Why Do Tornadoes Suck Things Up?

My six-year-old nephew spent the weekend at my house, which delighted my son to no end. The end result was the sort of excited chaos you might expect – lots of six-year-old bickering, and toys strewn everywhere, and strange non sequitur laced conversations. At one point, tornadoes came up. I don’t know why, because I wasn’t paying attention to the start of that conversation. But my nephew declared that he’d seen a tornado, and that his mom had made them get in the closet. And my son responded that you didn’t get in the closet, you got in the tub, because we’d had a tornado warning once and our front bathroom is the safest place in our condo for that sort of thing. So they argue the merits of bathroom versus closet for a few minutes, and then my son looks at me. I smile, preparing my “it depends on the house” explanation for the question I’m sure is coming.

“Dad?” my son asks. “Why do tornadoes suck things up?”

All right. So that isn’t the question I expected.

What is a tornado?

The National Oceanic and Atmospheric Administration has a page titled The Online Tornado FAQ, and I’ll be referencing it a lot over the course of this question. To begin with, I’ll just quote their answer to the question:

According to the Glossary of Meteorology (AMS 2000), a tornado is “a violently rotating column of air, pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.” Literally, in order for a vortex to be classified as a tornado, it must be in contact with the ground and the cloud base. Weather scientists haven’t found it so simple in practice, however, to classify and define tornadoes (per this essay by Doswell). For example, the difference is unclear between an strong mesocyclone (parent thunderstorm circulation) on the ground, and a large, weak tornado. There is also disagreement as to whether separate ground contacts of the same funnel constitute separate tornadoes. Meteorologists also can disagree on precisely defining large, intense, messy multivortex circulations, such as the El Reno tornado of 2013, compared to the parent mesocyclone and surrounding winds of damaging intensity. It is well-known that a tornado may not have a visible funnel. Mobile radars also have showed that tornadoes often extend outside an existing, visible funnel. At what wind speed of the cloud-to-ground vortex does a tornado begin? How close must two or more different tornadic circulations become to qualify as a one multiple-vortex tornado, instead of separate tornadoes? There are no firm answers.

In other words, a tornado is a vortex, very much like an atmospheric whirlpool. In an oversimplified fashion they form under basically the same conditions – our atmosphere is described by the same fluid dynamics that describes the behavior of water, after all. Moving air hits a barrier – in this case, denser colder air – and twists back on itself. The air is still moving into the barrier, however, so the air that is deflected back picks up speed thanks to the conservation of angular momentum, creating a vortex.

But, like I said, that’s the oversimplified explanation. Here’s what NOAA says on the subject:

The truth is that we don’t fully understand. The most destructive and deadly tornadoes occur from supercells–which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone. [Supercells can also produce damaging hail, severe non-tornadic winds, unusually frequent lightning, and flash floods.] Tornado formation is believed to be dictated mainly by things which happen on the storm scale, in and around the mesocyclone. Recent theories and results from the VORTEX programs suggest that once a mesocyclone is underway, tornado development is related to temperature changes across the edge of downdraft air wrapping around the mesocyclone (the occlusion downdraft). Mathematical modeling studies of tornado formation also indicate that it can happen without such temperature patterns; and in fact, very little temperature variation was observed near some of the most destructive tornadoes in history on 3 May 1999. The details behind these theories are given in several of the Scientific References accompanying this FAQ

What this means is that they’re vortices, and they form just like any other vortex. But, like most things in nature, they’re super complicated and we don’t really quite understand what makes them start.

So how do these tornados suck things up?

The famous “sucking tornados” are “multiple vortex tornados“, and they create what is called a “suction vortex“. Interestingly, the suction vortex has little to do with air pressure, and everything to do with wind speed. That is, the lower air pressure within the vortex isn’t low enough to “suck” things up. Tornadoes aren’t straws. Instead, the speed of the winds traveling up the vortex funnel create the “suction” effect.

