According to About Education, typical snowflakes range in size from the width of a human hair to nearly the size of a penny. Some reports claim that individual snowflakes can be as large as 3 to 4 inches across. One eye witness from Fort Keogh, Montana in January 1887, claims to have measured an aggregate snowflake that was 15 inches across.
The size and shape of a snowflake depends on the temperature in the clouds and the humidity level in the air. Generally, high humidity and low temperatures cause intricate or elaborate snowflakes to form. In the photos above, the small snowflakes fell at the beginning of the storm and the fully-formed snowflakes fell later in the day as the temps outside dropped.
If you are interested in how snowflakes form and grow, check out Snow Crystals. It provides several videos of snowflakes as they form. The snowflakes in the videos have been grown and filmed in a lab.
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