Snowflakes form when water vapor in cold clouds changes directly into ice on tiny airborne particles. This matters because snowfall is part of the water cycle, affects weather hazards, and helps scientists understand conditions inside clouds. Each snowflake records a small history of temperature and humidity as it falls through the atmosphere.
The familiar six-sided shape comes from the molecular structure of ice.
Key Facts
- Deposition is the change from water vapor directly to ice without becoming liquid first.
- Snow crystals usually begin on a nucleation particle such as dust, pollen, or sea salt.
- Ice has hexagonal symmetry because H2O molecules arrange in a six-sided crystal lattice.
- Supersaturation means relative humidity with respect to ice is greater than 100 percent.
- Common snowflake growth range: about 0 °C to -20 °C inside clouds.
- Fall time estimate: t = h / v, where h is height and v is average falling speed.
Vocabulary
- Deposition
- Deposition is the phase change in which water vapor becomes solid ice directly.
- Nucleation
- Nucleation is the first step of crystal formation when molecules begin to arrange around a tiny particle.
- Supersaturation
- Supersaturation is a condition in which air contains more water vapor than the normal saturation amount for a given temperature.
- Crystal lattice
- A crystal lattice is the repeating three-dimensional arrangement of molecules in a solid.
- Dendrite
- A dendrite is a branching snow crystal shape that grows when humidity is high and temperatures favor rapid edge growth.
Common Mistakes to Avoid
- Thinking snowflakes are frozen raindrops. Frozen raindrops form sleet, while snowflakes usually grow by vapor depositing onto ice crystals inside clouds.
- Assuming every snowflake has a perfect six-point star shape. Many snow crystals are plates, columns, needles, or irregular clumps depending on temperature, humidity, and collisions.
- Ignoring humidity when predicting snowflake shape. Temperature is important, but the amount of water vapor controls whether crystals grow simple, faceted, or highly branched.
- Believing snow can only form below 0 °C at the ground. Snow forms in cold cloud layers, and it can reach the ground if the air below is not warm enough to melt it completely.
Practice Questions
- 1 A snow crystal falls from a cloud base 1200 m above the ground at an average speed of 1.5 m/s. How long does it take to reach the ground, and what is the answer in minutes?
- 2 A cloud layer is at -12 °C, where branching growth is common, and a snow crystal spends 8 minutes in the layer while growing at 0.03 mm/min along each branch tip. How much length is added to a branch tip?
- 3 Explain why two snowflakes that start with the same hexagonal ice structure can still end up with different final shapes as they fall.