Download CLOUDS, CLASSIFICATION, AND

CLOUDS: KEYS TO UNDERSTANDING WEATHER,
CLIMATE, AND THE HYDROLOGIC CYCLE
-Jack R. Holt
A MEMORIAL DAY TRIP
If you don’t like the weather in Oklahoma, just
wait a minute.
-Will Rogers
I recall a particular windless, sultry
Memorial Day in Tulsa. A group within my
extended family had decided to go to a reservoir
outside of the city for a day of picnicking and
swimming. We secured a van for the purpose,
and spent the day trying to be comfortable under
a nearly cloudless sky. After a day in the
Oklahoma sun, we returned in late afternoon. I
was elected to drive. From a distance I could see
a large anvil-topped thunderhead building over
the city, and the first thing that came to mind
was “maybe it will cool things off.” As we
approached, we saw that the cloud had grown to
monstrous proportions and periodic lightening
flashes punctuated its blackness. The whole
scene looked as though an angry god was about
to take vengeance on the city, a notion that was
not far from being wrong.
Rain began to fall in unrelenting sheets so
that the windshield wipers became completely
ineffective. When I could see, the water had
filled the street to above the curb and formed
shallow streams that were growing. We made it
to my mother’s home, which stood on relatively
high ground and took shelter in the basement.
There, we sat, sometimes by candlelight, and
told stories, ate popcorn, and waited. The storm
raged throughout the evening and began to die
down in intensity only near midnight. However,
the authorities urged listeners by radio to stay in
their homes, because most roads were flooded
and very dangerous.
By morning, the rain had stopped, but Tulsa
had become a shallow lake, and the small hill on
which our house sat was one of many islands.
That morning, I went for a walk in the
neighborhood and saw downed trees, cars in odd
places, and even one car sitting atop another.
The news and weather programs that day (and
for days after that) announced that we had
received 11.5 inches of rain in a single storm.
The morning began with pillows of fair weather
cumulus clouds and gave way to another hot day.
Weather and clouds are related to each
other. Furthermore, the discovery of clouds as
physical entities that could be described, labeled,
and categorized, was a crucial link in
understanding weather and the great Hydrologic
Cycle of water from the oceans to the
atmosphere, and back, ultimately, to the oceans
again. The description of the general features of
the great cycle had been known since before
Greek Natural Philosophy, but an understanding
of clouds, cloud formation and the physics of
clouds was absolutely necessary to connect the
atmosphere to the ocean to help explain why
parts of the earth were deserts and others were
replete with rain.
A CLASSIFICATION OF CLOUDS
It is not in the least amiss for those who are
involved in meteorological research to give some
attention to the form of clouds.
-Lamarck
Jean Baptiste Pierre Antoine de Monet
chevalier de Lamarck (1744-1829; France)
was one of the very first to attempt to use a
classification system as a way to support a theory
of evolution. He had been made the curator of
worms (invertebrates) at the Musee Nationale in
Paris. In that position, he attempted to make
sense of the diversity of all animals without
backbones and build on the classification system
of Linnaeus, many years earlier. He also tried to
apply a method of classification to clouds, other
natural chaotic structures. Lamarck saw that
clouds seemed to occur in five different types:
• Hazy clouds (en forme de voile)
• Massed clouds (attroupes)
• Dappled clouds (pommeles)
• Broom-like clouds (en balayeurs)
• Grouped clouds (groupes)
He published his classification system of clouds
in 1802 and expanded it to twelve types in 1805.
Lamarck wrote that the purpose for such a
classification system was to group them
according to their causes. That is, he expected
that their shapes were related to how they were
formed. Thus, his categories were linked to
potential explanations of their formation.
In 1802, an Englishman named Luke
Howard (1772-1864) also saw a need to classify
clouds. He had been raised a Quaker, sent to a
Quaker school, and later in life became one of
the leading members of the Society of Friends.
Beyond that he did not have special education in
the sciences. Indeed, he was a successful
pharmacist. Although he was only an amateur
scientist, he had become a very prolific one. It
was at a meeting of amateur scientists that
Howard presented his classification system for
clouds. He categorized them into four basic types
and gave them descriptive Latin names:
• Cumulus (Latin, heap)
• Stratus (Latin, layer)
• Cirrus, (Latin, curl)
• Nimbus (Latin, rain)
The first three were named for their forms while
the last one was named for its action,
precipitation. He recognized that clouds by their
nature change and can change from one form to
another. While others had seen this as a problem
with a descriptive taxonomy, Howard expanded
his system to accommodate the transitional
stages. For example, cumulus clouds could
begin to bunch together. As they did so, they
formed a kind of broken layer of bunched clouds,
which he called cumulo stratus.
