We live at the bottom of an ocean of air. It is a huge
volume -- a 3-dimensional space. But people rarely float in this
ocean. Looking up from below, it is hard to imagine what the
atmosphere is like, even a small distance away from the surface. All
we can see is blue sky and perhaps some clouds, which produce rain or
snow if conditions are right. What is it like among the clouds?
We get few clues from everyday life. Even aboard
airplanes, we fly in climate-controlled luxury; we are sheltered from
the wind, and the cabin pressure, temperature, and humidity are
carefully adjusted for our comfort. Opening the window for a breath of
"fresh air" is out of the question.
But balloons float in the atmosphere. They move with
the air, becoming part of the sky. As it attempts to circumnavigate
the globe, the Solo Spirit balloon will tell us about its
surroundings. It carries instruments to measure conditions aloft, and
to report them back to Earth. This gives us the chance to participate
in an experiment -- to explore the atmosphere high above the surface
-- as we follow the flight of Solo Spirit.
For any experiment, it is important to be very clear
about what we actually measure. Here is a list of the names and
symbols of quantities we measure aboard Solo Spirit:
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h
|
height, or altitude, of the balloon
|
|
L
|
horizontal location (latitude and
longitude)
|
|
p
|
pressure
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T
|
temperature
|
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q
|
relative humidity
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V
|
balloon speed and direction relative
to the ground
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These measurements are made by the instruments in a
package attached to the side of the Solo Spirit gondola .
A real measurement is never that simple. For example,
we need to ask which "temperature" are we measuring. Is it
the temperature of the air in a cubic meter volume around the
thermometer? Perhaps it is the average for air in a 10 meter volume.
Maybe the balloon stirs up air from other places, or heats the
thermometer or the air around it directly? How long does it take for
the thermometer to 'forget' a colder or warmer temperature it may have
measured earlier? And even if we find the answers to all these
questions, how accurate is the thermometer itself?
We can ask such questions about each of the quantities
we measure. When we do experiments, much of our time is spent asking
and trying to answer the overall question: "What are we actually
measuring?"
There is another question, however, which saves us
from spending our whole lives on one measurement, and that is:
"How well do we need to understand the measurement in order to
find out what we are interested in learning?"
In the case of Solo Spirit, we just want to learn
something about what it's like high in the atmosphere, as compared to
our familiar place on the surface. Once we look at the real data, and
discover something that seems interesting, we may want to ask the
overall question: "What have we actually measured?" We could
make a list of all the factors that might affect the measurement. Then
we could try to estimate the size of each factor, and decide how
significant each one is to our conclusions.
Hint: The balloon is likely to affect T and q a
great deal under some circumstances. Parts of the balloon may be much
warmer than the surroundings; the cargo, the burner, and the pilot may
each be significant sources of water vapor, and the radiometer might
see part of the vehicle along with the atmosphere and surface below.
Here is one way to organize how we look at the
measurements, so each topic builds on what we did previously. This is
just an outline. More detailed explanations are given below.
Topics:
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L (horizontal location) -- h (vertical location)
-
p(z) (+ history of learning
about the atmosphere using balloons) -- gas properties
-
T(z), q(z) (+
"environmental conditions" at altitude)
-
V(z) (How does the balloon's
location change?) -- and the
"Global" Picture of Weather (balloon data in context)
-
Climate (seasonal and
longer-term changes in weather)
[We use 'z' for 'height above the surface.' So p(z)
represents pressure values over a range of heights. Such values can
be presented in a line graph or a table.]
This is a lot of material to cover in a pre-college
class. You may want to emphasize just a few of the points in 1 to 4
that relate to other material being covered in your class.
Topic 1. Location
How do we know where Solo Spirit is? -- Clues
Don't take for granted that the GPS (the
satellite-based Global Positioning System) is right. Use whatever
creative ideas you come up with to learn something about where the
balloon is located. Compare the results, and try to explain any
differences in your conclusions. Here are a few things to try:
Horizontal:
-
Using: distance = rate x time, try to predict the
balloon's location. How does the "rate" change over time?
