Why's That: What Makes Snow Wet or Dry?

Feb 10, 2017

The Mallinson Institute for Science Education's Todd Ellis sorts snowflakes on his sleeve.
Credit Sehvilla Mann / WMUK

It’s easy to see snow as a nuisance when you’re trying to remove it. But perhaps a look at the science of snow can restore your sense of wonder. Maria Drouillard of Kalamazoo wants to know: why is it that the texture of snow varies so much from powdery to damp? In other words, what makes snow wet or dry?

“That’s a really great question,” says Todd Ellis. He teaches weather and climate instruction at Western’s Mallinson Institute for Science Education.

Even if it’s all made of water, Ellis says snow can be wetter or drier. It all depends on how much snow you can get out of one inch of water.

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“An average I would say for around here and most of the eastern seaboard would be about 10 inches of snow per inch of water,” Ellis says.

Ten inches of snow might seem like a lot for one inch of water. But in a place like Colorado, where Ellis was a grad student, you could get much more than that.

“The snow was so dry you could get almost 30 inches of snow out of that same inch of water,” he says.

Ellis says that’s helpful for the ski resorts. But why is some snow much less moist than other kinds? Ellis starts with the clouds where snow crystals grow.

Every cloud is pretty damp, but the moisture does vary by a few percentage points. And the temperature varies too.

“So if you’re on the dry side of that, and you’re on the cold side, well below zero degrees Fahrenheit you can get these really fine crystals,” he says.

But if a cloud is relatively warm and – more importantly – moist, it can grow dendritic snowflakes.

“These are the ones that everyone makes in kindergarten, by cutting out the six sides and making the really fancy shapes.”

And those are just two kinds of snowflake. Ellis says besides the fine crystals and dendrites, you’ll find lots of other shapes too.

“And in fact there’s a whole series of diagrams that people have mapped out trying to figure out the different combinations of temperature and humidity that cause different-shaped crystals to form,” he says.

Snow on WMU's main campus
Credit Sehvilla Mann / WMUK

The shape of a snowflake can play a big role in how powdery or wet the snow becomes. That’s because some crystal shapes easily gather moisture or melt a bit on the way down, and then they stick together, creating a wetter snow. Ellis says fine snow crystals tend to resist melting, but it’s common for dendrites.

“They’re really susceptible to that kind of melting because they have all these appendages that are very easily melted as the air temperature gets pretty close to freezing they can start to melt on their own,” he says.

Dendrites are also just big – which makes it more likely that they’ll stick to each other.

And Ellis says right around freezing, snowflakes can change a lot as they fall.

“Believe it or not, the actual process of freezing gives off its own energy. Gives off something called a latent heating, which can momentarily actually melt snowflakes nearby, so it’s just – there’s lot of little things when you’re right near freezing that can cause the phase change to go from solid back to liquid, and liquid back to solid,” he says.

To summarize: powder snow really is drier than wet snow. There’s no single wet or dry snowflake shape. But some crystals – like dendrites – are more prone to melting as they fall.