Feature August 09, 2010
Send in the Clouds
The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-262&cid=release_2010-262
Gaze up at a cloud-filled sky, and you may spot the white, fluffy shape of a dragon, fish
or elephant. Looking at the same sky, Graeme Stephens sees a different vision -- a
possible future for Earth's climate.
Stephens, a professor at Colorado State University in Ft. Collins, is principal
investigator of NASA's CloudSat mission, launched in 2006 to improve our
understanding of the role clouds play in our complicated climate system. Stephens
says that as Earth's global temperature continues to rise, water vapor -- the most
abundant greenhouse gas on Earth, which traps heat much as carbon dioxide does --
will continue to build, with uncertain results.
"We're seeing that now," Stephens said. "We just don't know what this will mean for
how clouds might change, and for Earth's temperature and climate. Although a small
change of clouds--for example, more low clouds--in the right direction would mitigate
the effects of increased carbon dioxide, a small change of clouds in a different
direction--for example, more high clouds--would amplify the warming caused by
increasing carbon dioxide."
Calculating the balance between the cooling or warming effect of clouds and the
warming effect of greenhouse gases is a complex problem for researchers, given their
current understanding of clouds on Earth. And it's just one of many questions
Stephens and fellow scientists are working to address with observations from
CloudSat, an experimental satellite built and managed by NASA's Jet Propulsion
Laboratory, Pasadena, Calif. CloudSat's goal is to learn about clouds and their effect
on climate by studying them from space.
Floating Facts of Life
Clouds are an inescapable, and necessary, part of life. Aside from making for
spectacular sunsets, they also create weather as we know it, from drizzly spring
afternoons to the dark, dreary days of winter. "In all ways, shapes and forms, clouds
influence life on Earth -- including our climate," says Stephens.
They also play a major role in making Earth habitable. As the sun's rays shine on our
planet, flat, low-altitude stratus clouds reflect most of this heat back into space, keeping
Earth cool with their shade. At the same time, thin, wide cirrus clouds high in the
atmosphere trap heat on Earth's surface, keeping the planet warm. This delicate
balance helps to create a comfortable climate, where life flourishes.
Clouds also play a primary role in how life-giving water circulates around our planet.
As water on Earth's surface heats, it evaporates into water vapor and rises. As this
vapor cools in the atmosphere, the molecules begin to clump together around stray
particulates and condense to form clouds. When the clumps become too big, they drop
back onto Earth's surface in the form of rain or snow. The never-ending global process
of evaporation, precipitation, freezing and melting circulates water around the world --
while also providing the freshwater we need to live. This cycle, which is closely linked
to natural exchanges of energy among the atmosphere, ocean and land, helps define
our climate.
It's difficult to say what our world would be like if there were no clouds. But, says
Stephens, "It's certain that our world without clouds would be nothing like what we
know today."
Mars: A World Without Clouds (Mostly)
In fact, it might be much like Mars, says JPL planetary scientist David Kass. The Red
Planet today has relatively few clouds compared to Earth. That's because the Martian
atmosphere contains less than a tenth of a percent of the amount of water vapor found
in Earth's atmosphere. Without much water vapor, and with temperatures averaging 80
degrees Celsius (176 degrees Fahrenheit) colder than on Earth, only thin ice clouds
form. They tend to look like a thinner version of Earth's wispy cirrus clouds.
"We don't think that clouds on Mars get to the point where you couldn't see the sun
through them, but they might get thick enough that you could look at the sun through
them without hurting your eyes," sats Kass.
Mars also has thicker clouds made of frozen carbon dioxide -- commonly called dry ice
--that form both high in the atmosphere and at the poles during winter, where the sun
never rises for half the Mars year. These clouds are dense enough to dim the sun's
light by about 40 percent (although the polar clouds are never actually illuminated by
the sun), but because they are found only in limited regions near the planet's poles
and equator, they are unlikely to affect the Martian climate as a whole.
Scientists theorize that the relatively sparse clouds on Mars allow temperatures to rise
and fall dramatically. Without the cooling effect of significant cloud shade or the
insulating effect of thick cloud blankets, the surface of Mars heats drastically during
the day -- reaching temperatures around 18 degrees Celsius (65 degrees Fahrenheit)
at the equator -- before the temperature plummets at night -- to equatorial surface
temperatures as cold as 130 degrees Celsius below freezing (minus 202 degrees
Fahrenheit).
