More than ten feet (three meters) of snow fell last week in parts of upstate New York, and more is forecast for the U.S. Northeast in the coming days.

In all that snow, however, scientists believe the chance that any two flakes are exactly alike is virtually zero.

Why?

The answer, according to New York-based writer Mariana Gosnell, is in the way snowflakes form and fall to Earth. The process is detailed in her book Ice: the Nature, the History, and the Uses of an Astonishing Substance.

A snowflake begins to form when water vapor condenses around a speck of dust high in the clouds—up to six miles (ten kilometers) up—and then crystallizes.

How the water vapor keeps on condensing and where the snowflake falls