Archive for November, 2009

Just for fun!

The Latin students among you will readily recognize what sort of ideas are expressed by all these syllables, which are very clear and definite.

Stratocumulus perlucidus below Cirrus intortus

Upper level cirrus in various tangles with a large sheet of cumulus that still allows the light to come around and through.

Stratocumulus means a sheet of lumpy clouds. Stratus is sheet; cumulus is lumpy or humpy. Perlucidus means the light goes around it — the sheet does not cover the sky. In fact, convection in the center of the sheet makes it high there while the edges are not growing.

Above the cumulus — in the upper section of the photo, but also thousands of feet higher in the sky, the cirrus, the wispy clouds, are tangled in various ways. Having words for it makes it easier to remember what you saw and how it was laid out. Figuring what weather is implied is a different exercise, but The Weather Identification Handbook gives some basic clues.

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One of the things I had hoped to accomplish during this semester was to add to my own ability to recognize what I see in the sky.  I have added some vocabulary, but one book which has delighted me, and which  wish I had started out with, and which is my second nomination for the book to use as a spine for a weather course, is The Weather Identification Handbook by Storm Dunlop. (How did he ever get that name?) It is a beautiful and clear list of cloud types and cloud names — ten genera of clouds with fourteen species and nine varieties, also three accessory forms and then six more particular forms. These, with beautiful color illustrations, are the backbone of the book.

Genera (plural of genus), species, and varieties are not altogether like their biological counterparts, however. A particular species, such as castellanus (which means turret-forming) may appear within several different genera, so that you have stratocumulus castellanus (Sc cas), altocumulus castellanus (Ac cas), cirrocumulus castellanus (Cc cas), and cirrus castellanus (Ci cas). Or, one of the accessory clouds is called pileus, which means having the form of a hood or cap above a rising air mass. Cumulus (Cu pil) and cumulonimbus (Cb pil) clouds may each have pileus forms. In other words, a species is not a specific form within a single genera; rather it is a type of form which may appear as a modification within one or more of the genera.

Another important point about cloud names is that they are, like Linnaeus’ original biological classification, based on appearance, not on origin or function. And just as, over time, biologists realized that certain similar forms were less related than other very dissimilar forms, meteorologists have realized that similar cloud forms and names do not always imply a similar source or similar weather. Names, then, are just the beginning, important because you don’t easily remember — indeed sometimes you barely notice — the things for which you have no name. It would be a fun challenge to see how many cloud types you could identify and photograph within a semester, or even within a month. But a course on weather needs to be more.

The Weather Identification Handbook also has descriptions of the major light displays, precipitation types, and some of the identifying effects of wind speed. However, because the emphasis is on weather identification, the book is not systematic in its presentation of these other things, and there is no discussion of more fundamental things belonging to climate, the larger reality in which weather is set, such as Hadley cells (you can’t see them) or the Coriolis effect. An understanding of weather requires that those topics be covered, and other sources would be necessary. Nevertheless, the principal value of Storm Dunlop’s book is that it would encourage students to look at the sky. If you look, you get curious; if you get curious you learn; if you learn, you notice much more and remember much better. It’s a beautiful little book.


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Just a quick note here, but I have been skimming The Cloudspotter’s Guide by Gavin Pretor-Pinney, and am about to nominate this as the book I most wish I’d used for my course. It’s written with a happy mix of science, observation, and reflection. Unfortunately, the photos are black and white, but you can always look up to get your fill of color.

And, yes, there really is a Cloud Appreciation Society, and of course they maintain a website, which has, without any competition, the most extraordinary cloud photos I have ever seen.

Did you know that there is a small town in Australia which is home to a unique cloud formation called the Morning Glory? It is a kind of cloud roll which forms in the spring and which is the very Mecca of gliders because you can ride its wave as surfers ride the waves of the sea. Take a look!

There is also a wonderful gallery of photos of all the principal cloud types, and as these are sent in by members from all over the world, they are breathtaking in unexpected ways. Various kinds of displays within clouds are also illustrated with extraordinarily beautiful hotos. Here’s a photo of some fallstreaks, cloud holes that are caused by the passing of an airplane whose exhaust has seeded a portion of the cloud and caused its droplets to fall out.