But researchers don't yet know for certain how exactly Martian clouds affect the
planet's climate. "It's not clear yet how big a role clouds play in Mars' climate," says
Kass. "This is really on the cutting edge right now." As planetary climate models
become more sophisticated, they will include the radiative effects of the clouds seen in
data from the Mars Climate Sounder on NASA's Mars Reconnaissance Orbiter. Kass
says the modelers will be able to incorporate that data and examine cases with and
without clouds to see their impacts. "We hope to know more soon," Kass adds.
Venus: A Greenhouse Girl Gone Wild
If Mars is what an Earth without many clouds might look like, then Venus shows what
our world might look like with far more.
Venus' skies are stuffed with brilliant white clouds that stretch around the entire planet
without a single break. As a result, they -- and other molecules in the atmosphere --
reflect more than 80 percent of the sun's light back out into space. For many years,
planetary scientists thought this would keep the surface of Venus relatively cool. Yet
when the Russian probe Venera 4 landed on the Venusian surface in 1967, it
measured a temperature of 482 degrees Celsius (900 degrees Fahrenheit). That's hot
enough to melt lead.
"At that point, we realized two things: Venus' atmosphere is very thick -- about 100
times thicker than Earth's -- and greenhouse gases are important to climates," said
Kevin Baines, a planetary scientist at JPL and senior research scientist at the
University of Wisconsin-Madison.
Venus' thick clouds are surrounded by carbon dioxide, a greenhouse gas that traps
heat on the planet's surface. The little heat from the sun that makes it through the
reflective cloud barrier has little chance of escape, and as that heat builds -- if only a
little bit at a time -- the surface of Venus gets hotter and hotter.
The heating of Venus' clouds could also cause the planet's extreme air circulation.
The excess heat, Baines says, seems to whip the entire atmosphere up to hurricane-
force winds, causing the atmosphere at cloud level to circulate 60 times faster than the
planet rotates.
"Venus is a planet of extremes," says Baines. "It's very hostile and very hot; you can't
survive very long there."
Titan: Partly Cloudy, With a Chance of Methane Rain
There is a middle ground between Mars' relatively clear skies and Venus' cloud-
choked heavens. Scattered clouds float above the icy surface and liquid lakes of Titan,
the largest of Saturn's many moons. These clouds, which are made mostly of
methane, punctuate the sky more in the winter than in the summer, just like clouds on
Earth. By trapping in the little heat that makes it through Titan's upper level of thicker
atmospheric clouds, the scattered clouds warm the surface to a frigid minus 183
degrees Celsius (minus 297 degrees Fahrenheit) on average, keeping the moon's
methane lakes and rivers liquid.
NASA's Cassini-Huygens spacecraft studies Titan and its climate, in part to learn more
about how cloud cover and other variables affect climate.
CloudSat: Revealing the Inner Secrets of Earth's Clouds
So what have the first four years of CloudSat operations taught us about our
mysterious friends in the sky? Stephens says the mission has already yielded a
number of important findings.
Among the highlights, the satellite has gathered the first statistics on global vertical
cloud structure, including overlapping clouds, to create three-dimensional maps of
Earth's cloud cover. It measured the percentage of clouds giving off rain at any given
time (13 percent) to better understand how efficiently clouds convert condensed water
into rain. It has monitored nighttime storms at Earth's poles from space for the first
time. And it has revealed connections between storms at the poles and very high
clouds that help create ozone.
"Before CloudSat, we essentially had photos of the tops of clouds from other satellites
and photos of the bottoms of clouds from ground-based telescopes," says Deborah
Vane, CloudSat deputy principal investigator and JPL project manager for the mission.
"CloudSat's advanced radar slices into clouds and looks into their inner structure."
By viewing this complete picture of how clouds operate both inside and out for the first
time, and monitoring it on a global scale, CloudSat is offering climatologists the data
they need to create better models of Earth's climate -- and help predict what the
surface of our planet will probably look like in the future.
So could Earth ultimately turn into a steady inferno like Venus or a fluctuating icebox
like Mars? Fortunately, says Stephens, data from CloudSat and other sources show
that Earth's clouds are not about to shrink drastically or engulf our skies anytime soon.
"With CloudSat, we're getting information that's critical to understanding how changes
to clouds will ultimately take place," said Stephens. "If we can confirm that the
assumptions climate models make are right -- or wrong -- then we can have a major
influence on their ability to predict the future."
For more information on CloudSat, visit: http://cloudsat.atmos.colostate.edu/ and
http://www.nasa.gov/cloudsat .
#2010-262
-END-
Media contact: Alan Buis 818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif.
Alan.buis@jpl.nasa.gov
Written by Kelen Tuttle
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