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GPS
-
Other ideas -- radio stations received,
triangulation on ground control points, storm locations, star
locations, location of the terminator, warnings from hostile air
forces.
Vertical:
-
Sizes of surface features -- they appear smaller
as you go higher -- try to think of ways to determine how much
smaller things appear, depending on how far away they are. If you
know some geometry, that could help.
-
p(z) -- pressure changes systematically with
height; see Topic 2.
Topic 2. Pressure
How does pressure, p, change with height, based on the
observations?
Look at the data from Solo Spirit. If you can, plot
pressure against vertical location. What patterns do you see? Is there
a simple relationship that seems to hold true?
Generally, an equation called the Hydrostatic Relation
describes how pressure changes with height in the atmosphere. It
involves the force of gravity acting on gas and the way an ideal gas
is compressed by the force of gas above it. Simply put, the pressure
decreases very much (exponentially) with height. The reason: not only
is there less atmosphere pushing down on you as you go up, but the air
above you is also less dense because it is squeezed even less by the
air above it. A discussion of the hydrostatic relation can be found in
meteorology books, or in any good encyclopedia.
There are some subtleties, including the way
temperature affects the density of the gas, and the effects of any
vertical wind currents, which can become particularly important within
storms.
Some activities:
-
Discuss the compressibility property of gases, and
the ideal gas law.
-
Compare p(z) with a prediction based on the
hydrostatic relation -- Are there features in the data related to
measurement error in (p) or (z)? Any evidence for vertical winds?
Other unexplained features?
-
Study the history of learning about the atmosphere
using balloons -- In 1784, barely a year after the first aeronauts
showed how one could travel in the sky beneath a balloon, Dr. John
Jeffries, with the help of his pilot, Jean-Pierre Blanchard, began
exploring the upper air. He measured pressure, temperature,
humidity, wind, electric and magnetic fields, and collected air
samples for chemical analysis. Jeffries and Blanchard are
sometimes credited with discovering that pressure decreases with
altitude.
-
Discuss how the process of boiling works, and why
it takes longer to cook things in boiling water at high
elevations.
-
Included in the ideal gas law, and also in the
hydrostatic relation, is the effect of T on p(z). There may be
features in the p(z) data related to the change of temperature
with elevation [T(z)] -- go to Topic 3.
Topic 3. Temperature
How does temperature, T, change with height, based on
the Solo Spirit observations?
The basics of atmospheric heating: Most of the sun's
energy that reaches the Earth passes through the atmosphere, and heats
the ground. So the atmosphere is mainly heated from below. (There are
some exceptions, such as the heating of the stratosphere by
ultraviolet light that is absorbed by the ozone layer, and the heating
of the thermosphere by extreme ultraviolet light. But Solo Spirit
flies in the troposphere, which is below all that.)
The surface loses its heat in several ways: some heat
is emitted as infrared radiant energy; some heat evaporates liquid
water at the surface, which goes into the atmosphere as water vapor;
some heat is conducted into air near the ground -- since hot air
rises, this air may move upward, to be replaced by colder air -- a
process called convection. When convection and radiation operate in
the atmosphere, with the heat source at the ground, the resulting
temperature decreases with height.
When convection dominates, the rate at which T(z)
decreases is called the adiabatic lapse rate. In Earth's atmosphere,
the adiabatic lapse rate is about 9.8 degrees C/kilometer (28.3
degrees F/mile). However, if there is water vapor in the air, as there
often is, it may condense as the air rises and cools, releasing some
heat. So moist air cools more slowly as you go up -- at a rate called
the moist adiabatic lapse rate, which is about 6.5 degrees C/kilometer
(18.8 degrees F/mile). A discussion of convection, adiabatic lapse
rates, and atmospheric heating can be found in meteorology books, or
in any good encyclopedia.
Some activities:
-
Discuss what happens to sunlight that reaches the
Earth.