Here’s what happens. The secondary vortices of a multiple-vortex tornado orbit the axis of the primary vortex, increasing the wind speed around the primary vortex. When the wind from and around the secondary vortices “turns the corner” – that is, enters the primary vortex and suddenly changes from horizontal to vertical flow – angular conservation causes the wind to pick up an enormous amount of speed. It is this wind speed that lifts objects – cows, trucks, people, roofs, whatever – and hurls them into the air. Note that all tornadoes have this “turn the corner” effect, but it takes the secondary vortices to really get the wind moving fast enough to lift really heavy objects.

Waterspouts and fire tornadoes

My son had heard of waterspouts, and guessed that they were water tornadoes. He was right. A waterspout is literally just a tornado that forms over water and that sucks up water.

Fire tornadoes are a little different. They are similar to actual tornadoes in appearance – except, you know, for the fire – but are formed by rising surface winds (usually generated by the heat from the fire) that meet turbulent winds to form a spiral of rising flame. They aren’t formed by supercell thunderstorms and aren’t tornadoes. Doesn’t make them safe, mind.

 

What If The Oceans Froze?

My son has a fascination with ice. He loves it in drinks (something he gets from me, more than from his mother), and he loves to look at it. We play games with it, like the time I put an ice cube in a bowl for him so he could watch it melt. Or the time we left a cup of water outside to see it freeze. I think he likes the idea that water can turn into a solid, and the fact that it’s cold is just a bonus bit of entertainment.

So we’re driving to church one Sunday, and looking at the snow that’s covered everything – one of the few days this winter where we’ve actually had snow – and he asks me “what if the oceans froze?”

Well, that sounds like an apocalyptic scenario if I’ve ever heard one. “Froze solid?” I reply?

“Yes! So we could ice skate on them!”

Bear in mind that my son has never gone ice skating. So I have no idea where that came from. But the question is interesting. And, sadly, nowhere near as much fun as he’d hope.

When Does Salt Water Freeze?

To start with, ocean water has a much lower freezing point than freshwater. In “Can the ocean freeze?“, NOAA informs us that seawater freezes at 28.4 degrees Fahrenheit (which is -2 degrees Celsius), because of the salt. They also tell us that the average temperature of all ocean water is about 38.3 degrees Fahrenheit (3.5 degrees celsius). So, in theory, to freeze the oceans we’d simply need to reduce the average ocean temperature by 9.9 degrees Fahrenheit (5.5 degrees Celsius).

How cold would it have to get?

Interestingly, the simple truth is that all you’d have to do to freeze the ocean is get the air below the freezing point of the ocean. Then, eventually, you’d manage it. That would require bringing the average equatorial temperature down to that level, and the best figure I could find for that average temperature is 77 degrees f (25 degrees C). That’s a 38.7 degree F (21.5 degree C) difference. This would bring average global temperatures down to 22.3 degrees F (-5.5 degrees C), so things would be terribly cold. To put it in perspective, polar climates have an average temperature of 50 degrees F (10 degrees C).

Killing Frost

A killing frost is a temperature that will kill a plant entirely – not just damage the extremities. Corn and soybeans will die below 28 degrees F. Wheat is a little hardier, depending on the growth stage, but will pretty much die at 24 degrees F (-4 degrees C). So if it got cold enough to freeze the oceans, we’d be living in a permanent killer frost.  Which is another way of saying that we’d be in huge trouble.

Snowball Earth

Interestingly, this may have happened before. There is something called the Snowball Earth hypothesis, that says the Earth may have been frozen like this some 650 million years ago. Equatorial temperatures would have been around what present day Antarctic temperatures are like now – which means an average range of -67.18 degrees F (-55.1 degrees C) in Vostok to 22.46 degrees F (-5.3 degrees C) in the Antarctic Peninsula. There’s a whole lot of disagreement about this theory, though, so take it with a grain of salt until and unless more information comes along.

Ice And Snow

It’s winter, and it finally snowed in these parts, and these two facts have combined to make my son quite happy. Every day after I pick him up from his preschool, he asks if we can go walk in the snow – something that turns into us hiking across snow-covered fields and throwing snowballs at each other, and which ends with us having to leave snow-caked shoes in the hallway and hanging snow-covered coats and gloves (and sometimes pants) in the bathroom to dry.

