pressure gauge
That’s right! The added energy will increase the
evaporation rate because heat is absorbed by the
liquid molecules (remember evaporation is dependent
on temperature!)
Pressure gauge
And what will
happen to the
vapor pressure?
The vapor pressure will increase…until…
Pressure gauge
a new, higher equilibrium vapor pressure is reached.
Pressure gauge
a new, higher equilibrium vapor pressure is reached.
Pressure gauge
Until a new, higher equilibrium vapor pressure is
reached.
Pressure gauge
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 won’t
reach the equilibrium level!
Cold air doesn’t “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!)
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!
TEMPERATURE (oC)
Equilibrium mass of
water vapor (g) per
kilogram of dry air
-30
0.3
-20
0.75
-10
2
0
3.5
10
7
20
14
30
26.5
40
47
Data taken from
Lutgens and Tarbuck
IMPORTANT TO KNOW BEFORE WE
MOVE ON…
The evaporation rate is dependent on TEMPERATURE
The condensation rate is dependent on the HUMIDITY
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…
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
More water molecules may leave the surface of a
drop of water than return to it:
(NET EVAPORATION or “drying”)
Liquid water
molecules
More water molecules may leave the surface of a
drop of water than return to it:
(NET EVAPORATION or “drying”)
Water vapor
molecule
More water molecules may leave the surface of a
drop of water than return to it:
(NET EVAPORATION or “drying”)
EVAPORATION (E) > CONDENSATION (C)
The liquid drop gets smaller and will completely evaporate!
OR…More water molecules may condense on the
surface of a drop of water than leave it:
(NET CONDENSATION or “wetting”)
More water molecules may condense on the surface
of a drop of water than leave it:
(NET CONDENSATION or “wetting”)
More water molecules may condense on the surface
of a drop of water than leave it:
(NET CONDENSATION or “wetting”)
More water molecules may condense on the surface
of a drop of water than leave it:
(NET CONDENSATION or “wetting”)
More water molecules may condense on the surface
of a drop of water than leave it:
(NET CONDENSATION or “wetting”)
CONDENSATION (C) > EVAPORATION (E)
The liquid drop gets bigger!
In order for clouds to form, we need C>E!
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!)
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:
EQUILIBRIUM
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:
CLOUDS DRY UP!
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:
Cloud
droplets can
grow!
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
RELATIVE =
HUMIDITY
Air’s water vapor content
Amount of water vapor
required for equilibrium
vapor pressure
X
100
Since the condensation rate is proportional to
the air’s water vapor content we can substitute
into the formula below…
RELATIVE =
HUMIDITY
Condensation
Air’s water vapor Rate
content
Amount of waterRate
vapor
Evaporation
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
X
100
Relative Humidity indicates how near the air is
to the equilibrium (when C=E)
RELATIVE =
HUMIDITY
Condensation Rate
Evaporation Rate
When the condensation rate and the evaporation
rate are equal (equilibrium), the relative
humidity is 100%.
X
100
Relative Humidity indicates how near the air is
to the equilibrium (when C=E)
RELATIVE =
HUMIDITY
Condensation Rate
Evaporation Rate
When the condensation rate is 1/2 the
evaporation rate, the relative humidity is 50%.
X
100
Relative Humidity indicates how near the air is
to the equilibrium (when C=E)
RELATIVE =
HUMIDITY
Condensation Rate
Evaporation Rate
When the condensation rate is 1/4 the
evaporation rate, the relative humidity is 25%.
X
100
How do we determine how close we are
to a relative humidity of 100%? (cloudbuilding levels)
By determining the difference between
the air temperature and the
DEW POINT TEMPERATURE
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 C=E!
Above this temperature, the evaporation
rate will outpace the condensation rate
CLOUDS DRY UP!
Above this temperature, the evaporation
rate will outpace the condensation rate
CLOUDS DRY UP!
Above this temperature, the evaporation
rate will outpace the condensation rate
CLOUDS DRY UP!
Above this temperature, the evaporation
rate will outpace the condensation rate
CLOUDS DRY UP!
Below the dew point temperature, the
condensation rate will outpace the
evaporation rate
Cloud
droplets can
grow!
This means there will be more water vapor
in the air than the equilibrium level.
Cloud
droplets can
grow!
Balance will be obtained again as this
“extra” water vapor condenses out to form
clouds.
Cloud
droplets can
grow!
Balance will be obtained again as this
“extra” water vapor condenses out to form
Will decrease as
clouds.
condensation
RELATIVE HUMIDITY =
Air’s water vapor content
Amount of water vapor required
for equilibrium vapor pressure
This is why relative
humidity doesn’t usually
exceed 100%!
increases
X
100
Dew Point Temperature, unlike
Relative Humidity, measures the actual
moisture content of the air.
