When students at Charleston Middle School in Charleston, Illinois, walk into their science classroom, they enter a winter wonderland. Model snowflakes hang from the ceiling, lighted plastic snow crystals twinkle on and off, and wall posters explain how a mind-numbing variety of snowflake shapes form.
The festive decorations in science teacher Tim McCollum's room hint at the lessons that are to come. With the first snowfall of the year, students will capture close-up pictures of snow crystals and then use the images to piece together the puzzle of how each snowflake got its unique shape.
LET IT SNOW
There are three common states of matter: solid, liquid, and gas. For snowflakes to form, water vapor in the air must change state. First it condenses, or changes from a gas to liquid droplets, to form a cloud. The water droplets in the cloud continue to cool, and a few of them start to freeze. As the ice crystals grow, they take water vapor out of the air, causing it to change directly from a gas to a solid. Through this process, called deposition, the little crystals of ice eventually grow into snowflakes. "It takes about a million little droplets to make one snowflake," says Kenneth Libbrecht, a physicist at the California Institute of Technology who studies crystal formation and photographs snowflakes.
Snowflakes have long fascinated photographers and scientists alike (see Snowflake Bentley, p. 12). But taking a snapshot of snow isn't easy. "Depending on how cold it is outside, you have anywhere between 10 seconds and 30 seconds before the snowflake melts," says Brooke Homann, one of McCollum's former students who photographed snowflakes in her eighth-grade science class.
In wintertime, McCollum keeps a supply of digital microscopes called DigiScopes at the ready. The microscopes hook up to a computer, allowing students to capture close-up images on-screen with the click of a button. When someone in class spots the first snowflakes falling from the sky, students hurriedly put on their winter coats, gather up their microscopes and computers, and head outside.
The students hold out microscope slides that had been in the freezer, and let snowflakes fall onto them. Then, they put the slides under the digital microscope and quickly take a picture of the crystals before they melt. "I loved the pictures," says Brooke. "It was so cool how none of the snowflakes were alike. Everybody always says that, but you can't really understand it until you actually start taking pictures of hundreds and hundreds of snowflakes and really see that they're totally different."
SHAPE-SHIFTERS
Every year, approximately 1 septillion (that's 1 followed by 24 zeros!) snowflakes fall worldwide. Despite their vast numbers, odds are low that any two will look alike. That is because certain weather conditions, together with the path the snowflake takes from sky to ground, dictate the shape of the snow crystal.
"There's a tremendous number of different types of snowflakes," says Libbrecht. For instance, there are complex-looking stellar dendrites, with their six perfectly identical branching arms. Snow crystals can also come in shapes like simple prisms, simple needles, capped columns, hexagonal plates, and more.
By studying the shape and size of a snowflake, scientists can estimate the humidity and temperature of the atmosphere where the crystal formed. Humidity determines complexity: The higher the humidity, the more complex a snow crystal will be (see Nuts & Bolts, p. 11). Temperature mainly determines whether snow crystals grow into plates or columns. "Right below freezing, at -2[degrees]C (28[degrees]F), you get plates growing. They're usually kind of small because it's still warm. If you go colder, to -5[degrees]C (23[degrees]F), you get columns and needles that form," Libbrecht explains. "If you go colder still, you get really beautiful stellar crystals. Those tend to form at about -15[degrees]C (5[degrees]F). If it's really cold, you can get different combinations of columns and plates again."
SNOW STUDIES
In addition to learning about snow-crystal formation, McCollum's students are involved in a worldwide effort called the Global Snowflake Network, by NASA Goddard Space Flight Center's History of Winter program.
As participants in the Global Snowflake Network, the students record their observations while they're outside studying snow crystals. They mark down their latitude and longitude, the date and time of day, the temperature, humidity, and wind speed and direction. Then, the students record the various snow-crystal shapes they see. They enter all of this information into an online database. As more and more people around the world take part in the Global Snowflake Network, they add to scientists' understanding about snow formation, weather, and climate.
"There's a lot of information about snow that has been documented by myself and by others, but only poorly," says Libbrecht. "By going outside and looking around, you might find something new. Every time you see something, ask yourself: How does it work? What's it doing?" That is how science advances.
