Archive for October, 2009

Crepuscular rays

In response to the image offered by Thomas, Ana’s students have worked on portraying crepuscular rays. These have the particular beauty of combining the curves of a convection cell with the disciplined lines of the light rays which, being the lines of a cast shadow, are perfectly straight.

Here are a few of the results, also available on her blog today:


Oil pastels after Thomas's image of crepuscular rays

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I’ve been reading a new book — an old book really: From Raindrops to Volcanoes by Duncan Blanchard, first published in 1967 and presently being reprinted by Dover. It’s a kind of personal journal of Blanchard’s scientific adventures going from the study of how raindrops generate bubbles in the sea, to the way that these bubbles break and send tiny droplets into the air, to the possible relationship between these droplets and atmospheric charge, to the question of how undersea volcanoes send charges into the atmosphere.

This is the kind of book that shows you how scientists really think: An idea forms; in the course of checking that idea, other thoughts form and other experiments suggest still further avenues of thought. Along the way, other people have similar questions and sometimes more ingenious exercises to test them, and then, charmingly, a visit to the library unearths the works of men who raised these questions hundreds of years ago and made their own guesses, wise or foolish.

So, when a drop causes a bubble, and the breaking of the bubble causes the forceful ejection of new and incredibly tiny droplets into the air, how fast do you suppose that ejection really starts them off? It has to have some momentum because the air is quite thick compared to these tiny droplets and to get them up above the sea requires considerable force… The large ones, of course, we actually see; they might be traveling as fast as we swing our arms, and that can’t be much more than a few miles an hour, the speed at which we walk and swing our arms, right?

But would you believe 180 miles an hour for the little ones? No way! Please read his book and explain to me what he did wrong; there must be something. I didn’t follow every equation, I confess.

Actually, I am sure he was careful; and even though there are some math parts that will not catch everyone’s attention, he does give amazingly clear explanations of his work; it’s not just answers, but how he got there. He has the heart of a teacher; he wants to share his fascination with the physical world, all full of surprises.

Volcanic lightning from Chaiten Volcano May 2008

Here is an image of volcanic lightning from another source. It’s the Chaiten Volcano in Chili, which erupted May of 2008. Notice that the cloud is black and smoky, not grey like a rain cloud. The picture is from a news outlet.



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USA Today has a weather map that is interesting. It has the fronts, the highs and lows, and the isobars, though it does not label or explain them or give the barometric readings for them. At the top of the map, you will find that you have some other map options, such as satellite and radar views. But the view of fronts is the most helpful for our present purposes.

Of course the Jet Stream map is always interesting.

And the NOAA weather satellite map is more interesting that USA Today’s because you can get an animation over a 24 hour period.

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Chapter 10 of Eric Sloane’s Weather book explains the reading of a weather map. It’s one of those things that’s very jumbled and confusing when you first see it, but if you take the symbols one at a time, you can work it out and find it interesting.

  1. First, you see long curves with spikes on them, like the teeth of a saw. That’s the sharp edge of a cold front coming in, and the teeth point which way it is moving. Remember that a cold front isn’t necessarily “cold” as in freezing. It’s just colder than what’s already in place. It probably stormed along this line.
  2. Second, you see lines with scallops on them; they certainly look gentler than the cold front! This is the warm front, and again, the scallops show which way the air is moving; not as dramatic as the pointy teeth of a cold front, but the same idea. There may be low dull clouds here.
  3. Third, you may also see a line with points one way and scallops the other. Here, the warm and cold fronts have met and can’t go anywhere; they’re stuck for the moment. There may be low clouds and some wind. Sort of a useless wind that doesn’t take the weather anywhere…
  4. Fourth, you may see a line with points and scallops both pointing the same way. Here, a cold front has come up behind the warm front, and both are now moving together. Here there will be wind (like a cold front) and a long rain (like a warm front). It’s called an occluded front.

So that’s for the bold lines on the map. What about the big letter H and the big letter L?

  1. “H’ is for High, of course. It means a high barometer, and that means high pressure, which means dense air and no rain. When the air is dense, the water in it warms and evaporates.
  2. And “L” is for Low. When the barometer is low, the air is thin and water more readily condenses into rain and snow.
  3. Related to these two letters are some light lines that make concentric circles or beany shapes round about them. These lines are a connect-the-dots exercise, and the dots they connect are cities (or weather stations) where the air pressure is the same. As you move across these lines away from an “L”, the air pressure (the barometer) is rising. As you move across these lines outward from an “H”, the air pressure is falling. Simplified weather maps don’t use them, lest people be confused — but you won’t be!

