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.

 

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What is Plankton?

One night, we’re reading a book about whales. It was the prize in a kid’s meal from Chik-Fil-A, and chock full of pictures of whales and dolphins, and he loved it. So we get to the page about the Blue Whale, and he asks what it eats. “Plankton,” I tell him.

He thinks about that for a moment. “What’s plankton?”

I think about that for a moment. “Little tiny plants,” I say. “And tiny shrimp.”

“Ew,” he says, wrinkling his nose.

So there you have it. Whales eat gross stuff.

Plankton

plankton

It turns out that I was wrong. Plankton is not a type of creature, but a lifestyle – it’s defined as “the aggregate of passively floating, drifting, or somewhat motile organisms occurring in a body of water, primarily comprising microscopic algae and protozoa.” This is contrasted with nekton, which is “the aggregate of actively swimming aquatic organisms in a body of water, able to move independently of water currents.” The word comes from the German word Plankton from the Greek word plankton, meaning “wandering, drifting”. That words derives from the proto-Indo-European word *plak-, “to strike, hit”.

There are two different ways to divide plankton:  either by the kingdom the plankton falls into (phytoplankton, zooplankton, and bacterioplankton), or by whether or not the organisms are permanently plankton (holoplankton or meroplankton).

Phtoplankton

Phytoplankton, also known as microalgae, is “little tiny plants”.  They’re chlorophyll-containing organisms that float in the upper reaches of the ocean where sunlight can penetrate – generally the euphotic region, which is about 200 meters deep.  For the most part, phytoplankton is found along the coasts, in the upper northern and southern latitudes, and along the equator.

There are two magor groups of phytoplankton, dinoflagellates and diatoms.  According to NOAA, “Dinoflagellates use a whip-like tail, or flagella, to move through the water and their bodies are covered with complex shells. Diatoms also have shells, but they are made of a different substance and their structure is rigid and made of interlocking parts. Diatoms do not rely on flagella to move through the water and instead rely on ocean currents to travel through the water.”

Zooplankton

Zooplankton include the “tiny shrimp”, just like I told my son.  However, there is a whole lot more to them than “tiny shrimp”.  They include shrimp, worms, water fleas, isopods, tunicates, the larval form of larger organisms, and any other sort of poorly-swimming oceanic ocean life.  This includes jellyfish.

Zooplankton are further classified by size:

Size categories include:  picoplankton that measure less than 2 micrometers, nanoplankton measure between 2-20 micrometers, microplankton measure between 20-200 micrometers, mesoplankton measure between 0.2-20 millimeters, macroplankton measure between 20-200 millimeters, and the megaplankton, which measure over 200 millimeters (almost 8 inches).

Bacteroplankton

Many sources classify bacteroplankton as part of zooplankton, but others classify them as seperate because bacteria are in one of two kingdoms (Archaebacteria or Eubacteria) that is entirely separate from Animalia.  In brief, bacteroplankton are ocean-living bacteria, and there are a lot of them.  Estimates are that the ocean contains 3.1 x 1028 of them, which is a number I won’t type out because it is long and tedious to do so.

Holoplankton and Meroplankton

These two classifications merely describe whether the planktonic organism remains planktonic throughout its entire lifecycle.  Holoplankton are larval organisms, eventually maturing into nektonic creatures, while meroplankton are organisms that remain planktonic throughout their entire lifecycle.

So, next time you go… have fun at the beach.

How Long Would It Take To Drive To The North Pole?

About a week ago, we’re in my car running a few errands. My son is chattering away, happily talking about how Winnie-the-Pooh didn’t really discover the North Pole because he was just pretending. (The Winnie-the-Pooh books are our current bedtime story, if you’re wondering.) Something about the combination of these two facts must have sparked what happened next.

“Dad? How long would it take to drive to the North Pole?”

“Magnetic, or true?” I counter.

“What?” he responds.

The Two Poles

There are two North Poles (and two South Poles as well), the “true” (or geographic) North Pole and the magnetic North Pole. And they do not really match up. So what’s the difference between the two of them? Here’s how National Geographic describes the North Pole:

The North Pole is the northernmost point on Earth. It is the precise point of the intersection of the Earth’s axis and the Earth’s surface.

The North Pole sits in the middle of the Arctic Ocean, on water that is almost always covered with ice. The ice is about 2-3 meters (6-10 feet) thick. The depth of the ocean at  North Pole is more than 4,000 meters (13,123 feet).

