Every so often during a snowstorm, flakes start falling that look more like feathers than crystals – thick, clumpy, almost unreal. like small pancakes.
So what causes really big snowflakes?
It comes down to three main ingredients: temperature, moisture and gentle motion.
Snowflakes start small
Every snowflake begins as a tiny ice crystal forming on a microscopic particle, like dust, high in the clouds. Most are six-sided (hexagonal) crystals. On their own, these crystals are delicate
To grow large, they need help.
The sweet spot: Near freezing
The biggest flakes usually fall when temperatures in the cloud and just below it are hovering close to 32 degrees.
When the air is very cold, snow tends to stay powdery and fine. But when temperatures are near freezing, snow crystals become slightly sticky, as they partially melt. That stickiness allows them to clump together as they fall.
This process is called aggregation.
Instead of falling as individual crystals, they collide and bond into large clusters, sometimes the size of quarters or larger.
Abundant moisture
Big flakes also require abundant moisture. A moisture-rich storm produces thicker clouds and higher snowfall rates, which increases the odds that crystals will bump into each other on the way down.
Heavy snow bands, especially in coastal storms, are prime environments for oversized flakes.
Light winds
If winds are too strong, flakes break apart. Light winds allow aggregates to grow and remain intact as they descend.
That’s why the biggest flakes often fall during steady, almost serene snowfall, not during howling blizzard conditions.
When we see them in DC
In the DC area, the fluffiest, fattest flakes tend to show up in marginal-temperature storms, especially early or late in the season, when the atmosphere is just cold enough for snow but warm enough for flakes to stick together.
They’re common in heavy bursts along the Interstate 95 corridor during coastal storms, when temperatures hover between about 30 and 33 degrees.
A sign of a change
Sometimes the presence of large snowflakes signals rising temperatures and a change to rain because they’re falling through a moist, near-freezing layer that lets them clump together and partially melt.
