Archive for September, 2009

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

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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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


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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