Feeds:
Posts
Comments

Posts Tagged ‘Jack Williams’ Weather Book’

Chapter 5 has some mistakes and some interesting considerations.

Mistakes:

p. 64: molecules in motion collide all the time

Williams has a diagram of a water molecle and a discussion of ice, liquid, and gaseous activities. In the gas part, he says that these fast moving molecules are so far apart that they seldom collide. Nonsense! They collide all the time, but they mostly bounce off each other so that – as he correctly concludes — they don’t become attached very easily. And why do they bounce? Mainly because they are moving very fast, 1000 feet per second or more.

p. 66: relative humidity is the ratio of water to total volume, not water to “dry air”.

Here is a chart of the amounts of water to be found in saturated air at different temperatures. It gives the number of grams of water in one kilogram of saturated air. So, in 95° air that is saturated, there are 37.5 grams of water. In  86° air that is saturated, there are 28 grams of water in vapor form.

Note: this is saturated air, not “dry air” as he labels it. Minor, but confusing.

p. 67: the state of a molecule is its relationship with other molecules

Williams refers to “a liquid molecule” – meaning one molecule of liquid water.

[Probably you realize what he means, so just keep reading. But a molecule is just a molecule; you can’t tell if it’s liquid or solid until you look around it for some other molecules and see how they are attached, if at all.]

Then he talks about that molecule breaking its bonds, and if a lone molecule of water breaks its bonds, you might think it was breaking its internal bonds between hydrogen and oxygen. No, he’s just saying that a single molecule of water needs energy to break out of a liquid environment.

p. 67-68: a linguistic reflection

Sublimation means the change from a solid to a vapor without becoming a liquid in between. Sometimes when the air warms after an icy night, you see “sea smoke,” tendrils of fog rising from the air just slightly above the ice. This fog is actually condensed from invisible vapor that is leaping off the ice. This change from solid to gas is called sublimation. It then becomes liquid (tiny drops in the visible fog) for a short time before evaporating again.

Using the word sublimate to mean depositing vapor as ice is new to me. I don’t mean it can’t happen; it certainly can. But it’s a new use of the word sublimation to go from vapor to ice as well as from ice to vapor. It’s a pity because it obscures the metaphorical use of the word sublimation to mean the fulfillment of the human person by spiritual means, rather than by material and then social or other secular/human pursuits. So the person goes from his animal (material) nature to his angelic without the transition of social pursuits that most of us experience. Some of the saints may have lived so. But the metaphor is lost.

Alas!

Page 78 – 79: Thoughts about raindrops and clouds and diversity…

If raindrops fall at the rate of 14 mph, how long will they take to fall from a cloud that is 1 mile high? Two miles high? Three and a half miles high?

If a thunderhead in the middle latitudes were 7 miles high, how long would it take for a raindrop to fall from the top to the bottom? (Not that many would do that; most get pulled up again, and those that free fall are likely to evaporate halfway down…)

Besides, a drop it might fall faster part of the time, because the drops in high clouds get larger and then fall faster; but then they break …  it does make you think differently about rain, right? It’s not instantaneous; it has a history.

If you don’t have a project, you can write a story such as: The Story of a Drop: the saga of a drop of rain from a thunderhead to my head.

p. 79: How many differences in clouds

Thomas Henderson says that the whole difficulty of studying clouds is that they’re all so different. No two alike. Shall we have a contest? Shall we offer a prize if anyone can find two clouds alike? Did your Mom ever ask you to look for two blades of grass perfectly alike?

p. 66, 81 How much water up there?

If you have ever wondered how much water is up there, this chapter gives you two chances to figure it out.

One is on page 66 where we learn how many grams of water you could expect to find in saturated air at a certain temperature. Of course saturated air doesn’t mean a cloud; it’s just before the cloud forms. But it can give you an idea.

On page 81, we find an actual amount of water in a “small thunderstorm”  — whatever that is. Presumably it means a single thunderhead as opposed to a super thunderhead which pulls others into itself.

33 million gallons. (Glug!)

Are you surprised? Of course, not all the water available to a thunderstorm falls to the ground, right? 19 million gallons condense into clouds that just blow away afterwards; 4 million gallons fall on the ground, and the rest – out of 33 million gallons — evidently remains as uncondensed vapor, ongoing humidity.

Advertisements

Read Full Post »

When you come to chapter 5 of Jack Williams’ Weather Book, you will find that he starts with a crucial consideration whose importance long eluded me because the mechanism was so very mysterious. It is called phase change. Let me underline it for you:

Phase change, particularly in water, involves enormously more heat (or chill) than any ordinary change of temperature. To wit:

Of course you know that water has to absorb heat in order to raise its temperature (get warmer). Duh.