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Iridescent clouds

Iridescent clouds, which often appear close to the sun, have a more subtle color scheme than rainbows, but it’s very beautiful and worth watching for. These colors usually appear in alto-cumulus clouds near the sun, and the clouds need to be young because the physics of iridescence requires very uniform drop sizes. The longer clouds hang around, the more their drops interact and the larger the variety of drop sizes.

Here’s an image from November 2:


Iridescent clouds hang out near the sun and have the subtle colors of subtraction.

Iridescent clouds hang out near the sun and have the subtle colors of subtraction.

You can see that the sun is at the upper right. I think if you click on the image, you will get stronger colors, but they are still not pure like a rainbow.

They are called colors of subtraction because they are formed when white light enters into some situation that causes one set of wavelengths to cancel itself. In a rainbow or prism, you see red because of a single group of wavelengths. But the pink in iridescence is due to the loss of a single group of wavelengths — presumably the green ones. But it has a very different quality; it’s not just like the red of a rainbow only lighter. It’s a different quality of color altogether.

Here’s an oil slick from the parking lot of the main library. The colors are much stronger, but can you see that they are the iridescent family colors, not the rainbow family? In the lower left of the picture, the colors are particularly strong, but they are still these brassy, metallic colors, not the more pure and serene hues of the rainbow.


Oil slick showing the beautiful colors of subtraction

Oil slick showing the beautiful colors of subtraction




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I have sometimes mentioned a question about the naming of the horse latitudes. Here is a little research on that topic, but first, let’s review the major wind cells of the world:

Hadley cells

We have spoken of the Hadley cells, in which air rises at (or near) the equator, rides outwards (north or south) about 30° and then falls to the ground and slides back towards the equator to rise again. This motion completely encircles the Earth with two rings of moving air, and because of the Coriolis effect, these are actually two tipped slinkies of moving air:

The northern Hadley cell moves low-lying air southwest until the heat at the equator lifts it up; then the same cell moves it northeast until it sinks again near the Tropic of Cancer.

The southern Hadley moves low-lying air northwest towards the equator where it rises and then moves southeast until it sinks again not far from the Tropic of Capricorn.

Polar cells

Another pair of slinkies governs the motions of air near the poles, where it shrinks and sinks into the deep cold, slides away from the pole, warms and rises, and is then pulled polewards to fill in where the air is sinking.

If you think about it, you can see that the meeting of Hadleys and Polars would be a battleground: the northern reach of the northern Hadley is sinking while the southern reach of the northern Polar is rising. So will the air sink or rise at the meeting?

The gear cells – Ferrel cells

To compose this difficulty, there is a gear cell, a third pair of slinkies, a northern Ferrel cell and a southern one. The northern Ferrel brings air from the high cold around latitude 60° and drops down when it meets the Hadleys dropping down; then it races away along the ground towards the Polar cells until they force it upwards again. Rather than confuse you with the mirror description of the southern Ferrel, let me show you one of the nicest illustration of these cells, one printed without attribution in several places on the internet. Note that only the ground movement — the bottom part of each slinky — is shown on the globe. You have to look at the side bubbles to see the upper circulation. Thanks to whoever did it:

Major circulation cells of the world

Die Rossbreiten

Now let’s talk about the horse latitudes.

At the meeting of the Hadley and the Ferrell cells is an area where air is descending both from the north and from the south. Descending air cannot form clouds, as you can well understand knowing that clouds are convection cells of rising air. No clouds, no rain. No rain, desert. And if you are over the water, you won’t have a desert, but you won’t have storms, you won’t have wind, and you won’t be sailing very well. These latitudes are called the doldrums or the horse latitudes.

Horse latitudes?

Why? Because the ship won’t gallop along?

Most sources say that these terrible calms are named for the horses who were thrown overboard because they were dying of thirst when the voyage got too long. A grim title!

Guy Murchie suggests that they are named for English explorers John Ross and his nephew James. German maps call these latitudes the Rossbreiten, which could equally mean horse latitudes or Ross latitudes.

It would be an odd choice of honors for the Ross family, because these men were arctic explorers and are not famous for doing anything near the tropics. In fact, on his second arctic trip, John Ross’s ship got hemmed in by ice in Baffin Bay and he was stuck there for four years before he was able to get back to England. He made a lot of interesting discoveries, and his nephew, who was along, discovered the magnetic north pole (which is not the same as the geographic north pole).