“Why does it snow?” he asked yesterday, after we were back inside and bundled up under some lovely fleece blankets.

Uhm… I haven’t the slightest idea. I mean, I know it must be related to the concept of rain. Both rain and snow are precipitation, after all. But I don’t really know why snowflakes form instead of, say, sleet. Which makes this a great question! And a great question to combine with another one he asked, which came up while he was drinking some water: “why is ice made out of water?”

So let’s find out.

What is “Freezing”?

To understand freezing, we’ll need to take a quick look at matter. For help with this, we’ll turn to LiveScience.com and Matter: Definition & the Five States of Matter. They define matter as “anything that has mass and takes up space). The state of matter is the form the matter takes, with the most common states being solids, liquids, gasses, and plasma. (The article lists Bose-Einstein condensate as the fifth state; Wikipedia lists twenty-one different states). Matter can also be said to be in a phase, which is “a region of space (a thermodynamic system) throughout which all physical properties of a material are essentially uniform”.

Using ice as an example, an ice cube is water in the solid state in a phase the size of the cube. Ice cubes in a glass of water then represent a system which has water in two states (solid and liquid) and two phases (the liquid volume, and the volume of the ice cubes).

With that in mind, we’ll turn to Purdue University and Freezing:

When a liquid is cooled, the average energy of the molecules decreases.

At some point, the amount of heat removed is great enough that the attractive forces between molecules draw the molecules close together, and the liquid freezes to a solid.

The temperature of a freezing liquid remains constant, even when more heat is removed.

The freezing point of a liquid or the melting point of a solid is the temperature at which the solid and liquid phases are in equilibrium.

The rate of freezing of the liquid is equal to the rate of melting of the solid and the quantities of solid and liquid remain constant.

What happens when water freezes?

When water freezes, it undergoes a phase change from liquid to solid. Energy is removed from the water molecules, slowing them down and making them become more dense. However, it hits maximum density at 4 degrees Celsius (39.2 degrees Fahrenheit) – below that temperature, water starts getting less dense.

Why?

As water begins to freeze, the molecules crystallize into open hexagonal structures.
waterhex
This hexagonal structure contains more space than liquid water, making it less dense. So, by the time it has fully hardened into a solid, it floats on top of the liquid water.

How do snowflakes form?

Snowflakes start off just like rain – as water droplets forming around pollen or dust. The distinctive shapes arise because of crystalline hexagonal structure I mentioned above. A single water crystal will have six sides, and this causes the crystals to build up into a symmetrical pattern as they grow in size. The specifics of the shape are determined by the temperature and the atmospheric conditions:

Ultimately, it is the temperature at which a crystal forms — and to a lesser extent the humidity of the air — that determines the basic shape of the ice crystal. Thus, we see long needle-like crystals at 23 degrees F and very flat plate-like crystals at 5 degrees F.

The intricate shape of a single arm of the snowflake is determined by the atmospheric conditions experienced by entire ice crystal as it falls. A crystal might begin to grow arms in one manner, and then minutes or even seconds later, slight changes in the surrounding temperature or humidity causes the crystal to grow in another way. Although the six-sided shape is always maintained, the ice crystal (and its six arms) may branch off in new directions. Because each arm experiences the same atmospheric conditions, the arms look identical.

Sleet, Hail, and Frozen Rain

All of this made me wonder, though. If snow is literally created by the same process that creates rain, where to the other types of “winter weather” come from? That is, sleet, and hail, and frozen rain. Fortunately, NOAA has the answer.

Snow generally begins life in the “dendritic growth zone” (aka the “snow growth zone”), a layer in the atmosphere with temperatures between 10.4 and 0.4 degrees Fahrenheit (-12 to 18 degrees Celsius), and cannot form if the atmospheric temperature rises above 32 degrees Fahrenheit (0 degrees Celsius). Additionally, the relative humidity of the atmosphere must be at 70% or greater. Note, however, that the dendritic growth zone is not a formal layer of the atmosphere (hence the term “zone” instead of “layer”) – it can form at different altitudes, depending on the weather.