-
Discuss convection and the dry adiabatic lapse
rate. Predict T(z).
-
Compare the plot of T(z) from Solo Spirit with the
prediction.
-
As temperature decreases in the atmosphere, the
amount of humidity in the air usually goes down very rapidly,
since cold air cannot hold nearly as much water vapor as warm air.
(An equation called the Clausius-Clapeyron relation, which can
also be found in standard references, describes how much water
vapor can be held by air as a function of temperature.) Using T(z),
you might calculate the maximum amount of water vapor the air
could hold at each elevation.
-
What is the relationship between T(z) and q(z), as
measured by Solo Spirit? How does it compare to the predicted
maximum based on the Clausius-Clapeyron relation (Call this
quantity qmax(z)).
-
Discuss the "environment" at elevation,
using your knowledge of p(z), T(z), and q(z). Is the lifestyle of
people living high in the Andes or the Himalayas adapted to these
conditions?
-
Are there differences in the relationships between
T(z), q(z), and qmax(z) in the Solo Spirit data that relate to the
presence of clouds or storms? -- Even at the same elevation,
conditions change from place to place in the Earth's atmosphere.
The wind determines how quickly and in what direction the balloon
visits new places. -- go to Topic 4.
Topic 4. Wind and the Global
Picture of Weather
What can we say about the winds aloft, based on the
changing location of Solo Spirit?
In the 1830's, aeronaut Charles Green noted that at
10,000 feet elevation, there seemed to be a strong, steady westerly
(west to east) wind blowing across Europe. He planned his November
1836 flight aboard the Royal Vauxhall, which took him and two
companions from London to Weilburg, Germany, based on his knowledge of
this wind. The flight set the world distance record of 380 miles.
By 1842, John Wise, an American, had also discovered a
strong westerly wind at about 12,000 feet. He relied on this wind for
his July 1859 trip from Washington Square, St. Louis to Henderson, New
York, an adventure that set the world distance record at 809 miles. By
this time, Wise, Green, and others had speculated about the
possibility of ballooning across the Atlantic -- from west to east, of
course.
Not much was known about the inner workings of the
atmosphere. It was impossible to get a synoptic view of the planet.
There were no satellite images or soundings, and even communication
among ground observers at different locations was difficult by today's
standards. Balloons were at the cutting edge...
The modern picture of the atmosphere is based on a
combination of computer programs called general circulation models (GCMs)
and regular observations from satellites, unmanned weather balloons,
airplanes, ships, and weather stations. These days, balloon
adventures, including Solo Spirit's, make use of the best available
data from all these sources. But there are still uncertainties.
The west-to-east winds in mid-latitudes are a part of
the global circulation system, driven ultimately by the change in
temperature from low and high latitudes, and mediated by the rotation
of the Earth. From about 9 to 18 kilometers elevation, in the middle
to upper part of the troposphere, cold polar air meets warmer
mid-latitude air unimpeded by surface friction. The boundary, called
the polar front, can be quite sharp, and winds can reach speeds of 75
meters per second (165 miles per hour) or more. The rate at which
temperature changes with latitude is greatest in winter, leading to
the strongest winds. But on weather maps, you can see that the jet
stream changes intensity and moves around; it contains wiggles called
Rossby waves that change its shape over a period of days, and it tends
to steer the movements of storms. To get around the world in a few
weeks, Solo Spirit must ride the jet stream.
A discussion of atmospheric circulation, the
'prevailing westerlys,' Rossby waves, the polar front, and jet streams
can be found in meteorology books, or in any good encyclopedia.
Some activities:
-
If you did not do this for the activities under
"Topic 1: Location", plot the locations of Solo Spirit
at different times during its flight, and calculate its speed.
-
On weather maps, find the locations of the jet
stream along the path of the Solo Spirit. How do the observed wind
speed and direction compare with those at the appropriate times
and places on the weather maps?