Still, they were sailors, and were famous and interesting, and perhaps they deserve to have something named in their honor. I suppose the only way to figure out whether the original name was horse latitudes or Ross latitudes would be to see whether the phrase “horse latitudes” is found in English before the Rossbreiten in German, or only afterwards.

If you want to read about John Ross, here’s a link. It’s a Canadian biography site; he was born in Scotland and served in the English navy, but he explored Canada; that’s where Baffin Bay is. He wasn’t, perhaps, a perfect person, but he must have been a perfectly fascinating guest.  That’s what London thought, anyway.

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Scattering of short wavelengths

Much confusion has attended the business of why the atmosphere scatters blue and purple wavelengths more than red or yellow ones. Yes, it is true that oxygen has a blue color if you condense it into liquid form, but that is not why the sky is blue. Yes, it is true that water has a blue color because its bonds absorb red wavelengths; no, that is not why the sky is blue, though it may make a slight contribution.

The principal reason for the blueness of the sky is that the shorter wavelengths of light are deflected more than the longer wavelengths, and this concept is perfectly simple and obvious when properly proposed.  Here is a simple diagram in which red waves, which have a longer wave form, are contrasted with blue waves which have a short distance between their more frequent peaks and valleys. We say blue light has a shorter wavelength and a higher frequency than red light.

The more wavy the line, the more chance it will intersect with the dots.

The more wavy the line, the more chance it will intersect with the dots.

Look at how often the red lines intersect with the dots. They may not intersect at all! But the blue lines are so wavy it is difficult to find even one position in which there is no intersection.

The dots represent air molecules, which are, in fact, even smaller and more separated than the image suggests. The red lines represent red light waves, which are so long they seem almost straight. If they get through the air without meeting a molecule, they will come straight from the sun to the eye and we will see the red sun, just where it is in the sky.

But the blue lines are just never going to make it, and when they meet a molecule, they are not going to continue their journey as drawn, but are going to be deflected. They are then likely to meet another air molecule and go off some other direction. And then another. And another. By the time one blue photon comes to the eye, there’s no telling which part of the sky it’s going to come from — wherever it last met a molecule before that final deflection into our eyes. As a result, we have the impression of blue light coming from every direction in the firmament, while red and yellow light come from the sun.

I have diagrammed this second situation for you, with a very flattened sun on the left and a very large and flattened blue eye receiving its light on the right.

Blue light arrives from all over the sky, while red and other long wavelengths come directly from the sun.

Blue light arrives from all over the sky, while red and other long wavelengths come directly from the sun.

Dotted straight blue lines show where the eye (or rather the person behind the eye) supposes that the blue light has come from — two entirely different directions. But the red light obviously comes from the sun. If you click on the image, you can see it better; I have emphasized the  air molecules that get hit and how the wave then bounces away from that molecule; it cannot push past. Also, I have made one blue path winding upwards and around, and one purple path dancing downwards to help you distinguish them.

Of course thousands of different wavelengths of light come from the sun, and many of the red and yellow waves get bounced and perhaps a few blue and purple ones get through. What about green light? Some of these waves get through and some get bounced, making skylight a very characteristic soft blue with a surprising amount of green. But the principal point is this: the sun generally appears red or yellow because we see its longer light waves as coming directly from its place in the sky while its shorter wavelengths are deflected so many times that they finally arrive to our eyes from every different direction in the firmament .

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This is a way of understanding the density of gases, and why Avogadro’s number is the same for large or small molecules, and also some other things about molecules in a gas. First of all, I would like you to see how large Avogadro’s number is, and what proportions it implies about molecules and the space they inhabit in a gas.

6.02 x 1023 is Avogadro’s #

There are 6.02 x 1023 molecules of gas in 22.4 liters of gas at STP (which means Standard Temperature and Pressure, which means 0º Celsius or 32º Fahrenheit and “one atmosphere” which means the air pressure at sea level.)

So, how many molecules in one liter of gas?

Avogadro’s number means as 602 x 1021 molecules of gas per 22.4 liters at STP. (I just moved the decimal and subtracted from the exponent.)

And that means there are 26.875 x 1021 molecules of gas per liter. (Divide 602 by 22.4.)

Or to put it another way, there are 26,875 x 1018 molecules of gas per liter. (Again, I just moved the decimal — three places forward — and subtracted  3 from the exponent.)