Finally, there may be arrows on your map, and these tell the wind direction. The number of fletches on the arrows tell the strength of the wind.

If you take these items one at a time, you will soon find that the weather map is all sorted out and as readable as anything else written in a language that you know. I notice that many of the weather maps now in use are so careful to use pictures (of rain and snow) for example, that they only give information about “now” and no forecast or sense of direction at all. The announcer makes the forecast, not the map.

I guess they don’t think people can understand the symbols, and most can’t. But once you understand the symbols, there is actually more information in the symbols than in the pictures.

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Weather Doctor

In the course of searching for information on the web, I recently came across this 12-year blog by a weather watcher named Keith Heidorn who calls himself the weather doctor. A writer and painter as well as an observer, he has a wealth of information on his blog. For a moment, I thought I should just bow to him and close down. On second thought, I have my own purposes and will continue writing, but do take a look. It’s very comprehensive, and, as blogs are, composed of short, digestible pieces.

For a pleasant journey into thoughts about weather, go to his home page, then to the site map, and then to whichever article interests you. He has weather history, personal observations, seasons essays, book reviews, definitions, artwork (his paintings for sale as notecards), poetry (his own and others’ material).

For example, the section called weather eyes has a Basic Cloud Atlas, as good a summary as you’ll find. His list of topics in the weather almanac has an account of a the cold wave of 1899: Freezing America: The Cold Wave of 1899; I never heard of it, but now’s as good a time as any to learn. The weather and the arts section has a piece called John Newton: Amazing Grace: Surviving A Storm At Sea; beautiful story; if you don’t know it, you should. Or if you want an interesting typhoon story, try The Largest of Them All: Typhoon Tip. And here’s an intriguing book title: A Grain of Sand — Nature’s Secret Wonder by Dr Gary Greenberg. I was happy to see many of the books I have reviewed on his list, but this was completely unexpected! And if you want to see his artwork, visit:

The Weather Doctor’s Nature Gallery

Have a good day!

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Chapter 8 of Jack Williams’ Weather Book is about thunderstorms and tornadoes. Here are some reflections on the chapter.

Page 115 – 116

This chapter opens with the setting for a series of related tornadoes in 1974, called the Super Outbreak which is well described in various places online, with beautiful pictures.

Williams offers a four-part setup:

First, the jet stream was very strong, 100 mph, and it was traveling from Texas towards the northwest, an unusual route which was set up to carry any weather system pretty fast.

Second, a strong storm was crossing the Rockies and its low pressure center was bound to pull winds together from every possible direction as soon as it reached the plains and could look around for more energy.

Third, the Gulf of Mexico was very warm and the air above it was unusually humid. As you know, humid air rises; then its water condenses and the air rise still more. This intensely, upwardly-mobile air was certainly going to be pulled into the low pressure system.

Fourth, air out of the dry southwest was also racing in, partly under the influence of the jet stream and partly seeking the low-pressure center. (JW says that the low pressure system “pushed” dry air across the Mississippi. Low pressure centers never push; they only pull; you know that. But because of the Coriolis effect, the same low pressure center can pull some winds east and others north and indeed every other direction. Hard to remember this fact.) Anyway, the dry southwest air naturally parked in the upper atmosphere because it was warm, but in doing so, it kept the moist Gulf air from climbing easily to storm heights and dissipating its energy. As a result, the Gulf air became more and more oppressive, more and more energetic, more and more determined to punch through.

When it got enough energy to break through the southwestern dry air, the wet stuff was bound to be violent, and so it was. The total system spawned 127 tornadoes, one of them ½ mile across. I didn’t know a tornado could be that wide, ½ a block is about right. This was the greatest outbreak in recorded history.

JW has some very nice graphics on the storm setup and the tornado tracks.

Page 117

Normally, thunderstorms thrive where the air on the ground is warm and the air aloft is cold; this is called “unstable air”. It favors storms because the warm air rises, the water condenses out; the air rises more, the thunderhead forms and the rain falls. Straightforward. Remember the term unstable air.

Page 118

A graphic on the formation and dissipation of a single-cell thunderstorm dominates this page. It is well worth studying. If you want a second image, go to the National Weather Service’s website and look at their drawing. Note particularly that while a thunderhead has a strong updraft from the rising of warm, moist air, it also develops a strong downdraft when rain forms. The falling rain pulls air down with it, and when this cool package reaches the ground, the rain just soaks in, but the cold air flows out away from the storm, sometimes for many miles. That’s the source of the cool breeze that you feel as a storm draws near. It is the outflowing of a cold downdraft from a storm cell in the distance.