Other transitional clouds included:
• Cirro-Stratus (small, rounded clouds in
a layer)
• Cirro-cumulus
resembled
the
very
successful
Latin
classification system that Carolus Linnaeus had
created for living things. When meteorologists
all over the world had a means to categorize
clouds by formal definitions, then they could
communicate aspects of clouds.
FIGURE 2. . High waves of cirrus and
developing mid-level clouds as the leading edge
of a warm front moves from south to north at the
end of February 2005.
FIGURE 3. . This is a cumulus cloud that a
cumulo-nimbus (note the snow shower falling
from the left end of it).
FIGURE 1. Luke Howard. An image from the
John Day website at .
http://www.cloudman.com/luke/luke_howard.htm
His classification system was almost
immediately successful and almost universally
adopted. The reasons that the world adopted
Howard’s system, as opposed to that of Lamarck
likely are complex. However the consensus
seems to be that Lamarck used French phrases
while Howard used the more universal (and
neutral) Latin terms. Also, Howard’s system
CLOUDS AND WEATHER
The sky, too, belongs to the landscape. The
ocean of air in which we live and move, in which
the bolt of heaven is forged, and the fructifying
rain condensed, can never be to the zealous
Naturalist a subject of tame and unfeeling
contemplation.
-Luke Howard
Clouds
form
when
air
becomes
supersaturated with water, which then condenses.
There are several ways in which air can become
supersaturated: it can gain water, it can become
cooler, or it can change its ability to hold water
due to change in air pressure. Cloud formation
usually involves all three components. Fog is a
special type of cloud whose formation is
indipendent of air pressure because it develops
on the ground from air that is saturated and then
cools off (usually through radiative cooling).
mountain (see Figure 5). As air is forced up and
over the mountain, water condenses so that the
windward side of the mountain has a wet climate
with frequent rainfalls. As the air passes up and
over the mountain, it descends on the other side.
The warmer, denser air can hold much more
water, but most of that has been wrung out of it.
So, the other side is very dry, often desert, sitting
as it does in the rain shadow of the mountain.
FIGURE 5. A diagram of the influence that a
barrier like a mountain can have in a on regional
climates. The windward side of the mountain
has frequent rainfalls while the leeward side is a
desert in the rain shadow of the mountain.
FIGURE 4. A Diagram of cloud formation from
thermal convection. The column on the left
illustrates the relative density of air molecules
with height due to pressure changes. The arrows
indicate the movement of warmer, moistureladen air into higher, less dense, cold levels
where the moisture condenses to form clouds.
Most clouds develop as air rises. A simple
case would be that of a thermal (see Figure 4).
Air near the ground is heated through the day,
and begins to rise displacing the cooler air above
it. It carries moisture that had dissolved at the
temperature and pressure near the ground. As it
rises, air carries moisture from near the ground at
near sea-level atmospheric pressure. However,
air higher in the atmosphere has less air over it
pressing down; so, the air molecules are farther
apart and the air column is cooler, a mechanism
called adiabatic cooling. The water dissolved in
the air at the temperature and pressure on the
ground condenses at the more rarified, cooler
temperatures at higher levels, and clouds are
made.
This is not always benign.
The
thunderstorm that Tulsa experienced on the
Memorial Day was due to the thermal convection
from the heat island of the city. In a way, the
city brought down destruction on itself.
A similar situation can be seen when
moisture-laden air encounters a barrier, like a
Of course, the global situation that
influences weather and cloud formation usually
is much more complex (go to Ice Ages for a
more complete explanation). The atmosphere is
made of air masses that have different
temperatures and interact with each other. The
line along which air masses collide is called a
front. If a mass of warm air collides with a mass
of cold air, the warm air tends to override the
denser cool air (see Figure 6). The outcome is a
gradual change from high cirrus clouds followed
by midlevel stratus that gives way to low level
stratus that frequently becomes a nimbus.
Cold fronts have a different structure from
warm fronts. Because the approaching cold air is
denser than the warm air that it is moving into, it
tends to form a bulge. Along the leading edge,
warm air is forced upwards. There, immense
towering thunderheads that give rise to many of
the spring and summer severe storms can form
(see Figure 7).
