Low Pressure

So I’m leaving for my daughter’s giving birth. We thought it was imminent, and all’s been quiet, so I just thought I’d check the weather forecast for New York.

Weather.com for New York: some sun, some rain — oh, here it is! The ten-day forecast says that Wednesday October 7 has a 60% chance of showers and Thursday October 8 has 70% chance of being a bad day for aches and pains. As you know, this means that they are predicting a low barometer. I don’t understand why the aches come after the showers instead of before, but it’s the best I can do for a low-pressure forecast.

So what?

Well, any midwife will tell you that when the weather is “good” for an extended period of time, all the pregnant women start running late, and then if a big storm comes in, they all go into labor at once.  This isn’t exactly a “big storm” but it’s the biggest one in sight. Stay in touch, and pray for pregnant women everywhere.

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Anticrepuscular rays

Ruth shares her discovery of a wonderful APOD image of anticrepuscular rays. A brief description is given at the APOD site, and also here on sept 12. It’s a wonderful image. Thanks, Ruth.

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Roll Cloud

Last week, as the cold and turbulent air was roiling around, I took this picture of a roll cloud. A mere photo doesn’t do the sky justice, because the amazing thing about this type of cloud bank is that it generally comes in from the west as a white bank of clouds across an entire quarter to half of the horizon, and it usually comes into a fairly clear sky.

Last week, however, the skies were never fully clear, and besides that, my camera couldn’t handle enough of the horizon to make it clear how far it stretched. Anyway, for what it’s worth, here it is:

Roll cloud from the south into South Dakota

Strangest of all, and invisible in a photo, it came in from the south. This type of white cloud bank, a roll cloud, is a classic cold front. How could we possibly get a cold front in from the south, never mind it was already quite cool?

The answer is to be found in the jet stream animation for September 25. There was a certain point when the little wind creature — we’ll call him the flying squirrel — had a paw reaching directly up into eastern South Dakota, bringing cold air from that curious gyre that formed in the middle plains between September 20 and September 25 or so. It has since flown away, replaced by a regular leviathan of wind that stood directly over our state for a few days, while the wind was quite strong, cool, and turbulent. It had to be 30 mph, judging by the size of the branches that were swaying in the wind.

Then, quite suddenly during the night, the leviathan arched its back into Canada and left our skies clear and our air gentle. I don’t know if we have so much as a whisper of wind.

To build an intersting animation of the jet stream, go to the blogroll at the side, and click on the jet stream. Find the flying squirrel and the leviathan for yourself.

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Links restored

The links to the jet stream and to a site that gives satellite images of what amounts to cloud cover have been restored to the site. They are under blogroll at the side, and make it easy to check on the jet stream at any time. Just click on the link.

The satellite images are actually infra-red images, but the clouds come out strongly in such imagery because of their reflectivity. As a result, you get better cloud images than the visible light imagery which goes dark when the sun sets. You can get a single still image or up to a 24-hour loop.

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The Man Who Rode the Thunder

Sam kindly lent me his library copy of William Rankin’s amazing account of ejecting from a jet into a thunderstorm. It is just as amazing as the title suggests, and it is a true story. Chapters 11-14 are the account of the ejection, the free fall, the thunderstorm, and his rescue. Here’s a bit about thunder:

Throughout the time I spent in the storm, the booming claps of thunder were not auditory sensations; they were unbearable physical experiences — every bone and muscle responded quiveringly to the crash. I didn’t hear the thunder, I felt it…

After each flash of lightning, everything turned completely black. I was lost in a pool of ink. During the intense brilliant light, when bolts shot by, the clouds seemed to boil around me, sending up huge vaporous balls of grayish cotton. Even when I kept my eyes closed the lightning had a blinding effect.

It’s an interesting description of the effect of lightning within a cloud. That’s something we don’t think about — we know that the heat of a bolt will rip along a tree trunk, and we know that many bolts of lightning go from cloud to cloud, but we don’t consider that within the cloud also, such a blast of heat must make the vapors boil.

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Assignments #4

Assignments:

First of all, notice the weather. Record your observations for us to discuss.

Then, because the more you know the more you notice, study the next chapter(s) in your books.

Eric Sloane’s Weather Book ch 4-5

These two chapters are so short, you can read both. Since you have 14 chapters, we’ll be doubling several times.

Chapter 4

Sloane compares isobars — lines going along the spatial geography of similar barometric pressure — to elevation lines on a topographic map. Since you probably haven’t seen topo maps, that may not be a completely simple idea, but I think he explains it well. After you read the chapter, find a newspaper or other printed weather source and find the isobars. Bring a copy to class so we can look together at the maps and be sure everyone has the idea clear.

Chapter 5

Sloane describes the Coriolis effect. Be sure to study the cartoons — notice that he divides his drawings between the folktales and some unexpected observations. Sloane’s cartoons have lots of information. What does he say about riverbanks? Is there a place near Sioux Falls where you can get a picture of a riverbank eroded on the right more than the left? (On the river’s right as it comes downhill.)