Page 119

The term “dryline” refers to the place where warm wet air meets warm dry air. JW wonders why this is a boundary that triggers storms. The reason seems to be that the dry air (out of the southwest) is already high and it puts a lid on the warm moist air so that it can’t rise easily and make normal, small clouds and storms; so when it finally does punch through, it is violent.

The graphic on page 119 shows how the downdraft of one storm cell be transformed into a bust that goes out to meet a body of rising warm air, plows under it or smashes against it, and causes a strong updraft to form, thereby building a new storm cell.

Page 120

JW lists the conditions for a the worst storms, “killer storms”. This list seems contradictory (for example, it requires stable and unstable air) but the different weather conditions he describes must come together from different directions as he outlined for the Super Outbreak at the start of the chapter. The important new information is that conditions such as a jet stream above a storm cause the updraft to tilt; in this way, they keep the downdraft away from the updraft. Cold downdrafts running into the updraft of a storm cell are the death of storms; separating the two allows the storm to keep going. This is very important to keep in mind as we go on.

Page 121-122

The top of page 121 is a graphic of a squall line. A squall line is a series of thunderstorms several miles apart, lined up along a cold front or often several miles ahead of a cold front. He does not give their distance apart, but I believe it is about 10 miles.

The individual thunderheads in a squall line are higher than usual, about 50,000 feet, which means they have pushed well above the usual limit of the troposphere. Remember that one mile is about 5,000 feet, so this is close to ten miles high; the troposphere is that high only over the equator. This means that pilots cannot fly above this storm but must go around it. Airplane require “air” to fly, and at this altitude there is not enough.


In almost a casual manner, the text now introduces an important new concept: the downdraft. You already know that when warm, humid air rises, the water vapor condenses and releases new heat into the atmosphere, causing the air to rise yet faster. But now we learn that when cold wet air (full of small droplets) falls into dry air, the droplets evaporate, causing the cold air to get colder and therefore denser, and making it fall yet faster.

Page 123

This graphic about tornados is more statistical then meteorological, but look it over and see what you find interesting. Note: he shows the direction of the tornado’s spin, so look at it. Is it clockwise or counter-clockwise? Why? If you don’t know, ask Corey.

Page 124-125

Here is a two-page diagram on the mesocyclone, the mid-size storm system, larger than a thunderstorm, smaller than a hurricane. Look over this diagram (or the one on the National Weather Service page given above) and then go to the storm chaser’s log and look at the pictures. This brings the diagram to life in a most unexpected way. How beautiful; how awesome!

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Some answers

For the chapter seven questions, here are some answers:

How does rain become “freezing rain”?

Water packs more closely (contracts) as it approaches close to the freezing point, but the molecules must jump away from each other to form an ice lattice, an ice crystal. For this reason, it takes a little energy for water to freeze, and a gently falling rain is sometimes unable to find this energy. But at the moment of impact, it is jolted, and as it flows around the object, the crystals form, coating whatever object it has touched.

This is a good time of year to watch for an opportunity to see the beautiful effect of supercooled droplets turning to ice. If you leave a car outside, and if water collects on the windshield and sits there very quietly while the night cools down, the drops may be supercooled by morning. Then when you tap the windshield, the water crystallizes right beside your finger and this disturbance crosses the window in a few seconds – crystal formations going out from their starting point by your finger.

Where does a western storm get new humidity for continued life?

As storms cross the Rockies, they need moist air from the Gulf to renew their strength; as they cross the Appalachians, they may get a new supply of water from an Atlantic storm.

Winter storms out of the west reach the east coast and then northern winds (A) send these storms southward to meet new storms from the southerly Atlantic and make them worse for a while; then the northern winds sweep it all away and bring clear skies.

Freezing drizzle on planes:

Marcia Politovitch and Ben Bernstein were able to calculate the exact conditions of humidity, pressure and temperature which indicated imminent danger of riming on the wings of an airplane.

Donner Snow

During the winter of 1889-1890, 65 feet of snow fell on Donner Pass. Such a quantity of snow would hardly be manageable even today.

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It has been my dream of many years that home school support classes — or any educational community, for that matter — would seek to support the family by teaching a single topic in science, and a single period in history each semester, through all levels of the school. Certainly when we think of something like the life of Columbus or the science of weather, we can easily find resources for every grade level. How much easier it would be for the family to participate in the education of children if they had only one history and one science topic at a time, rather than different ones for each child! Is there really any reason for teaching astronomy in fifth grade and earth science in sixth instead of the other way round? Does American history have to be in fourth grade and world history in sixth? Not at all!

