Cloud Formation &
Fronts
Ingredients Required for Clouds:
Water vapor (water as a gas)
Conditions favoring the change of state
(from gas to liquid or ice)
CONDENSATION
A surface for water vapor to
condense on (condensation nuclei)
How does water vapor
get into the air?
T
E
E
T
T
By evaporation and transpiration
E
Moisture Capacity
• Temperature controls the amount of
moisture that COULD go into the air
• The amount of moisture that COULD get
into the air is called its CAPACITY.
• The greater the temperature the greater
the moisture capacity of the air.
Temperature
How do you get the water vapor
out of the air?
• The vapor in the air must CONDENSE!
• This happens when the air is “Filled to its
capacity” or saturated (100% rel
humidity).
• These terms above are not really accurate,
but will serve as a good starting point.
• A surface is also needed.
How do you get the water vapor
out of the air?
• Morning dew is a perfect example of moisture
getting out of the air.
• Dew forms on cool surfaces only.
• Dust, smoke, ash, and salt particles in the air
serve as the surface for water vapor to
condense on.
• When the vapor hits these surfaces, it clings to
them and forms super tiny water droplets
around them
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?
•The air must rise
•As the air rises, it encounters less
pressure
•The air molecules expand and cool
(Adiabatic Cooling)
•When the air is cooled to the dew
point temperature condensation
occurs and clouds begin to form
What causes the air to rise?
• Forced up over a mountain range
• A mass of relatively low density is forced
up and over a mass of cooler, more dense
air
• Carried by a convection current
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
WINDWARD
SIDE
WET Air rises, expands,
cools, condensation
occurs, clouds form and
it rains.
LEEWARD
SIDE
DRY Air descends, is
compressed, and
heats up.
The second way to get air to rise
is for less dense air to be force
up and over more dense air.
• This happens at the boundaries between AIR
MASSES
• AIR MASS: A region of air that has similar
temperature and moisture.
What is an Air Mass?
• Air masses are large bodies of air which have
similar temperature and moisture
characteristics.
• Air masses form when air stays over a region
(called the source region) for several days.
• Air masses that form over water will be moist.
• Air masses that form over land will be dry.
AIR MASSES
• Air takes the characteristics of the surface over
which it formed.
• An air mass that forms over the ocean will be
moist. These air masses are called MARITIME
air masses
• An air mass that forms over land will be
relatively dry. These air masses are called
CONTINENTAL air masses
Air Masses
• An air mass that forms over a cold portion
of the Earth is called a POLAR air mass.
• An air mass that forms over a warm
portion of the Earth is called a TROPICAL
air mass.
• An air mass that forms over an
extraordinarily cold portion of the Earth is
called an ARTIC air mass.
Open your ESRT to page 13!
Air Masses
Air Mass Classification
Source Region
Identifiers:
• A for Arctic,
• P for Polar,
• T for Tropical;
Moisture Content
Identifiers:
• c for continental
(meaning the air is
relatively dry),
• m for maritime (meaning
the air is relatively moist);
•
•
•
•
cT
cP
mT
mP
Continental Tropical – dry & warm
Continental Polar – dry & cold
Maritime Tropical – humid & warm
Maritime Polar – humid & cold
• cA Continental Artic – very dry & very
cold
What happens when air masses
meet?
• The boundary between two air masses is
called a FRONT.
Fronts – boundary that separates 2
different air masses
• Either a warm front is advancing or a cold
front is advancing.
Kinds of Fronts
•
•
•
•
Cold Fronts
Warm Fronts
Stationary Fronts
Occluded Fronts
Go to page 13 of your ESRT!
Cold Front
• A cold front occurs when a more dense air
mass pushes under a less dense air mass –
short–lived thunderstorms
Cold air advancing toward
warmer air
Warm Front
• A warm front occurs when a less dense air
mass rides up over a more dense air mass –
longer, steady precipitation
Warm air advancing toward cold air
Stationary Front
• A stationary front occurs when the air masses
on either side of the front are not moving
toward each other.
Occluded Front
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)
A continental polar air mass comes down
from Canada and meets a maritime
tropical air mass from the Gulf of Mexico.
Watch the cloulds form.
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)
The third way in which air rises to
form clouds is by riding a rising
convection current
Remember convection cells in the Mantle
driving plate tectonics?
The same process occurs in the
atmosphere!
CONVECTION: Energy Transfer due to
differences in density of a fluid.
The rocks in the mantle can flow and so can
the air.
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
Air will rise when it
is heated, because it
will become less
dense. Air moves
in to replace the air
that rose up. The
converging air will
further rise. As it
rises, it cools and
water vapor will
condense. This will
release latent heat,
and further heat
the column of air.
What caused this band of clouds to
form over the equator?
In Summary
What is needed for clouds to form?
How is air forced to rise and cool (3 ways)?
What is the difference between the windward
and the leeward sides of mountains?
What is an air mass?
What is frontal uplift?
How does convection create clouds?