-
The balloon data is a "worm" of
information in the 3-dimensional space that is our atmosphere. Use
the visualization tools, and your own ingenuity, to get as good a
picture as possible of the location of Solo Spirit in the overall
pattern of winds, temperatures, and storms.
-
Discuss the factors that determine the overall
wind pattern. Why does the jet stream flow from west to east? What
determines its shape, and the way its shape changes from day to
day? Why is December-January the flight window for Solo Spirit?
-
Use the wind speed downstream of the balloon to
predict its course. Develop a strategy for the remainder of Solo
Spirit's journey, to get the balloon home as quickly and safely as
possible. One tricky aspect of this activity is that the wind
blows at different speeds, and in different directions, at
different elevations. Aeronauts can change the elevation of their
balloons, so they use the differences in winds to gain some
control of their flight paths.
Ballooning Strategy -- There are two
ways to get a balloon into the sky. One is to fill it with hot air,
which is lighter than cold air; it therefore rises, carrying the
balloon up. The other way is to fill the balloon with a gas that is
inherently lighter than air, such as hydrogen, coal gas (mostly
methane), or helium. Both methods were invented by Frenchmen, and were
first demonstrated within weeks of each other, in Paris. The first hot
air balloon to carry human passengers was built by the Montgolfier
brothers, and was launched on November 21, 1873. Jacques Charles built
the first hydrogen balloon used for transport, and flew it on December
1.
Each technique has advantages and drawbacks. Modern
hot air balloons carry propane burners. The pilot can shoot a 10-foot
flame into the balloon's interior to heat the gas and lift the vehicle
higher. But this only works as long as there is fuel -- hot air
balloons must carry with them all the fuel they need for an entire
flight.
A hydrogen or helium balloon does not need to burn
fuel to remain aloft. But balloons leak, and to stay up, or to move
higher, the pilot of a lighter-than-air balloon must jettison
"ballast" from the gondola. Sand is usually used as ballast,
but if the balloon has used up its ballast and is also low on gas over
terrain unsuitable for landing, food, clothing, furniture, or
equipment must be thrown overboard.
During the day, with the sun shining on a
lighter-than-air balloon, it may heat up and rise too much. The only
way to lower the balloon is to "vent" gas. This can create
problems at night, when the gas envelope cools, and the balloon may
then settle too low. Cost is another consideration for such balloons
-- helium is expensive, and hydrogen, though cheaper, is highly
flammable.
For a flight around the world, a balloon must stay
aloft for at least a few weeks, and must also be able to move up and
down, to make use of winds blowing at different elevations. So Solo
Spirit is a hybrid vehicle, a "de Roziere" balloon, named
after the pilot of the Montgolfier's first aerostat. It contains an
inner sack of helium, to provide buoyancy without the need to burn
fuel. It also has an outer envelope of air that can be heated, so the
pilot can maneuver up and down without using up helium or ballast. The
challenge for the pilot is to position the balloon so it rides the
best winds, while maneuvering as little as possible to conserve
resources.
Topic 5. Climate
The 'Global Picture of Weather' changes with seasons,
and from year-to-year. With what you learned in Part 4, you have a
good start on understanding the longer-term changes in the atmosphere,
which we usually refer to as "climate" changes.
Jackson, Donald Dale, The Aeronauts, Time-Life Books,
The Epic of Flight series, Alexandria, VA, 1980.
The Encyclopedia Britannica, CD-ROM edition, 1997.
Special Issue on the Vega Balloon Mission, Science
(Journal of the American Association for the Advancement of Science),
Vol. 231, pp.1349, 1369, and 1407-1425, March 21, 1986.
Lorenz, Edward N., The Nature and Theory of the
General Circulation of the Atmosphere, World Meteorological
Organization, 1967.
* * *
Ralph Kahn is a research scientist at the Jet
Propulsion Laboratory in Pasadena, California, who frequently writes
on Earth and Space Science issues. His writing has appeared in The Los
Angeles Times, The Denver Post, Sky and Telescope, and elsewhere.