At some point – one which is determined by the size of the snowflake and the weather conditions – the snowflake falls. It may partially melt as it falls, but as long as it passes through no more than “a very shallow melting layer” (no more than 1,500 feet thick) that is no more than 33.8 degrees Fahrenheit (1 degree Celsius) and then refreezes. In fact, partially melted and refrozen snowflakes help produce the wet snow that is so beloved of people who build snowmen and pack snowballs.

Sleet starts life as snow, and goes through a similar process to the creation of wet snow. However, it hits a melting layer that is less than 2,000 feet thick and has a temperature between 33.8 and 37.4 degrees Fahrenheit (1 to 3 degrees Celsius). All of which is a fancy way of saying that sleet is partially melted snow.

Freezing rain also starts life as snow. However, it hits a melting layer with a temperature above 37.4 degrees Fahrenheit (3 degrees Celsius). This makes it melt completely. The exterior cools below freezing as it falls, but it doesn’t fall long enough to turn into sleet before it hits the surface. But, since the exterior is at the freezing point of water (or extremely close), it freezes on contact with the surface.

Hail is as unrelated to snow as anything that starts out life as “ice crystals in the upper atmosphere” can be. It israin that gets blown up into extremely cold layers of the atmosphere and freezes. Then it falls when the wind won’t hold it up, only to (possibly) get blown upwards again and have more layers of ice freze around it. This can continue until the updrafts that keep throwing it upwards are no longer able to do so.

What Is A Hurricane?

I had a house full of kids, recently. My son’s friends came over, so that gave us four people under the age of eight running around my house. And as kids do, they talk. One of the children, a seven-year-old girl, was telling everyone else about the tornadoes and recent flooding in Texas. This got them to talking about storms, and soon enough my son stops and asks me a question.

“What’s a hurricane?”

Ah… a big storm? You’d think I would know more. I used to live in Maryland and, while we didn’t get hit with hurricanes the way that (say) Florida did, we still got them coming ashore. I’ve watched the rain hammer down and the wind howl, and then sat in the eye, and then watched the wind and the rain hit again. But it turns out I don’t have a clear idea beyond “a big storm”.

Fortunately, the National Hurricane Center of the National Oceanic and Atmospheric Administration() offers a lot more detail. According to them, a hurricane is one of four varieties of a “tropical cyclone”. A tropical cyclone is, of course, “a rotating, organized system of clouds and thunderstorms that originates over tropical or subtropical waters and has a closed low-level circulation.” These storms rotate counterclockwise in the Northern Hemisphere, and clockwise in the Sourthern Hemisphere. NOAA breaks tropical cyclones into four categories:

  • Tropical depression, a cyclone with maximum sustained winds below 39 mph
  • Tropical storm, a cyclone with maximum sustained winds of 39 – 73 mph
  • Hurricane, a tropical cyclone with maximum sustained winds of 74 – 110 mph
  • Major hurricane, a tropical cyclone with maximum sustained winds of 111 mph or more.

Confusingly, hurricanes are known as cyclones in the Indian Ocean and South Pacific Ocean, and as typhoons in the western North Pacific. Tropical cyclones have seasons, with the Atlantic hurricane season running from June 1 through November 30 and the Eastern Pacific hurricane season running from May 15 through November 30.

Intriguingly enough, the winds are not necessarily the most dangerous aspect of a hurricane. Storm surges get that award. These are swells of sea water 30 to 40 miles wide and up to 15 feet higher than the normal tide level that get driven ahead of the storms, acting like a miniature tidal wave when they hit shore.

Surf’s up!

How Many Snowflakes Does It Take To Make A Snowman?

Far from being a white Christmas around here, things are shaping up to be a sort of greyish-green muddy Christmas. I’m disappointed, my son is disappointed, and my wife is thrilled – she doesn’t like snow. But we did get a few flurries last week. At that time, my son stood at the sliding glass door of our condo and stared out. “It’s snowing, daddy!”

“I know,” I say, looking up from my book.

“But it’s not enough to make a snowman,” he adds, sounding a little disappointed.