What would that look like?

Suppose, just to visualize these things, we make an image that is 8 orders of magnitude larger, putting it into the realm of things we can see. (Hereinafter,  “orders of magnitude,” which increase by powers of ten, will be called ooms.)

Eight ooms up from an atom is the size of a large bug; let’s say a bee. (I know I’ve been saying molecule; but let’s talk about atoms for a while since most molecules in the atmosphere are just two or three atoms and therefore lie in the same size range as atoms. I’ll bring the molecule back at the end.)

Eight ooms up from one liter is the Earth.

So it’s like 26,875  x 1018 bees in the volume of the Earth.

What does that look like?

Well, it’s like 26,875  x 1015 bees in a California-width cube … Or it’s 26,875  x 1012 bees in Ireland up to the lower edge of the aurora. (Yes, auroras are rarely seen in Ireland, but I just mean 60 miles up, so we have an Ireland-size cube.)

And that is like 26,875  x 109 bees in a city such as Sioux Falls from the quarries up to the cirrus clouds.

That, in turn is like 26,875  x 106 bees in your half mile or one km neighborhood — but also the same height which is really higher than bees ever fly; hang in a minute to see if that’s really so many…

It would be like 26,875  x 103 bees in pair of football fields surrounded by sequoia trees — but bees don’t really fly that high either, so let’s make it just a tenth as high, surrounded by maple trees for example, and it’s just 26,875  x 102 bees flying around a couple of football fields within one 10 meters of the ground.

Now, a tenth of that (26,875  x 10) would be the same density of bees within 1 meter of the ground, which is their more usual flying zone, and a tenth of that (26,875) would be that density of bees at about the height of the clover which is where they get their honey.

Consider 26,875 bees in the clover

Now, I used football fields, whose length is 120 yards and width is 53 yards, so a pair of them have a length of 120 yards and a width of 106 yards. That means (120 x 106) there are 12,720 square yards of field, with 26,875 bees foraging in a football field? What does that look like?

It’s about two bees per square yard. I suppose if you had 2 or 3 bees on your table, it would make you nervous, but it’s really not so many, is it? In fact, if it were a little plot of clover and grass that large, 2 bees would hardly be noticeable until they started flying. That’s the density of bees in the football field. I don’t suppose you’d want to play football, but just to take a walk wouldn’t be so bad.

Bees and hummingbirds

Finally, let us imagine that our football field has a mixture of bees and hummingbirds (the hummingbirds represent the larger molecules, of course) in just the same numbers. It might even have a few robins (representing very large gas molecules.) The bees fly very fast and the hummingbirds, who fly a lot slower, are bigger, of course, so they do bump into stuff just as often as the bees. If you became a bee yourself and flew into the field blind, you would be about as likely to be struck by a bee as by a hummingbird, because the one is larger and the other is faster.

Anyway, the sum of it all is the peculiar fact that the vapor pressure of a gas (the power of molecules bumping) at STP (cold air by the ocean near Greenland) is about the same whether the gas is composed of big molecules or little ones. Unlike marbles in a bag, all the molecules really have a lot of room, just like the bees and the hummingbirds, and while the big ones bump heavier, the little ones bump with more zing, but if they weren’t moving, they would have a ridiculous amount of space.


I must admit, at the end of all this, that gas molecules move a few hundred times faster than bees and hummingbirds; so they bump all the time, but that’s another story.

…In fact… (ahem!) They all bump so much that if you gave a message to one of them, and if it were passed each time one bumped another, it would cross our paired fields in less than one second unless they were all hummingbirds.

That’s what happens when you shout.

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Why does humid air rise?

How can the addition of water vapor make it lighter? When I take a dishrag and wet it, it gets heavier; how can I take a parcel of air, add water, and thereby make it lighter? This is all nonsense!

(I suppose this lesson should have come before the one about phase change, but better late than never.)

Yes, if you add liquid water to a parcel of air it will be heavier, but humidity is not about liquid water; it is about water vapor. We confuse the two because a fog is obviously heavy and since the droplets in fog are so small as to be invisible, we think of them as vapor. This is an understandable mistake; we can’t see fog droplets and we understand that the air is saturated in a fog, so we think that the fog is the vapor. Besides, in poetry, fog is called vapor.

