Grades K–2 · Earth & Space
Why can a snow crystal grow six arms?
An enlarged snow crystal can look like a tiny star with six arms. Its shape begins with water molecules, the very small building pieces of water. When they join solid ice, they fit into a repeating arrangement that favors growth in six main directions.
In the diagramSnow crystals can grow a sixfold patternEnlarged ice crystalSix long arms spread around one center.Real falling snow has many shapes, not only perfect stars.
More water vapor, the invisible water gas in air, can become ice on the growing crystal. The arms of one small crystal pass through much the same cold, moist air, so they can develop similar branches. Changes in the air can change how the next part grows.
That does not mean every bit of snow is a perfect star. Some crystals grow as flat plates, and arms can break or join other crystals. The drawing enlarges one branched example. Its six directions offer a visible clue to an arrangement much too small to see directly.
About this explanation
Starting knowledge: Recognizes snow, ice, water, and repeating shapes around a center.
Sources
- NOAA NESDIS — How Do Snowflakes Form?
- NOAA — How do snowflakes form? Get the science behind snow