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 doesn’t need to be cooled much for
relative humidity to equal 100% (equilibrium.)
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
RELATIVE =
HUMIDITY
Condensation Rate
Evaporation Rate
Does not need to be lowered
much for C=E!
X
100
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.)
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.
RELATIVE =
HUMIDITY
Condensation Rate
Evaporation Rate
Must be lowered a lot for
C=E!
X
100
For every 10o C (18o F) increase in
dew point temperature, the air contains
twice as much water vapor.
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.!
How does Relative Humidity Change?
#1 By the addition or removal of water vapor
If water vapor is added to air this rate
will increase…and R.H. will too!
RELATIVE =
HUMIDITY
Condensation Rate
X
Evaporation Rate
100
How does Relative Humidity Change?
#1 By the addition or removal of water vapor
If water vapor is removed from air this
rate will decrease…and R.H. will too!
RELATIVE =
HUMIDITY
Condensation Rate
X
Evaporation Rate
100
INITIAL CONDITION
1 kg air
7 g water vapor
20o C
Vapor required for
equilibrium = ?
TEMPERATURE (oC)
Equilibrium mass of
water vapor (g) per
kilogram of dry air
-30
0.3
-20
0.75
-10
2
0
3.5
10
7
20
14
30
26.5
40
47
Data taken from
Lutgens and Tarbuck
INITIAL CONDITION
1 kg air
R.H. = ?
7 g water vapor
20o C
Vapor required for
equilibrium = 14 g
7g
RELATIVE =
HUMIDITY
Air’s water vapor content
Amount of water vapor
required for equilibrium
vapor pressure
14 g
X
100
INITIAL CONDITION
1 kg air
R.H. = 50%
7 g water vapor
20o C
Vapor required for
equilibrium = 14 g
Addition of 5 grams of water vapor by evaporation, NO
temperature change!
1 kg air
7 g water vapor
+ 5 g =12 g
E
20o C
Vapor required for
equilibrium = 14 g
Addition of 5 grams of water vapor by evaporation, NO
temperature change!
1 kg air
R.H. = 12 100
14
12 g water vapor
E
20o C
Vapor required for
equilibrium = 14 g
Addition of 5 grams of water vapor by evaporation, NO
temperature change!
1 kg air
R.H. = 86%
**in a sealed
container, we
will eventually
reach 100%
even without
heating!**
12 g water vapor
E
20o C
Vapor required for
equilibrium = 14 g
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. John’s
Pond: Cold Spring
Harbor, NY
St. John’s Pond
Foggy Morning at Judy’s Pond—Newcomb, NY
How does Relative Humidity Change?
#2 Temperature increases or decreases
RELATIVE =
HUMIDITY
Condensation Rate
X
Evaporation Rate
If temperature decreases, this rate will
too, and R.H. will increase!
100
How does Relative Humidity Change?
#2 Temperature increases or decreases
RELATIVE =
HUMIDITY
Condensation Rate
X
100
Evaporation Rate
In addition, the vapor pressure will decrease. This means less
water vapor is needed for equilibrium.
How does Relative Humidity Change?
#2 Temperature increases or decreases
RELATIVE =
HUMIDITY
Condensation Rate
X
Evaporation Rate
As a result, the amount of vapor present in the air may
become enough for clouds to form, even if it wasn’t before (at
higher temperatures.)
100
INITIAL CONDITION
1 kg air
R.H. = 50%
7 g water vapor
20o C
Vapor required for
equilibrium = 14 g
COOLING AIR TO 10o C
1 kg air
7 g water vapor
10o C
Vapor required for
equilibrium = ? g
TEMPERATURE (oC)
Equilibrium mass of
water vapor (g) per
kilogram of dry air
-30
0.3
-20
0.75
-10
2
0
3.5
10
7
20
14
30
26.5
40
47
Data taken from
Lutgens and Tarbuck
COOLING AIR TO 10o C
1 kg air
R.H. = ?
7 g water vapor
10o C
Vapor required for
equilibrium = 7 g
COOLING AIR TO 10o C
1 kg air
R.H. = 7 g 100
7g
7 g water vapor
10o C
Vapor required for
equilibrium = 7 g
COOLING AIR TO 10o C
1 kg air
R.H. = 100 %
7 g water vapor
A change in
R.H. without
the addition of
any water
vapor!
10o C
Vapor required for
equilibrium = 7 g
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.
Early morning view from the top of Mount Goodnow (Newcomb, NY)
How does Relative Humidity Change?