There is one difficulty, of course: it is demanding for the teacher to cover new topics year after year for a sequence of perhaps four or six years. But some teachers would welcome the variety anyway, and the relationships between the periods in history and the disciplines of science naturally emerge from such an arrangement where everyone is involved.

In any case, my own support group in South Dakota has generously engaged this challenge by having the art classes teach weather-related drawing while I teach meteorology. Here are some weather drawings from Ana’s class:


They were reading Sloane’s weather books,including one that specifically teaches the drawing of the sky and clouds. See now much motion they were able to put into their clouds and water?

Jannell also chose a weather theme for a print-making project with her younger groups of students.


More information about Jannell’s techniques can be found on her blog.

Brother Petroc’s Academy is the name of the school I imagine when I think of how schools might be. It is a great dream!

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JW Weather book: Chapter 7

The most important part of this chapter, from the perspective of understanding meteorology, is from pages 102-106, with a dubious diagram on page 106, about which I have already written. The rest includes a lot about snow removal and road maintenance, including an interesting historical sidelight on the Donner Pass. The one thing of scientific (not just practical) interest in these latter sections is the enormous difficulty of predicting anything with clarity. This is instructive – the weather is just incredibly complicated. There’s also a sidebar on the Knights, researchers on cloud ice, rain, and hail formation. Again – all this is much harder to understand than it first seems to be.

For the first few pages, I have some review questions for you instead of a summary. For the later pages, just a summary.

Page 102

How does rain become “freezing rain”?

Where does a western storm get new humidity for continued life?

Page 103 (continues a chart from page 102)

Sleet is frozen rain or partially frozen rain. Unlike graupel, it is transparent.

How does sleet differ from freezing rain?

Can you see why graupel is not transparent, but only translucent?

Winter storms reach the east coast and northern winds do which two things:

A        1) Northerly winds send winter storms meet new storms from the southerly Atlantic and make them worse.

2) Then sweep it all away and bring clear skies

B       1)  As winter storms reach the coast, northern winds strengthen them

2) Then those same winds blow them south to the Carolinas

C       1) As winter storms drop snow in the mountains, northern winds slacken

2) Then the weakened northern winds enter the southern storms and turn them clockwise.

Page 104

What is the problem with flying in freezing drizzle? Why would this be worse than snow or rain?

What three conditions make freezing drizzle at high altitudes? (Note: all weather is just the consequence of three things: temperature, humidity, and barometer…)

Page 105

What was the great accomplishment of Marcia Politovitch and Ben Bernstein?

Page 106

See post from October 19 on snowflake formation.

True or false: Snowflakes are always rimed.

True or false: Graupel is heavily rimed snowflakes.

Page 107-108

Mostly about the effect of ice on roads, this section is not really about weather; the salting of roads has substantial economical and environmental consequences.

Page 109

Here’s an interesting graphic about how sensors are imbedded in roads to inform highway personnel of the needs for snow or ice removal. Not really about weather; this is a public weather forecaster’s book, remember.

Page 110 -111

The Donner Pass is important in the history of western settlement in the US. It’s a very sad story; Williams reminds us of the story and says that the Donner Pass can still be treacherous.

No special weather insights here except to be careful of snow in mountain passes – and be astonished by the amount of snow that can fall in the right circumstances. How much snow fell on Donner Pass in the winter of 1889-1890?

Page 112

Interesting story about two researchers, Charles and Nancy Knight, who study cloud and rain formation. Makes you realize how hard it is to understand what is really going on. There’s a lot that we don’t know!

Page 113

This continues the discussion of road salting that began on page 109. Road salting decisions, again, are important to weather forecasters who are really in the business of forecasting the weather-related environment for people’s use. They are not actually so much about the causes of weather itself.

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In his chapter on snow and ice, Jack Williams has a diagram of snowflake formation. I think that his diagram is misleading and reflects a little confusion. The specific problem is the role of supercooled water droplets, which settle down on various surfaces as rime. Rime, however, is not essentially involved in the making of snow. Let me explain this amazing process, using numbers that correspond to Jack Williams’ diagram “How a snowflake is born and grows,” found on page 100 of my book. My principal resource is the work of Ken Libbrecht, surely the expert on this topic.