I think you will appreciate Sloane’s drawing of the sledder who slides to the right going down the mountain, and then, aiming to the right, finds himself in a left spiral as he goes into the circular valley. Look at it very carefully, so you can remember how it works.

While we’re talking about unexpected Coriolis effects, go to this website: http://tafkac.org/science/coriolis/coriolis_force_tyson_debunking.html

The whole thing is worth reading, but scroll down maybe 1/3 of the page for a little piece on an engagement between the British and the Germans during World War I, near the Falkland Islands. (You can do a word search for “Falkland” if that’s an easier way to find it.) What happened here?

Chapter 4: Storms and fronts

page 47-48 — if your book is two pages off, sorry. First two pages of chapter 4…

So what is a front? Very simply, it’s the front portion of a mass of moving air. Weather people talk about a front as if it were some kind of object – it’s just the front end of the incoming weather – and being in front, it’s where the action is.

Notice the interesting story about Benjamin Franklin’s investigations of a storm. He was a first-rate scientist and gave up that career to serve our country. Thanks, Ben.

page 49

Cold front, warm front, occluded front. The warm and cold fronts are easy. The occluded front is more of a challenge. He says that cold fronts don’t often catch up with warm fronts, but then what’s that on page 56-57 where that’s what’s on the map? Anyway, the point is that cold fronts travel faster, so… So I don’t know how often, but sometimes.

pages 50-51

What’s the Beaufort number for winds at 100 mph?

pages 52 – 53, then 54 – 55 (or so)

(I discovered in class that we have different page numbers.]

Anyway, Williams has a two page spread on  a warm front moving across the US, followed by a cold front. Take some time with this diagram. Then he has another two-page spread on the same storm becoming an occluded front.

pages 56-57

More discussion of weather that travels and of the interaction between upper and lower winds. Notice that the cold front seems to have caught up with the warm front…

pages 58-59

This is an inset piece on a specific storm which added high tides to its fury. In an interesting interplay between water and air, between meteorology and astronomy, the storm hit at the very time when tides were high anyway because of the positions of sun and moon. Really, you never know what’s going to be important in a storm.

pages 60-62

The chapter ends with a discussion of the interaction between upper and lower air during a storm. Upper and lower winds do not always go the same direction.

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One Drop of Rain

Another post that was inadvertently left in draft form. So many different kinds of things to do wrong!

Not long ago, Ann asked me why one occasionally feels a single drop of rain when the sky is perfectly blue and clear, with no ongoing rain of any kind. I said I did not know.

But now I have a hunch.

I was watching some clouds from my trampoline (best place to watch clouds) and then I got this big fat drop — and no it was not a bird. One drop. And the sky was perfectly blue. Well, not perfectly blue. Some blue, some white, some gray-brown. I didn’t have my camera, but I got a picture later, not as pretty, but the best I can show you for now.

Here, the background clouds were white and reflective, much more than they seem, while the one in front, and lower, was brownish gray. It’s not because it was in shadow; it just was a gray cloud. Rain clouds are gray, but there was no rain; the sky was mostly blue, a little windy, very pleasant.

So I was trying to figure out the brownish parts, and I concluded that since they weren’t crisp like cauliflower, they were decaying cumulus, and in fact, they were quite thinned out and no longer reflective like the bright white clouds around them and in other parts of the sky. Furthermore, closer inspection revealed virga at their edges – looks like a ragged skirt, or a ragged slip dragging below the hem of a skirt. Here’s a better image of virga:

Anyway, virga is actually precipitation that doesn’t reach the ground because it evaporates before it gets there.

Well, but maybe just one drop gets all the way down.

The sky is like the ground – it’s not perfectly even and featureless; it’s full of variation, little wisps of cloud and little puffs of wind all going in and out of each other like mice in the fields. Hard for one drop to make it because the chances are much greater that it will evaporate, but then here I am with just one drop on my head.

Notice the universe; don’t just assume you know it. It’s full of surprises!

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Beaufort Verse

Admiral Francis Beaufort spent enough time with both his own diary and his ships logs to recognize the importance of making naval reports more uniform and useful. Everyone really has a different idea of what sort of breeze is “gentle” or “strong”, so he thought it would be useful to choose terms that would be more universal by their relationship with specific observations — and after about 32 years, the Royal Navy agreed. This is the origin of the Beaufort Wind Scale, which has been adapted for land use by adding observations that apply to trees and houses — not just waves!

Of course modern weathermen — on land or at sea — use wind speeds in knots, miles, or kilometers per hour, but it sometimes happens that these more objective and specific measures — so important in official reports — have the effect of making us less observant. Somebody asks for the wind speed, and we run for the newspaper or the phone instead of looking out the window. In this way, we detach ourselves from experience. Notice the weather!

In your weather observations, use drawings as well as words, and describe the evidence of the wind, not just the numeric speed. You will see more that way. Here’s a very simple chart to use.

30 chart alone

Meantime, unable to resist the opportunity to make something more accessible through verse, I have put Beaufort’s work to rhyme.

Beaufort Wind Scale

Beaufort naught leaves ponds like glass

Reflected swans for real may pass.