275px-Arctic_Ocean
Here.

The magnetic North Pole is a little more difficult to pin down. Why? Well, let’s start by letting NOAA tell us what the magnetic poles are:

Magnetic poles are defined in different ways. They are commonly understood as positions on the Earth’s surface where the geomagnetic field is vertical (i.e., perpendicular) to the ellipsoid. These north and south positions, called dip poles, do not need to be (and are not currently) antipodal.

“Antipodal”, by the way, is your new word for the day. It means:

  1. Geography. on the opposite side of the globe
  2. diametrically opposite
  3. Botany. (in a developing ovule) of or at the end opposite to the micropyle

So why does the magnetic field move? Because it’s produced by the spinning of the Earth’s inner core – a solid iron ball almost as big as the moon and hotter than the sun that spins a little faster than the Earth’s crust in a massive sea of liquid (or, at least more liquid) metal called the “liquid inner core”. So far, so good. Right?

world-pr

Now, the full details of what generates the magnetic field are not understood – a fact that I’m certain makes scientists extremely happy. But, as Natural Resources Canada explains, we do understand the basic concepts:

For magnetic field generation to occur several conditions must be met:

  1. there must be a conducting fluid;
  2. there must be enough energy to cause the fluid to move with sufficient speed and with the appropriate flow pattern;
  3. there must be a “seed” magnetic field.

All these conditions are met in the outer core. Molten iron is a good conductor. There is sufficient energy to drive convection, and the convective motion, coupled with the Earth’s rotation, produce the appropriate flow pattern. Even before the Earth’s magnetic field was first formed magnetic fields were present in the form of the sun’s magnetic field. Once the process is going, the existing field acts as the seed field. As a stream of molten iron passes through the existing magnetic field, an electric current is generated through a process called magnetic induction. The newly created electric field will in turn create a magnetic field. Given the right relationship between the magnetic field and the fluid flow, the generated magnetic field can reinforces the initial magnetic field. As long as there is sufficient fluid motion in the outer core, the process will continue.

So why does it move? well, the magnetic field is created by a fast-spinning ball inside a slow-spinning ball. So it wobbles, at an average speed of 10 kilometers per year for the entire 20th century.

nmppath2001_med

So where is it right now? According to the World Data Center for Geomagnetism you’ll find it at 80.4° North, 72.6° west. Or, in other words, Nunavut, Canada.

Magnetic Pole
Right there

How long would it take to drive there?

According to the handy Computing Distances between Latitudes/Longitudes in One Step page, and checking the latitude and longitude of Cincinnati, Ohio (39.1000° N, 84.5167° W), I get the following figures:

  • Geographic North Pole: 3,524.64 miles
  • Magnetic North Pole: 4,144.18 miles

Now, looking at the map above, you would think that driving to either pole would be impossible. After all, there’s ocean in the way! But this is where living in Cincinnati comes in handy – we have a Ride the Ducks tour here!

duck boat

The “Duck Boat” is a World War II surplus DUKW, an amphibious vehicle capable of cruising at 35 mph (56 km/h) on roads and 4.41 mph (7.14 km/h) in the water with an operational range of 400 miles on land (or 58 miles/93 km in the water). I doubt it has the ring mount for the machine guns, though. So, we can do this. In theory, at least.

Eyeballing the map, I’m going to go with an estimate of 90% overland travel to the magnetic North Pole and 65% overland travel to the geographic North Pole. Further assumptions will be:

  • Driving 18 hours a day – my wife has agreed to go along with this mad idea, so we can drive quite a bit. But we still need to stop for bathroom breaks, meals, stretching our legs, and refueling.
  • We’ll be hitting our cruising speed.
  • We don’t get eaten by polar bears.
  • We’re not worrying about mountains.
  • We drive in a straight line, and we don’t get lost.

Getting to the geographic North Pole is roughly 2,291 miles over land and 1233.64 miles over sea. That’s (2,291/35) + (1233.64/4.41) = 345.19 hours. Based on our assumptions, that’s 19 days, 4 hours, 15 minutes. And we would have needed to stop for gas 27 times on the trip.

Getting to the magnetic North Pole is 3,729.762 miles over land and 414.418 miles over sea. That’s (3,729.762/35) + (414.418/4.41) = 200.54 hours. Based on our assumptions that’s 11 days, 3 hours, 23 minutes. And we would have needed to stop for gas 17 times.

Either way, we also get a complimentary duck whistle for the trip.  So my son is going to love this!

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!

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.