It takes about one calorie of heat to raise the temperature of one gram of water by one degree in the Celsius scale. Interesting. That’s what a calorie is, then. It’s enough energy to raise one gram of water by one degree, Celsius.

[Our Fahrenheit degrees are smaller. One calorie would raise one gram of water almost two degrees Fahrenheit, 1.8 degrees, actually. So it would raise a half-teaspoon of water one degree Fahrenheit].

But it takes about 80 calories to change ice at 32°F into water at 32° F or, looking at the less familiar Centigrade scale, to change ice at 0°C into water at 0°C. In other words, it takes almost as much energy to make that phase change as to heat the water all the way from there to the boiling point.

And then – get this — it takes about 500 calories of heat to change one gram of water at 212° F into steam or vapor at 212° F; or, returning to the Centigrade scale, it takes 500 calories of heat to change one gram of water at 100°C into steam at 100°C. In other words, water soaks up five times as much energy going from “hot” to “steam” as it does going from ice-cold to simmering hot.

This is just an astonishing thing, but it underlines for you that phase change isn’t just one more degree of temperature. It’s a big change and big amounts of energy (in the form of heat) are involved.

Just to drive the point home, let me turn this around and say one more thing, a direct consequence of what I already have said, but in case you didn’t think of it:

Therefore, going down:

When enough water vapor is cooled to become to a gram’s worth of water droplets, it leaves behind about 500 calories of heat in its former surroundings. And when a gram’s worth of those water droplets settles into ice, another 80 calories of heat are left behind in the old environment.

Therefore, whenever we see a phase change, we need to remember that heat is either being absorbed or being given off in huge quantities.

Clouds and phase change

So, with all that in mind, let’s think about clouds. Moist air rises, all full of water vapor. You are thinking, well, the water vapor isn’t 212°.

How do you know? You can’t measure it, one molecule at a time. It is water vapor, and it is just as energetic as that stuff that comes whistling out of your teapot.

So then it gets up high enough to cool into a cloud, which means high enough to condense into droplets of water, tiny droplets, but still droplets, not vapor. When it gets high enough, it condenses, and each gram’s worth of water means that 500 calories of heat have been abandoned. Where did that heat go? It has to go somewhere; it doesn’t just cease to exist. “Energy is conserved.”

It goes into the air.

And that makes the air warmer.

And that makes the air rise faster.

And that makes the air go higher, where it’s colder and where more water condenses, leaving more heat to in the air, causing the air to rise faster, causing it to go higher…

This is crazy! When is it going to stop?

Well, eventually the air is going to get either so warm (warm air holds more water than cold air), or so dry, that water stops condensing out of it.

And that is the top of the cloud.

Every cloud has a top.  Sometimes the top is temporary and another gust of rising air breaks through the top and goes higher (there’s your cumulus congestus from August 21 or your cumulus mushroomi from August 22) but it doesn’t rise forever and ever. Eventually, there is a top.

Keep these things in mind as you read this chapter, because nothing else will be clear if you don’t have this in mind.

One more thing: why?

Why does it take so much more energy to get the water to jump out of the pot than it takes just to warm it up?

First off, even for you it’s easier to roll over than it is to jump up, right?

But there’s more to it. Do you remember learning about surface tension? About water being little bear heads with positively charged ears and negatively charged chins? And do you remember that all these charged water molecules pull on each other like a bunch of magnets; and at the surface of the water there is a kind of skin that forms so that you can fill a spoonful of water right up over the top without spilling?

That skin has to be broken by each escaping molecule. That’s what’s hard about becoming steam. It’s as if you not only have to jump, but you have to roll over and then jump right up through a tight curtain.

That’s some muscle, eh? 500 calories to launch one gram of water. (A gram is almost a quarter teaspoon.)

What is hard about becoming a liquid instead of ice is similar, though not quite as tough a challenge. Ice is a crystal where each molecule of water has its exact place and is held there by exactly four molecules around it. Well, at the surface of the ice, it’s not held on four sides: on three, or maybe only two. So then it can jump off, but it’s a bit of extra work. That’s all.

It’s easier to sit in a circle and hold hands with your friends than roll away and go somewhere else, but sometimes you just shake off their hands and roll, right? Not too hard.

But not as easy as sitting still.

That’s enough for one day.

Read Full Post »