#2 Temperature increases or decreases
RELATIVE =
HUMIDITY
Condensation Rate
X
Evaporation Rate
If temperature increases, this rate will
too, and R.H. will decrease!
100
How does Relative Humidity Change?
#2 Temperature increases or decreases
RELATIVE =
HUMIDITY
Condensation Rate
X
Evaporation Rate
Now the vapor pressure will increase, and it will require
more vapor for equilibrium.
100
How does Relative Humidity Change?
#2 Temperature increases or decreases
RELATIVE =
HUMIDITY
Condensation Rate
X
Evaporation Rate
The amount of water vapor in the air will not be enough for
the condensation rate to balance evaporation.
100
INITIAL CONDITION
1 kg air
R.H. = 50%
7 g water vapor
Assume
no liquid
water in
flask
20o C
Vapor required for
equilibrium = 14 g
HEATING AIR TO 30o C
1 kg air
7 g water vapor
30o C
Vapor required for
equilibrium = ? g
TEMPERATURE (oC)
Equilibrium mass of
water vapor (g) per
kilogram of dry air
-30
0.3
-20
0.75
-10
2
0
3.5
10
7
20
14
30
26.5
40
47
Data taken from
Lutgens and Tarbuck
HEATING AIR TO 30o C
1 kg air
7 g water vapor
30o C
Vapor required for
equilibrium = 26.5 g
HEATING AIR TO 30o C
1 kg air
R.H. = ?
7 g water vapor
30o C
Vapor required for
equilibrium = 26.5 g
HEATING AIR TO 30o C
1 kg air
R.H. = 7 g
100
26.5 g
7 g water vapor
30o C
Vapor required for
equilibrium = 26.5 g
HEATING AIR TO 30o C
1 kg air
R.H. = 26%
A decrease in
R.H. without a
change in water
vapor
7 g water vapor
30o C
Vapor required for
equilibrium = 26.5 g
Fog and dew dissipate later in the morning,
once temperatures have risen above the dew
point temperature
View from Mt. Goodnow, several
hours after the previous picture
There is a physical problem to making
clouds that must be discussed, as we
have not yet discussed an important
ingredient of cloud formation.
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!
The rate of evaporation from a
water droplet increases as the
size of the molecule decreases.
Liquid Water Droplet Size and Evaporation Rate
EVAPORATION RATE INCREASING
Cloud droplets don’t 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.
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.
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.
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
Pollution
(First three pictures are not by the author)
Salt water droplets from
the ocean are carried by
updrafts into the
atmosphere. When the
water evaporates, the salt
is left behind.
The best condensation nuclei are
hygroscopic, or water absorbent
We can think of them as water-droplet
“magnets”
Water vapor
molecules
Condensation
Nucleus
The best condensation nuclei are
hygroscopic, or water absorbent
We can think of them as water-droplet
“magnets”
The best condensation nuclei are
hygroscopic, or water absorbent
We can think of them as water-droplet
“magnets”
The best condensation nuclei are
hygroscopic, or water absorbent
We can think of them as water-droplet
“magnets”
The best condensation nuclei are
hygroscopic, or water absorbent
We can think of them as water-droplet
“magnets”
The best condensation nuclei are
hygroscopic, or water absorbent
We can think of them as water-droplet
“magnets”
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.
Condensation nuclei hold the liquid droplets long
enough so another vapor molecule can condense on it.
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!
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!
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%!)
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%!)
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!)
What factors result in cloud formation?
Air rising and cooling to the dew point by expansion (adiabatic
cooling)
By forced lifting—such as when air is forced over a mountain:
Pictures from the National Audubon Society Field Guide to Weather
Air rising and cooling to the dew point by expansion (adiabatic
cooling)
By forced lifting—such as when less dense warm air is forced
above more dense cold air (when two air masses meet)
Air rising and cooling to the dew point by expansion (adiabatic
cooling)
By forced lifting—such as when less dense warm air is forced
above more dense cold air (when two air masses meet)
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
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
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)
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
Liquid water
Lower vapor
pressure above ice
Ice
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”
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
Liquid water
Lower vapor
pressure above ice
Ice
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
Liquid water
Lower vapor
pressure above ice
Ice
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
Liquid water
Lower vapor
pressure above ice
Ice
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
Liquid water
Lower vapor
pressure above ice
Ice
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
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
Liquid water
Lower vapor
pressure above ice
Ice
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
Liquid water
Lower vapor
pressure above ice
Ice
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
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
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
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/astropix.html
SEE “APOD”
explanation (in
speaker notes: right
click and select
speaker notes)
Required (and
enjoyable!) reading:
This book was
essential for
completing (and
understanding) this
presentation
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.