Glancing outside, I see that there isn’t even enough snow to dust the ground. It’s melting before it lands, in some cases. “No,” I agree, “there isn’t.”

“It’ll take six million!” he declares. “Seven, eight million!”

At this moment, I’m pretty sure I see where he’s going with this. But I ask anyway. “Eight million what?”

“It’ll take eight million snowflakes to make a snowman!”

Well, that’s not precisely a question. But I’ll run with it.

It turns out that there are a few ways to figure the answer. For laughs, I tried punching the question “how many snowflakes in a snowman” into Wolfram Alpha. Believe it or not, I got an answer: an average of 58 million (5.8 x 107), with a range of 54 million to 61 million (5.4 x 107 to 6.1 x 107). The calculations assumed that a “typical snowman” could be modeled by a cylinder with a volume of 40 cubic feet, and that a snowflake could be modeled as a sphere with a volume of 0.0000004 cubic feet (4.0 x 10-7) and a packing density of 0.56 to 0.64.

Another way to do this is to figure out the mass of a snowflake. Amusingly enough, The Physics Factbook actually has an entry titled Mass of a Snowflake. It tells us the following:

  • The mass of a water molecule is 2.992 x 10-26 kg.
  • A typical snow crystal may contain 1018 water molecules.
  • A typical snowflake is made of 100 snow crystals

The end result is that a typical snowflake weighs about 3 mg. There’s 28.3495 grams in an ounce, which is the same as 28,349.5 mg, so that gives us 9,449.8 snowflakes per ounce. There’s 16 ounces in a pound, so that gives us 151,197 snowflakes in a pound of snow.

Here’s where it gets tricky. As far as I can tell, there is no such beast as a “regulation size snowman”. So I’m going to do a lot of estimates here. Let’s assume a three-tier snowman, with the middle tier about two-thirds the size of the base, and the top tier about half the size of the base. And let’s assume my son is building it. My son weighs 49 pounds, and 15-20 pounds seems to be about the limit of what he can lift without help. So, let’s assume he starts by making a 18 pound base (near the upper end of what he seems to be able to handle, because rolling is easier than lifting). That gives him a 12 pound middle and a 9 pound top. So, 39 pounds of snow in all. 39 times 151,197 gives us 5,896,696 snowflakes. Less than the Wolfram Alpha estimate, yes, but I don’t think my son’s snowman is 40 cubic feet of snow, either.

Interestingly enough, he’s not far off from his own estimate of six to eight million snowflakes. Eight million snowflakes would only add another 13.9 pounds (2,103,304 snowflakes/151,197 snowflakes) to the snowman. That would give us a snowman with a 24.4 pound base, a 16.3 pound midsection, and a 12.2 pound head. It might be slightly tough going for my son, but I suspect he could still build that.

For laughs, the hypothetical 39 pound snowman is composed of 589,668,300,000,000,000,000,000,000 water molecules. Numbers like that are the clearest explanation of why exponents were invented.

Why does it have to rain?

So we’re all in the car yesterday, and it is pouring down rain. Buckets of it. And my son asks “why does it have to rain now?”

“Because the clouds are full,” my wife answers. 

“That’s no fair,” replies my son.

This got me thinking. I’m familiar with the basics of the water cycle – I had to learn it in second grade. But, I was a little fuzzy. What actually causes rain?  And so, invoking the power of the Internet, I went to NOAA.

If you don’t recognize the name, NOAA is the National Oceanic and Atmospheric Administration. They’re a federal agency tasked with, among other things, weather forecasting and climate monitoring.   And on a SciJinks page titled What makes it rain?, they reminded me of some stuff I’d learned back in second grade and forgot.

See, we all know that heat makes water evaporate.  And warm air can hold more water than cool air. This warm air condenses around dirt and dust and pollen in the air, making clouds (or fog, depending on altitude).  

How long those clouds last is a function of air temperature, because cooler air will make water condense faster. When too much water has condensed to remain airborne, the water falls.

Voila!  Rain!

So why did it have to rain yesterday?  Because there was a lot of water in the air, and it was cold.