Thoughts of winter often drift to the perennial question: Is it really true that no two snowflakes are alike? The answer depends on how alike "alike" is, and the definition of a snowflake, according to Kenneth Libbrect, a physicist at the California Institute of Technology who studies ice crystals. At the molecular scale, there are so many ways molecules of water can combine to form a typical snow crystal that the chance of two being the same is close to zero, he says. At the optical microscope level, it's quite possible that two simple hexagonal crystals will appear identical. But at the level of larger, complex snowflakes — many, many crystals stuck together — the possibility of finding two that are alike is once again about nil, Libbrect says.

    Click the "Next" arrow above for nine more looks at the winter wonderland of science.

    The stars come out on cold, dark nights

    Looking for the way to an astronomer's soul? Skip the hot summer night and head straight for the dark, icy glare of winter. That's because cold air is thin on moisture, giving the sky a crystal clarity that's untenable in summer's steamy atmosphere. Bundle up and step outside to be treated to prominent displays of the winter constellations such as Orion the Hunter, which includes three bright stars that form an unmistakable line known as Orion's Belt. Or head to Northern Canada, get cozy, and catch the northern lights, shown here.

    Time indoors + kids = the flu

    Got a fever, aches, pains, a sore throat and a dry cough? It must be flu season. Incidence of the sickness peaks from December to March because people spend more time indoors, which eases transmission of the virus from person to person. The virus is most commonly spread in droplets that are sent airborne by the coughs and sneezes of infected people. Recent research indicates that kids, such as the one pictured here, are key flu spreaders. That may be because they spend hours each day huddled together in crowded classrooms — and, as parents know, don't always cover their mouths when they sneeze or cough. In an effort to combat the virus, scientists now recommend that all kids, from 6 months to 18 years of age, get a flu shot.

    Frigid air spawns mother-of-pearl clouds

    Some of the most frigid temperatures on the planet spawn some of nature's coolest, and rarest, clouds. The nacreous clouds, sometimes called mother-of-pearl clouds for their iridescent appearance similar to the interior of mollusk shells, occur only at high polar latitudes and temperatures of less than minus 176 degrees Fahrenheit. Also known as polar stratospheric clouds, they form in the lower stratosphere — between six and nine miles high. Their iridescence stems from predawn or post-sunset light that passes through water ice crystals. This picture of nacreous clouds was taken from Australia's meteorological base in Antarctica. The clouds are also occasionally spotted in high-latitude Scandinavian countries, Northern Canada and Alaska.

    Mount Washington: Worst weather in the world

    The weather observatory atop Mount Washington in New Hampshire bills itself as the "Home of the World's Worst Weather." The 6,288-foot-tall, wind-scraped peak sits at the confluence of three major storm tracks. Wet, gusty, icy, foggy and generally miserable conditions are the norm. In fact, it holds a record for the highest recorded gust: 231 miles per hour, set in 1934. A team of meteorologists thrive on the nastiness, monitoring the conditions around the clock all year long. They also conduct scientific research such as monitoring cosmic radiation and ozone and studying air pollutants and climate variability.

    Nor'easter: The science of the East's winter storms

    Between October and April each year, the East Coast is all but guaranteed at least one bout of torrential rain, coastal flooding or a blizzard that results from a meteorological phenomenon called a nor'easter. It occurs when a northward-creeping low-pressure system in the Gulf Stream collides near the North Carolina coast with a high-pressure system of frigid, dry arctic air carried south by the jet stream. The systems' winds combine and form a potent front that travels along a northeastern track, dropping rain and snow on the mid-Atlantic states and New England. In this picture, a person walks along the New York waterfront in the midst of a powerful nor'easter.