十万个为什么·科学阅读30篇（2）

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是什么让一些雪花形成完美的六边形，而另一些却显得杂乱？

Snowflake symmetry begins with water molecules bonding at 60-degree angles in ice crystals.
Perfect hexagons form only when temperature stays near −15°C and humidity remains steady for minutes.
Slight shifts in air pressure or dust particles disrupt molecular alignment during descent.
Each arm grows identically because vapor diffusion rates match across the crystal’s surface.
Irregular flakes result from collisions, melting-refreezing, or fluctuating atmospheric layers.
Scientists use cloud chambers to recreate conditions and photograph real-time growth patterns.
Microscopic impurities act as nucleation sites, sometimes triggering branched or needle-like shapes.
Computer models simulate how tiny changes produce vastly different macrostructures reliably.
Photographing falling snow with high-speed lasers reveals why no two large flakes are identical.
Studying crystal formation improves weather prediction and aircraft icing safety systems.