Zero wind leaves water like a mirror; and smoke rises straight up.

Drifting smoke is Beaufort 1

Water scales; but waves are none.

A slight wind causes water to have the appearance of fish scales; smoke drifts a little.

I turn my face to Beaufort 2

While glassy waves touch my canoe.

A little more wind actually makes waves and can be felt on the face.

A gentle breeze is Beaufort 3

Rustling leaves and just-breaking sea.

As wind speed approaches 10 mph, the waves become strong enough to break, and on land, the leaves rustle without ceasing.

Branchlets move for Beaufort 4

And milk-white horses trot ashore.

The next sign of strengthening wind is a little white on the cresting waves; still not threatening, but getting brisk. Small branches move in this wind.

Beaufort 5, a fresh’ning breeze,

Lengthens waves and bends young trees.

Somewhere near 20 mph, the wind begins to bend young trees. Waves become longer, and the “white horses” become numerous and insistent.

Whistling wires are Beaufort 6

Men with umbrellas are in a fix!

If you use an umbrella, you know that sometimes the wind is just too strong. At sea, this is when the spray begins to come up.

With heaping seas and blowing foam

‘Gainst Beaufort 7, ’tis hard to roam.

Eventually, even the human body is so buffeted as to make it hard to walk in the wind. This is about 30 mph.

Snapping twigs — spindrift at sea

Beaufort 8 is the gale for me!

We really have a storm now — a gale. Small parts of trees and plants begin to break in the wind, and the foam is blown above the water. This gets us up to 40 mph. “Gale” has a specific meaning for Beaufort; not just big storm, but these specific observations.

Beaufort 9 is a howling gale

Testing the ship and the roofing nail.

Nobody likes to lose shingles, but it happens; and you better have a solid ship if you are at sea.

Trees by the root and ships in a wave

Beaufort 10 is a terrible knave.

Basically, this is just below hurricane force.

If you’re on board for B-11

Best prepare to knock at heaven.

Hurricane. Anyone can be hurt; any building is up for grabs.

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Cloud Viewer

Besides the problem of a stiff neck, one difficulty in cloud watching is that it’s hard to see how the clouds overhead are moving, particularly if they are a little slow. Also, if there are opposing winds, it’s hard to see whether they are really opposing or just going the same direction at different speeds.

The cloud viewer is simply a piece of glass with a dark backing — either dark paper or flat black paint. This makes a kind of darkened mirror, which is easier to view than a regular mirror because it’s not as bright. Ordinarily, the sky caught in a mirror might tire your eyes because it would be so much brighter than your table top, your lap, or the ground. So the darkened mirror.

Tape the edges of the glass with duck tape and fasten the whole thing to a piece of cardboard or fiberboard. If you have a small compass, that will make it look very professional and will help you to pay attention to the exact direction of the cloud movements.

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Opposing Winds

Thought I posted this last Tuesday or so!

Ann and I were talking last Friday, and she mentioned that once she looked up and saw two sets of clouds going in directly opposide directions. Isn’t that amazing?

Let’s think about how this could happen. Remember that our weather generally comes from the west. But storms — not just hurricanes, but ordinary thunderstorms — are spirals of wind, spiraling counter-clockwise around a low-pressure center. Now, if that spiral were all the wind there were, then the storm would never go away. Why does the storm go along to the east? Because there are other winds, and those winds take it. Unless a storm is very high indeed, the prevailing winds continue right above it, so all you have to do is superimpose a west wind on a spiral and you have a whole series of possibilities:

Now you can see that on the east side of the storm (on the right) the winds of the storm and the winds above the storm are at right angles. These crossed winds — winds at right angles — tell you that you are directly in the path of a storm.

On the south side of the storm, upper and lower winds would both be from the west; in that case, you are not in the center of the storm’s path and will not get its worst fury, whatever that is. If you are in a plane, you want to go round a thunderstorm on this side.

On the north side, the storm winds would be from the east and the prevailing winds from the west. Again, you are not really in the path of the worst of the storm, see? You might not even get wet; depends on the storm. In an airplane, however, you will get buffeted coming from the west; an eastbound plant wants to pass this storm on the south side. But on the ground, it’s just a neat show, and that’s where Ann was the day she saw opposing winds. In fact, all of us have been there many times.

Finally, look at the west side of the storm. Here the winds are crossed at right angles again — north and west — but the storm is leaving. So crossed winds come every time a storm passes right over you, first when it comes and then when it goes. You can figure out for yourself whether a storm is coming or going, right?

So if it’s so common, how come Ann thought it so unusual?

Well, number one, people don’t look up. ‘Specially when a storm is coming and there’s so much to pick up and put away before the rain. But I’m pretty sure Austin told me he had seen it too. How about the rest of you?

Number two, sometimes there are so many low clouds that you don’t see the upper ones. In the thick of a storm, you only see the lower clouds. But there’s often a time before a storm when blue skies mix with two levels of clouds. Watch for this.

The bottom line is that it’s not really all that uncommon to have crossed or opposing winds. What’s uncommon is looking up enough to notice. Thanks Ann.

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