During the winter many of the cold fronts
come down from the Arctic air mass. Such cold
air is very dense and tends to be associated with
high pressure cells. As such, it tends to descend
from the high, dry layers and produce those
clear, bright, cold winter days (see Figure 8).
This is similar to the descent of air on the rain
shadow side of a mountain.
FIGURE 6. A diagram of cloud formation and weather changes as a warm air mass slices intoa cold mass.
FIGURE 7. A cold front moves into a mass of warm air. The cold air, being denser, does not override the
warm mass, but moves into it along a bulging interface. It tends to force the warm air to rise along the
bulge , and can form severe weather.
FIGURE 8. A diagram of why high pressure
tends to form a cloudless sky. The high pressure
column tends to make the air descend from high,
dry air. Thus, there is no source of dissolved
water that would tend to give rise to clouds.
THE HYDROLOGIC CYCLE
The hydrologic cycle is the perpetual movement
of water throughout the various components of
Earth’s climate system.
Thomas Pagano & Soroosh Sorooshian
Clouds are among the most visible of the
components of Earth’s climate system, but the
amount of water actually contained in them is
diminishingly small compared to the amounts in
the oceans or on land (see Figure 9). Indeed,
most of the water held by the atmosphere is not
in the forms of clouds. Nevertheless, they are
necessary as conduits of water from the oceans
to land. Furthermore, evaporation removes heat
from the oceans and serves to transfer it from
warmer, tropical regions toward the poles.
A major pool of water, particularly
freshwater, can be found in glaciers and other ice
fields. Though some occur at high altitudes,
most occur at high latitudes. Glaciers can
contain as much as 70% of all freshwater
(though freshwater is only 2.5% of the total
amount of water on Earth). Although permanent
ice has so much water locked up, it does not
figure significantly in the hydrologic cycle,
because the exchange rates are so low. For
example, glacial ice renews itself at the rate of
about 10,000 years while atmospheric water
renews itself about every 18 days. So, although
there is less water in the atmosphere than in polar
ice, the contribution to the hydrologic cycle by
polar ice is negligible.
FIGURE 9. The Hydrologic Cycle. The three major reservoirs of global water and the rates and types of
movement between the reservoirs.
Water comes to land solely by means of
precipitation. Much of it flows on the surface, or
infiltrates into the ground to become
groundwater, but eventually makes its way back
to the ocean (37x1012m3/yr). As water returns to
the ocean, it carries nutrients and sediments with
it. Surprisingly, though, almost twice as much
water (37x1012m3/yr) goes back into the
atmosphere from land through evaporation and
the movement through plants called transpiration
The most impressive exchange occurs
between the ocean and the atmosphere where
nearly 80% of the water evaporated returns as
precipitation. Clouds thus generated increase the
albedo or reflectivity of the Earth and affect the
global heat energy budget (Glacier and ice reflect
light/heat about as well as clouds do). Heat
energy imparted to the atmosphere through
evaporation, serves to moderate temperatures
toward the poles. Indeed, in climate change
(global warming) scenarios, the temperatures at
the poles will see a much greater increase than
those at the equator for that reason.
A deeper understanding of the hydrologic
cycle will allow us to better explain questions
like:
• What happens to cause and maintain an ice
age?
• How do the cold periods of the ice ages end?
• What happened to bring about the end of the
snowball (or shush ball) Earth?
• How did the hydrologic cycle operate during
such periods?
• How did the hydrologic cycle operate during
times when the earth was much warmer?
For example, during the time of the midCretaceous (the last period in the great Age of
the Dinosaurs), the Earth was much warmer than
it is today. Estimates based on oxygen isotopes
in rock laid down at the time suggest that the
average global temperature was about 8C higher
than it is today, due in part to 3-4 times the
The
current levels of atmospheric CO2.
increased atmospheric temperatures also
produced increased evaporation rates and a much
more active hydrologic cycle. Consequences for
the planet were profound. Global precipitation
rates were up more than 25% over the present
day. This led to the formations of nutrientdepleted lateritic soils well into the temperate
zone.
Also, the differences between the
temperatures of the tropical and polar regions
were much smaller.
