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Identical snowflakes

We don't get a lot of snow in Australia. Snow tends to happen at the higher latitudes that are closer to the poles, and most of Australia is closer to the equator than that.

We don't get a lot of snow in Australia. Snow tends to happen at the higher latitudes that are closer to the poles (above about 35°), and most of Australia is closer to the equator than that. Even though we don't have a lot of experience with snow, like most of the rest of the world, we do think that “no two snow crystals are the same” – and you guessed it, that's another mythconception.

Snow can happen at low latitudes, if the mountain is high enough, such as Kilimanjaro, which is only a few degrees from the equator. (However, it seems that the equatorial ice may possibly soon, vanish, thanks to Greenhouse Warming.) Mount Ranier near Seattle holds the record for the greatest snowfall in a season (a thousand inches, roughly 25 metres, in 1971-1972), while Colorado holds the record for the highest single day snowfall of 76 inches (193 cm) in 1921.

Snow forms when water vapour is exposed to very low temperatures (such as at the tops of clouds) and turns into ice.

The “classical” snowflake has six spokes radiating out from a common centre, in the same way that the spokes of a bicycle wheel radiate out from the central hub. But this star-pattern is only one of the many possible shapes. The shapes vary depending on the temperature, closeness to other snowflakes, wind, and so on. For example, if you drop the temperature from 0°C down to -25°C, and keep everything else constant, you will see snowflake shapes running through hexagonal plates, needles, hollow prisms, plates, stellar dendrites (the “classical” star shape), back to plates again, and finishing up with solid prisms.

The forces that induce water molecules to leap from one ice crystal face to land on another ice crystal face, seem to be very reliant on the local temperature. This helps explain why there are so many different shapes.

As a snowflake falls, it tumbles through many different environments. So the snowflake that you see on the ground is deeply affected by the different temperatures, humidities, velocities, turbulences, etc, that it has experienced on the way. Most snowflakes would have slightly different flight paths, and so they would have different shapes. Of course, snowflakes that land near each other tend to have similar histories.

Wilson A. Bentley, a farmer who was born, lived and died in the small town of Jericho in Vermont was called “The Snowflake Man”. He supported this “all snowflakes are different” theory. Around 1884, at the age of 19, he became the first person to photograph a single ice crystal, by cleverly marrying a microscope to a camera, using an adjustable bellows mechanism. In 1920, the American Meteorological Society elected him to the state of Fellow. They also awarded him their very first research grant, in recognition of his “40 years of extremely patient work” - for which they gave him $25. He continued working in this field until his death in 1931, by which time he had taken 5,381 “photomicrographs” of individual snowflakes. Towards the end of his life, he said that he had “never seen two snowflakes alike”. And so the story arose that all snowflakes are different.

But in 1988, the scientist Nancy Knight (at the National Center for Atmospheric Research in Boulder, Colorado) was studying wispy high altitude cirrus clouds. Her research plane was collecting snowflakes on a chilled glass slide that was coated with a sticky oil. She found two identical (under a microscope, at least) snowflakes in a Wisconsin snowstorm.

They were hollow hexagonal prisms, rather than the classical six-spoked star-shapes – but as far as snowologists are concerned, they counted as snowflakes. But if you want to be pedantic, they probably weren't identical if you were to look at the actual molecules – but at this level, is anything identical?

Since the earth was formed about 4.5 billion years ago, about a million million million million million snowflakes have fallen – but Mr. Bentley made his pronouncement of the cold hard facts after looking at just over 5,000 of them.

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