2015职称英语理工类阅读理解文章及练习(二)

Youve probably heard that no two snowflakes are alike.Of course,nobody has ever confirmed that statement by examining every one of the estimated one septillion snowflakes that drift to Earth each year.still,Kenneth Libbrecht,a professor at the California Institute of Technology,is confident that the statement is true.

　　Several factors in the environment affect the shape and growth rate of a snow crystal.One factor is humidity.Crystals grow faster and in more intricate shape as humidity increases.A second factor is air temperature.A snowflake is born when several molecules of water vapor in a could land on a speck of dust and freeze to form a simple crystal.As the young crystal bops around in the cloud,it passes through air pockets of varying temperatures.If the crystal passes through a pocket of air that is,says,15 degrees Celsius,it will grow quickly and sprout six arms,says Libbrecht.If the crystal is then tossed into a warmer pocket,one about-10℃,the arms tips will stop growing quickly and form six-side plates.If the crystal then drifts into an even warmer pocket of about -℃,its top and bottom will grow more quickly than its sides and become more column like in shape.

　　Using his cooling tanks,Libbrecht has learned how to create snow crystals of different shapesplates,colhuns,needles etc.Libbrecht has even refined his techniques so that he can make crystals that look highly similar to one another.Still,he lacks the control to manufacture identical twin snowflakes.A slight difference in humidity and temperature can upset the growth profile of a crystal.