The Cretaceous scenario would seem almost
like paradise, particularly as I think about
scraping the windows of my car at the end of
another Pennsylvania winter. Shouldn’t we
welcome climate change that would bring about
increased global temperatures? Unfortunately,
the devil is in the details, most of which have yet
to be discovered. First of all, I am sure that the
anticipation would turn more to dread if you
were to read this in mid-July. The most
important questions, though, center around what
would a more active, energetic hydrologic cycle
be like? Clearly, a warmer atmosphere is a more
energetic atmosphere, which produces storms of
greater energy levels. Imagine an Earth in which
hurricanes come at category 6 over an ocean that
has expanded by the temperature increase. This
is just one type of disaster that I can anticipate;
an attribute of a complex system, like the
hydrologic cycle, is that most outcomes cannot
be anticipated by examining the components of
that system. That is why the amount of water in
the atmosphere is not as important as the
exchange rates between the atmosphere and the
land, the mediators of which are clouds. Thus,
the study and understanding of clouds and cloud
formation are vital to our understanding of
weather, climate, the hydrologic cycle, and what
the Earth might be like by the end of the 21st
Century.
Luke Howard provided the foundation by
which clouds could be categorized and classified.
This he provided the foundation that led to the
development of modern meteorology by
assigning a common set of terms and concepts to
a dynamic system by which others could
communicate and then begin to explain. The
aspect of common language is important because
science is a community of people who just won’t
shut up. They constantly evaluate and reinterpret
evidence as skeptics and “devil’s advocates”.
They communicate explanations for phenomena,
and all that must be done with the terminologies
of their respective disciplines to seek with
greater precision nature’s subtle truths.
Unfortunately, charlatans among politicians,
the business community, and policy makers prey
upon the minds of an uninformed public. They
present the discussion within the scientific
community as lack of qualified knowledge.
Nothing could be farther from the truth;
scientists seek a precision of knowledge that they
do not now have. Nevertheless a less precise
understanding is infinitely better for a society to
use than that of outright ignorance fueled by
greed. In 1995, the phrase “Global Warming” in
legitimate scientific literature was rare. Now the
phrase along with the more general variant,
Climate Change, is quite common. Certainly,
the community of scientists sees the tempest
ahead. Will we as a society ignore the signs and
drive headlong into the storm?
-March 2005
SOURCES CONSULTED FOR THE ESSAY:
Gedzelman, Stanley. 1989. Cloud Classification
Before Luke Howard. Bulletin American
Meteorological Society. 70(4): 381-395.
Hamblyn, Richard, 2001. The Invention of
Clouds. Picador. New York.
Holton, James. 1992. An Introduction to
Dynamic Meteorology. Third Edition.
Academic Press. New York.
Pagano, Thomas and Soroosh Sorooshian. 2002.
The Hydrologic Cycle. IN: Michael
MacCracken and John Perry, eds. Vol 1. The
Earth
System:
Physical
and
ChemicalDimensions
of
Global
Environmental Change. Encyclopedia of
Global Environmental Change (Ted Munn,
editor-in-chief). John Wiley and Sons, ltd.
Chichester.
Peterson, Larry, Gerald Haug, Konrad Hughen,
Ursula Rohl. 2000. Rapid Changes in the
Hydrologic Cycle of the Tropical Atlantic
During the Last Glacial. Science 290:19471951.
Pierrehumbert, Raymond. 2002. The Hydrologic
Cycle in Deep-Time Climate Problems.
Nature 419: 191-198.
White, Tim, Luis Gonzalez, Greg Ludvigson,
Chris Poulsen. 2001. Middle Cretaceous
Greenhouse Hydrologic Cycle of North
America. Geology. 29(4): 363-366.
http://observe.arc.nasa.gov/
QUESTIONS TO THINK ABOUT
1. What are the four major categories of clouds?
2. Why, although he presented a cloud classification system earlier, was
Lamarck’s system ignored?
3. Who created the cloud classification system that we use today?
4. What is the major difference in the way that a cloud forms and fog forms?
5. How can a thermal form a cloud? In what way is that similar to the way in
which a barrier like a mountain forms clouds?
6. How can a mountain be responsible for the formation of a desert? What is
that called?
7. Clouds frequently form at the margins of air masses. How do the structures
of warm and cold fronts differ?
8. Why does high pressure produce clear skies?
9. What is the hydrologic cycle? What are the major “reservoirs” of water
within the cycle?
10. Why are glaciers and permanent ice fields not considered important in the
hydrologic cycle?
11. How can the Cretaceous Period inform us about global warming?