Welcome to You Ask Andy

Martha Berry, age 12, of Portland, Me„ for her question:

How does snow form?

When Andy was very young, he thought that snowflakes ware made from raindrops frozen white in a chilly cloud. Later, he learned that thin was not so at all. Rain is formed by one weather process and snow is formed by an entirely different weather process. But both rain and snow are formed from gaseous water vapor in the air and both form only when the air contains a surplus of water vapor.

Snow forms when the air is below the freezing point of water, most often when the airs temperature is between ten degrees above and four degrees below zero. In addition to surplus vapor, there must also be a certain amount of dust aloft in the air. This dust may be soot or microscopic particles of sand or soil. These solid little particles form the kernels or nuclei, around which the fragile snowflakes form.

A microscopic speck of dust whirls and dances through the chilly air of a soft grey snow cloud. It charges into countless particles of vapor, chilled below freezing point. The collisions turn the vapor into fragments or ice. Countless numbers of these fine fragments arrange themselves to form one dainty little snow crystal. A sizeable snowflake that comes fluttering down from a soft white cloud is made up of a great many little snow crystals.

It is always surprising to discover what a lot of snow can be made from a small amount of water. Take a glass tube, sealed at one ends and put into it about ten inches of newly fallen snow, Scoop up the snow just as it falls and do not try to tamp it down. Now take the glass tube indoors and let the snow melt in the warm air. You will find that ten inches of fresh snow yields about one inch of water.

What is used to make up the rust of the snow? Just plain air. Each snow crystal is made from countless splinters of ice, too small for our eyes to see, arranged around countless tiny pockets of air. Together, the ice and air give the snowflake its frothy whiteness.

We have all heard that a snow crystal is a beautiful six‑sided design and that no two snowflakes are exactly alike. The six‑sided figures form because of the shape of the individual fragments of ice. They tend to slip and stick along hexagonal lines, There are countless millions of these icy fragments in a single snowflake and therefore there are countless millions of possibilities in forming the lacy hexagonal designs. It is just possible that two snowflakes this year will arrange themselves in identical designs ‑ but it is not likely. Even if identical twin snowflakes did form, it is not likely that anyone would catch them both, put them under the microscope and photograph them ‑ so we would never know about it.

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