But for scientific purposes, (and in sci-speak) vapor has a specific meaning; and fog is not vapor.

Fog and cloud droplets, for all that they are very small, are enormous compared to a molecule of water – and true water vapor means water floating in molecular form. Fog forms when saturated air cools and some of the water condenses. The remaining air is still saturated, and would be so even if you took away all the water droplets. The saturation is about vapor, not about droplets, however tiny.

How big is a water molecule?

Can a water molecule really be much smaller than a cloud droplet?

If the water droplets of an evaporating fog are a few 1/100ths of a millimeter wide, they are still a good five orders of magnitude larger than a molecule of water which is a few Ångstroms wide. Five orders of magnitude… That means like the difference between a softball and a city; the difference between a single grain of dandelion pollen and your kitchen table; between a beetle and a freight train, between a kitten and a thunderhead. The difference is simply enormous.

Look closely at the steam coming out of your teapot. Notice that right by the spout, the steam is invisible. An inch or two away, it is white. Right out of the spout, it is vapor; a short distance away, it has condensed into tiny droplets.


Now that we have an idea of what we mean by humid air – air full of water vapor – we can say something else which we learned from the research of a fellow named Avogadro and those who followed his lead; it is this:

Every parcel of gas, every volume of gas, holds the exact same number of molecules, no matter what kind of molecules they are. Well, that’s not quite right: of course a gas that is hotter expands and has fewer molecules in a specific volume; or a gas can have a wrapping around it such as a balloon and be squashed so more molecules fit into a specific volume. But if we have an imaginary box of gas of any precise size, then every other box of that size that has the same temperature and the same pressure has the same number of molecules, and it doesn’t matter whether the molecules are water, oxygen, nitrogen, carbon dioxide, or even much larger molecules such as vanillin or cinnamaldehyde. It will always be the same total number of molecules if the box size, the temperature, and the pressure are the same.

We don’t imagine this would be true, because we think of atoms and molecules as balls of different sizes, and we would certainly not get as many beach balls as we would marbles into any box. But the point about gases is that they are balls in motion, all banging against each other and bouncing away, and – this is the crucial point – the little ones move faster. Of course they move faster. Wouldn’t you move faster if a bear bumped into you as opposed to a rabbit or a fly? Because the little ones move faster, they effectively take up as much room as the big ones that move more slowly, and the sum of it is that a given volume of gas always has the same number of molecules. Some may be little speeding molecules that would condense into something quite small; others may be big galumphing ones that would condense into something moderately large. But if you count them, the number is the same.

And if you have 22.4 liters of cold gas down by the sea, you have precisely 6.02 x 1023 molecules in your box. That’s a famous number, called Avogadro’s number, and you can ask your chemistry teacher why they chose 22.4 liters instead of something more obvious. (There is always an interesting reason for such things.)

But always having the same number of molecules means that some gases are heavier than others. The heavy gases don’t take up any more room; they are just heavier. It follows that they are more gravity-challenged; they fall while others rise.

Saturated air rises:

So let us get back to the air with its water vapor. When water vapor gets mixed into the air, the oxygen and nitrogen have to move over to make room because only 6,02 x 1023 bits can fit into the box; ultimately, when you are out-of-doors, the gases move up, because all the space around is already filled with oxygen, nitrogen, argon, and the other components of air, and “up” is the only place where there is more room. But the water molecule is actually lighter than the oxygen molecule and also lighter than the nitrogen molecule.

Some airy weights:

Basically, you get the weight of a molecule by counting the protons and neutrons in its atoms:

That makes O2 have a weight of 16 + 16 = 32

It makes N2 have a weight of 14 + 14 = 28

Carbon dioxide (CO2) has a weight of 12 + 16 + 16 = 44

Argon is an atom that has a weight close to 40

Finally, water H2O has have a weight of 1 + 1 + 16 = 18

See how light the water is? It is amazing that it doesn’t float away altogether and become lost in space.

Anyway, you can see that a box of gas that is part water vapor will be lighter than one that is only oxygen and nitrogen. So it will rise.

In conclusion

When people talk about dry air soaking up water like a sponge, then, it’s not really like a sponge that gets heavier as it soaks up water. It’s more like potting soil that gets lighter (per cubic foot) as you add Styrofoam (or whatever that white stuff is).  So saturated air – air holding all the water vapor it can – naturally rises.

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