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FEATURE
Scientists Studying Wintry Ice In Summer Clouds

01.28.05

Winter is here, snow is falling in many areas of the country, and some of us are already wishing for the return of hot summer days. But, would you believe that even on the hottest summer day the temperature inside some clouds remains icy and winter-like, producing temperature readings as cold as negative 70 degrees Celsius (negative 94 degrees Fahrenheit)? Would you also believe that the ice crystals that form at the top of big summertime clouds may help scientists predict next winter's snowstorm?

Ice crystals come in thousands of different shapes and sizes. Image right: Seen here is a profile of ice crystals, showing the diversity of their shapes. The larger crystals toward the bottom of the image occur at warmer temperatures (approximately -37 degrees Celsius, or -34.6 Fahrenheit), while the smaller images at the top occur at colder temperatures (approximately -49 Celsius, or -52.6 Fahrenheit). At nearly 150 micrometers wide, some of the largest ice crystals in this image are about as thick as three strands of human hair. Image credit: Andrew Heymsfield

Last month, scientists from NASA's Langley Research Center in Hampton, Va. and Goddard Space Flight Center in Greenbelt, Md. published a paper in the Journal of Geophysical Research on the importance of classifying ice crystals within the big summertime clouds, or convective cloud systems, as observed during a Florida-based research campaign. In their paper, the scientists showed that their instruments can identify the ice crystals and now they can begin to classify the crystals. By learning to classify the ice crystals in clouds, these scientists hope to contribute to improving weather and climate models, the complex computer programs used to show future atmospheric conditions.

Weather and climate computer models are complex because they must account for hundreds of variables, including many that seem completely unpredictable. Vincent Noel, a research scientist with Analytical Services and Materials at NASA Langley and the author of the journal article, explains, "Usually climate prediction means predicting the evolution of temperature, pressure, relative humidity, and plenty of other variables, over small (a few days) and large (a few centuries) timeframes. However, to predict all this stuff with enough accuracy, we need to take into account clouds -- and for the time being, clouds are the most important source of uncertainty in climate prediction."

Known to interrupt hot summer afternoons with almost daily thunderstorms, convective cloud systems are very common in Florida. Image left: Known to interrupt hot summer afternoons with almost daily thunderstorms, convective cloud systems are very common in Florida. This image shows the unique shape of these systems, often called an "anvil" because of their potential to grow quite wide at the top and bottom, remaining narrow in the middle. Image credit: NOAA

Recognizing that clouds represent so much scientific uncertainty, some NASA scientists and other researchers decided to study tropical convective clouds in Florida, a type of large cloud system very common in that area. Their research project, called CRYSTAL-FACE (Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment), took place in the summer of 2002 throughout the state of Florida and the Gulf of Mexico, with the immediate goal of studying all aspects of the unique convection cloud formations from aircraft, land and satellite-based instruments.

If you have spent a day at Disney World in Orlando, or if you have relaxed on the beaches of South Florida, you have likely seen convective or heat-generated clouds. These clouds form when the Sun's rays warm the ground, causing hot air to rise, and condense into clouds. They are unique because they are massive in size, at times ranging from 100 to 200 km wide (62 to 124 miles); they form and dissipate very quickly, in as little as two hours; and they can be extremely thick, reaching 15 km (9.3 miles) in height, which is 6 km (3.7 miles) taller than Mt. Everest.

At the top of the convective clouds are cirrus clouds made of ice crystals. These crystals effect weather and climate in two ways: first, depending on the ice crystal's shape, it affects the amount of Sun's energy reflected or trapped near Earth's surface; and second, in their relationship with ozone destruction in the upper atmosphere (stratosphere).

Earth's energy budget is the balance of energy between the Earth and space. Image right: Clouds, particularly the high thin cirrus clouds, play a major role in the balance of (reflecting and absorbing) solar energy between the Earth and space. Scientists are trying to find out more about the ice crystals within the cirrus clouds and what role they play in this balance. Image credit: NASA

"Because of all this solar radiation, the Earth gets hot," said Noel. "When any body is hot, it radiates infrared light." Infrared is light at one end of the spectrum, and people use infrared goggles to see things in the dark (which is how you can see people in the dark using infrared goggles). "Clouds trap this infrared radiation, absorb it, and re-emit it later; this is called the greenhouse effect." Clouds, specifically cirrus clouds, are the reason that a lot of infrared radiation stays near Earth instead of going into space.


Katie Lorentz
NASA Langley Research Center

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