"I never really liked the snow, but [taking pictures of] the snowflakes made it a whole lot more interesting," says Brooke. So which would Brooke prefer: getting off from school for a snow day, or going to class? "I would rather be in school and taking pictures of snowflakes," she says.
nuts & bolts
SNOW-CRYSTAL FORMATION
Snow-crystal shapes vary with temperature and humidity, a measure of the amount of water vapor in the air, What crystal shape would you expect to see if it were -13[degrees]C out, with low humidity?
SNOWFLAKE BENTLEY
Wilson Bentley was a farmer in Jericho, Vermont, who lived from 1865 to 1931. When he was a teenager, he became interested in the structure of snow crystals and came up with the idea of attaching a camera to a microscope to get magnified pictures of the flakes. When he was just 19 years old, he became the first person known to photograph a snow crystal. Over the next 46 years, he took more than 5,000 snow crystal images, providing the world with its first up-close look at the intricate structure and symmetry of snowflakes.
PRE-READING PROMPTS:
* How do snowflakes form? What variables do you think would affect the formation of snowflakes?
* What conditions are needed to photograph a snowflake?
* It's said that no two snowflakes are alike; how can that be?
DID YOU KNOW?
* Snowflakes aren't always white. Particles in the air can combine with moisture in the air and change the color of the snowflakes. In the past, coal dust changed snowflakes to a gray color. On Canada's Prince Edward Island, red clay dust colors the snowflakes pink!
* The largest snowflake listed by the Guinness Book of World Records logged in at 38.1 centimeters (15 inches) in diameter and was 20.3 cm (8 in.) thick. This snowflake was found in January 1887 in Fort Keogh, Montana.
* The speed of an average snowflake is 5 kilometers (3.1 miles) per hour.
CRITICAL THINKING:
The Global Snowflake Network collects data about snowflakes from people around the world for a better understanding of snow formation, weather, and climate. This type of research, in which groups of volunteers collect data and make observations, is called citizen science. Would you like to be a citizen scientist? What observations or data collection could you do in your community?
CROSS-CURRICULAR CONNECTIONS:
LANGUAGE ARTS: Biographers provide accounts of the lives of people. These writers research their subject and conduct interviews. Imagine you have been chosen to write the biography of a snowflake. What questions would you ask during your interview? What facts could you include about its birth, life, and death? What stories could it tell about its life? Compile this information into a short story or PowerPoint presentation for the class.
RESOURCES
You can access these Web links at www.scholastic.com/scienceworld.
* Download Science World's PowerPoint about the states of matter at: www.scholastic.com/scienceworld.
* For a comprehensive guide to snowflakes, snow crystals, and frost, visit snowflake researcher Ken Libbrecht's Web site: www.its.caltech.edu/~atomic/snowcrystals.
* Want to know more about Wilson "Snowflake" Bentley? Read Jacqueline Martin Briggs's Caldecott Medal-winning book called Snowflake Bentley, Houghton Mifflin Company, 1998.
DIRECTIONS: Answer the following questions in complete sentences.
1. What is the role of condensation in snowflake formation?
2. What tools do the students at Charleston Middle School use to take pictures of snowflakes?
3. Needle-shaped snowflakes occur at what temperature? (Hint: Refer to the diagram on p. 11.)
4. Who was Wilson "Snowflake" Bentley?
5. What does the Global Snowflake Network hope to learn from people sending in their snowflake data?
Answer
1. Condensation is the first step in snowflake formation. Water vapor must condense from a gas into liquid droplets to form a cloud. As the droplets freeze, snowflakes begin to form.
2. The students at Charleston Middle School use digital microscopes, computers, and chilled microscope slides to take pictures of snowflakes.
3. Needle-shaped snowflakes occur when the temperature is around -5[degrees]C.
4. At age 19, Wilson "Snowflake" Bentley was the first person known to take an up-close picture of a snow crystal This Vermont farmer, who lived from 1865 to 1931, attached a microscope to a camera and took more than 5,000 pictures of snow crystals over the course of his life.
5. By observing snowflakes and recording weather and temperature data, the Global Snowflake Network hopes to learn more about snow formation, weather, and climate.
Source Citation
Janes, Patricia. "Secrets of a snowflake: students study snow crystals up close to gain insight into their formation." Science World 7 Dec. 2009: 10+. Academic OneFile. Web. 5 Jan. 2010.
Gale Document Number:A213956586
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