  1. It is correct that snow begins, often, with tiny cloud droplets. Tiny. Microscopic. So small that when they turn crystalline, they have facets either at once or very quickly. Sometimes, as I understand it, snow begins when water vapor builds directly onto a dust particle, and there never is a droplet. Sometimes a small crystal melts and refreezes and the building process goes forward.
  2. The further growth of cloud droplets is not particularly involved in snow-making for the simple reason that larger water droplets freeze as pellets, not as hexagonal crystals, and therefore a large size could actually inhibit the growth of a six-sided crystal.
  3. As you know from our discussion of phase change, cooled water does not automatically crystallize at 32°F (or 0°C), partly because water has to give up a lot of energy to freeze and partly because the water molecules which pack more closely as they cool down to the freezing point have to move apart again to make the lattice of an ice crystal. Therefore, water can drop to a temperature well below its natural freezing point of 32°F or 0°C.  Such “supercooled water” freezes when disturbed. Supercooled water droplets may freeze as pellets, however, not in crystal form.
  4. One way or another, snow begins with crystals in the form of hexagonal plates, and these plates grow at the corners, making the branched crystal we know so well. Williams says that this happens if the temperature is about 5°F and if the cloud has a high saturation of water. Ken Libbrecht says that the process begins at about 10°F. A cloud is always saturated; that is why the vapor turns to water droplets; but it can become super-saturated when vapor is cold enough to condense but remains vapor for lack of a condensation nucleus. Not every cloud droplet becomes a snowflake; read on.
  5. In step five, Williams’ confusion comes closer to the surface. In fact, snow crystals grow because water vapor settles onto the growing crystal – this water is added to the crystal, as for any crystal growth, molecule by molecule. It may also happen that supercooled cloud droplets become attached to the snow crystal at this point, and they are called rime; but rime is not an essential part of “snowflake” growth any more than sleet is an essential part of snow. It’s something that happens sometimes, and, in small amounts, it may be decorative, but it is never part of the 6-sided symmetry of a snow crystal.

Here is a snow crystal from the wonderful images of Ken Libbrecht at snowcrystals.com. The tiny irregular dot in the lower right sector is the clearest example of rime. That’s how small it is, and how totally not part of the 6-sided symmetry.

LIbbrecht snowflake with rime

LIbbrecht snowflake with rime

6.  Riming continues, says Williams in step seven of his diagram. Since the topic of this sidebar is snowflake growth, riming, the settling and sticking of supercooled water droplets, is simply off topic. The way that a snowflake grows is that water vapor – that is, H2O gas — condenses onto the crystal, molecule by molecule, in precisely the orientation that the electro-magnetic nature of water allows or even demands at a particular temperature. At some temperatures, the growth is needle-like branching off the corners; at other temperatures growth is all around the outer rim, making plate-like portions. The particular history of each snow crystal’s travel through cloud areas of different humidity and temperature is thus written into the shape of the crystal. And each journey is unique!

As you know, condensation leaves the air warmer, and what with one thing and another, nearby cloud droplets warm sufficiently to evaporate and this new supply of vapor continues to build the snowflake. So the other droplets in the cloud do contribute to the snow crystal, but not by riming, rather by evaporating and depositing. It’s just amazing how much physics is sitting here; and it was very tough to discover!

7.  At the seventh point of his diagram, Williams recognizes that continued riming will form graupel, not a snowflake, but he imagines that this will happen “if the cloud is thick enough” – suggesting that he may not be aware of the evaporation-condensation cycle I just described. A thick cloud can give you perfectly good snowflakes, not just graupel! I got my information from Ken Libbrecht’s website, particularly from a paper (of his) that is linked there.  I won’t pretend I read the whole thing; I did not. Much of it was beyond me. But I read enough to get this part clear.

8.  Now Williams says that when the flake falls out of the cloud, it continues growing “for a while”. The delicacy of a snow crystal is so great that it is very subject to evaporation, and once it leaves the saturated atmosphere of a cloud, I’m guessing it’s just lucky to make it to the ground. Depends on the humidity, of course, but the point is precisely that the relative humidity is much lower outside the cloud. One of the reasons snow is so different the day after it falls is that the dendritic (branched) crystals just don’t survive. You don’t go out the day after a snowfall and find the stuff that people photograph for that lovely symmetry. It’s not there. I doubt that snow crystals grow outside their clouds, and I don’t trust Williams enough to take his word for it.

9.  In step nine, Williams states (correctly) that if the individual crystals fall into warmer air, they may stick together. This is the way we sometimes get enormous flakes, ½ inch in diameter, or even more. Technically, the word “snowflake” refers to these clumps of crystals, while the correct word for single crystals is “snow crystals.” But of course we call them snowflakes, and that’s fine.

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