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 (144 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."

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