Why does 1 inch of rain equal 10 inches of snow?


Is this conversion even right? I found it on google with little explanation. One inch of rain seems minuscule to 10 inches of snow but maybe I have a fundamental misunderstanding of how much one inch of rain actually is. Please help.

In: Chemistry

I believe it’s density. Snow is water yes, but it’s full of air. Imagine a pool filled with water and a pool filled with snow. The pool will be full with both, but if you were check the mass of both, the water will have many more particles then the snow due to all the tiny air pockets within the snow. The air is a big reason why snow is fluffy/crunchy/squishy etc.

This is correct. Partially.
That is Ten inches of fresh fluffy snow.
It’s a combination of organization of molecules and the fact that water expands when freezing.
The fluffy flakes are able to stay spread out as they stack due to their shape. The molecules themselves are spread out as the freeze and crystallize. Each flake has plenty of ‘free space’. And then all the space in between each snow flake.
You ever play tetris and missed your mark and blocked your lowest open spot? Well snow is like that except there’s more than one block falling and they are all different shapes and only the wind has any say in where or how they land. As it melts the bonds that keep them in place are broken and the molecules are allowed to flow into the open spaces.
The resulting water has much less ‘free space’ between molecules

water molecules pack together way better than individual ice crystals. snow is more air than liquid water. that’s why it’s so fluffy, and why it’s such a good insulator (compared to liquid water or solid ice)

Think about how heavy water is vs fluffy snow, vs dense slush. Fluffy snow has a ton of air mixed up in there.

Like letting a full glass of crushed ice melt. The water you’re left with after won’t be as high as the ice.

That sounds about right, assuming the snow is pretty fluffy. When the snow is fluffy, it’s because the snow flakes aren’t all lying perfectly flat and all squished together tightly. Some are tilted, and they have lots of air in between them. Suppose you had a coffee can or milk carton open at the top, and it got filled up (about 10″ high) by falling fluffy snow outside. If you then brought it inside, the snow would melt — but you’d only have about 1″ of water, because the water does NOT have air mixed in with it.

If you’ve never seen lots of fluffy snow, that might be harder to visualize. If you’ve never seen it, let’s try a different situation. Popping popcorn. If you put a small layer of popcorn in the bottom of a pan, it’s only about a quarter of an inch tall. But when you pop it, you might get 8 inches or more of popcorn. That’s because the popped corn now has lots of air mixed into it and also in-between it.

Or think of a piece of soft foam: you can squish it down to a lot thinner, when you squish the air out; fresh snowfall has a lot of air mixed in between the flakes.

It’s all about water content of that 10” of snow. Fresh snow is pretty close but there are factors I’ll get into later that create some variables.

Your statement talks about height of water or snow. Think in volume but not density. More specifically water content for a volume of snow. One crystal of snow (flake) is more or less equivalent to water density for the same amount of molecules, allow for some expansion when freezing. As one user stated the shape of snow crystals doesn’t allow for tight compaction. Therefore the volume (space it occupies) of 1” of water to 10” of snow is vastly different but the water content of 10” of snow to 1” of water can be similar. With time the snow crystals change shape and snow generally shrinks in volume and therefore 8” or 6” of old snow is closer in water content to your 1” of water. Consider that glacier ice is really, really, really old snow. It’s the closest you may get to 1” of each.

Also consider that there are variances on the water content of snow depending where you are on the continent assuming USA. 10” of snow in Colorado would typically have a lower water content (colder and lower relative humidity) than 10” of snow in Washington state (warmer and more humid) which makes for a more dense 10” of snow. Likely resulting in a higher water content in Washington over Colorado for the same 10”.

On a more micro level each snowfall comes with its own unique environmental conditions (humidity, wind,temperature) and the water content for volume can vary greatly with each individual storm.

Melt down 10”x10”x10”= 1000 cubic inches of different snow and see how much water you get. Don’t boil it as that will cause some loss of volume to evaporation.

Hope that helps. I study snow for avalanche purposes.

Snow isn’t very dense, meaning there’s a lot of air between the water molecules. Liquid water is a lot more dense so the molecules are much closer together.

If there is snow on the ground where you are, try an experiment. Fill a stove pot full of snow – don’t press pack it in, but make sure it’s filled well. Then put it on the stove to cook. When all the snow is melted, but before it starts to boil, take it off, let the pot cool, and then measure the depth of the water. It should be around 1/10th the depth of the pot.

If you packed the snow down as you were filling the pot, the water will be higher, because you put more snow in the pot by making the snow more dense.

The snow ratio also changes with the atmosphere, if there’s a particularly dry layer the snow falls through and the air is incredibly cold, the ratio goes up. It’s not uncommon during strong winter storms to see 1:20 or 1:25 ratios. You just don’t see 25 inches of snow because cold air holds less water and you generally won’t get an inch of precipitable water in the air, so the .25 inches of rain you can get will translate into a couple of inches of snow

It is difficult to predict how much snow you will get because you have to predict how much water will fall AND how fluffy the snow will be. Snow can be 2x to 15x as fluffy as plain liquid water, so forecasters just predict the amount of water and then multiply by 10.