Global Snow

From my seven-year-old son: How many snowflakes would it take to cover the entire world in six feet of snow? (I don't know why six feet...but that's what he asked.)

—Jed Scott

It's been too hot where I live, so I like thinking about this question!

Snow is fluffy because it has a lot of air in it. The same amount of water that makes an inch of rain would make a lot more than an inch of snow.

An inch of rain is usually equal to about a foot of snow, but it depends on what kind of snow it is. If the snow is light and fluffy, an inch worth of rain could make over 20 inches of snow!

All the clouds in the world, combined, hold about 13 trillion tons of water. If all that water were spread out evenly and all fell at once, it would cover the Earth with an inch of rain—or a foot of snow.

Most of the Earth is ocean. If we only made water fall on land, there would be enough for three or four inches of water. That's how much falls in a very big rainstorm.

So three or four inches of water should add up to three or four feet of snow, right?

Almost, but there's a problem. When snow piles up, the snow on the bottom gets squished. If a foot of snow falls, then another foot falls, the snow on the bottom gets squished, which means the whole pile is shorter than two feet tall.

If you leave the snow there, it will slowly get less and less deep as it settles down and compacts. This means that even if six feet of snow fell everywhere, it would only be six feet at first. Before long, it might be five feet. (This happens to humans, too. You get shorter throughout the day as your body compresses a little!)

This can make it hard to record exactly how much snow falls, and sometimes even weather experts have a hard time! If you wait until the end of a snowstorm to measure snow, maybe it will have all squished down, or some of the snow might have melted, so your measurement will be too small.

Instead of waiting until the end of the storm, you can measure the snow in parts. You let some snow fall, measure it, then clear it away and wait for more snow to fall.

You have to decide how much snow to clear away. If wait too long, the snow might become too squished, but if you measure it too often, it will all be light and fluffy and you'll get a number that's way too high.

Believe it or not, the National Weather Service has written special guidelines for how often to clear away snow, so everyone can measure it the same way. They use a special snow-measuring board, which is probably just a regular piece of wood, but I like to imagine that they treat it like a precision instrument and store it in a special locked case until it's needed.

The official guidelines say that you should clear the snow-measuring board every six hours. A few years ago, there was a big snowstorm, and the Baltimore airport measured 28.6 inches of snow. That would have been a new record. But then the National Weather Service learned that the person measuring the snow had cleared the board every hour, instead of every six hours. So they didn't know whether to count the record or not.

I didn't see what they ended up deciding, because four days later, another blizzard hit Baltimore and everyone suddenly had more important things to worry about. (Then there were more after that one. It was a snowy winter.)

Still, people have never seen a winter with six feet of snow across the entire world. A snowfall like that would—to answer the original question—take a total of about a mole of snowflakes, give or take a few zeros. With that much snow, every one of the 70 million kids in the United States would be able to make enough snowballs to hit every other kid with a snowball three times over.

Or you could keep some of the snowballs for yourself. Right now, in the hot weather where I live, that sounds wonderful.

Disappearing Water

What would happen if all the bodies of water on Earth magically disappeared?

—Joanna Xu

As is often the case with these questions, everyone would die.

The first people to notice would be swimmers and boaters, for obvious reasons.

To avoid a glass half empty scenario, we'll assume the water is replaced by air.

Most people swim in water which is relatively shallow, so most of them would survive the fall to the bottom, albeit with a few broken bones.[1] People out on the ocean, on the other hand, would be in trouble.

The ones in shallow water would hit bottom first, since they wouldn't have as far to fall. Within the first second, a large fraction of the boats in lakes, rivers, and harbors would crash into the bottom, and many of those on board would survive.

Boats out on the ocean would take longer to fall. Over the next five seconds, a wave of crashes would spread outward from the continents, as boats struck the continental shelf farther and farther from shore. These boats would be smashed to tiny fragments, killing everyone on board.

After the first six or seven seconds, there would be a brief lull in the ship destruction rate. Continental shelves drop off steeply, and most of the ships out over the deep sea would take a little longer to fall.

The Titanic sank in about two miles of water. After it disappeared beneath the surface, the two halves of the ship took between 5 and 15 minutes to reach the bottom.[2] Without the ocean there, it would have reached the bottom in about 30 seconds, striking it at airliner cruising speed.[3]

Within the first minute, just about every large ship would be on the bottom. The final boat to reach the bottom would probably be a small sailboat or life raft that was crossing an ocean trench when the water vanished. Thanks to low weight and/or drag from the sails, one of these vessels could take many minutes to reach the bottom.

If there were a seaplane floating on the deep ocean, it could conceivably survive, although it would take some luck and quick thinking by the pilot. The plane would initially drop, but as it gained speed it would tend to pull into a glide. After the initial shock, the pilot would have a reasonable amount of time to try to start the engine. Thanks in part to the thicker air, it's possible a seaplane could successfully land on a smooth patch of seabed. If the engine got started, the pilot could also try to fly to shore and land on a runway.

Fish, whales, and dolphins, and nearly all marine life would die immediately. Those near the bottom would suffocate or dessicate, while those near the surface in deeper water would suffer the same fate as boats.

Then the really weird stuff starts.

Without evaporation from lakes and oceans feeding the water cycle, it would stop raining. Without pools of water to drink from, people and most animals would dehydrate and die in a matter of days. Within a few weeks, plants would start withering in the ever-drier air. Within months, mass forest die-offs would begin.[4]

Huge amounts of dry, dead vegetation lead inevitably to fire, and within a few years, most of the world's forests would have burned. Forests store huge amounts of CO₂, and this burning would roughly double the amount of greenhouse gas in the atmosphere, accelerating global warming.

All in all, Joanna's scenario would result in virtually all life dying out pretty fast. But then things would get even worse.

Without a water cycle to weather rocks, the carbon-silicate feedback system which acts as a long-term thermostat to stabilize climate[5] would shut down. Without this feedback, volcanic CO₂ would build up in our atmosphere, leading—in the long term—to scorching temperatures similar to what's happened on Venus.[6]

We were going to lose our oceans anyway. As the Sun gets hotter, eventually water will start escaping through evaporation, and—one way or another—the planet will dry out and heat up. However, the loss of the oceans never seemed like something worth worrying too much about, since it's a billion years in the future. The oceans will be here long after our species is gone.

Unless Joanna ruins everything.