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Wednesday, September 9, 2020


Look, there is only so much one can say about this wretched political situation we are in. It is now after Labor Day and I am in full freak out mode, so rather than obsess about today’s polls I will take a moment to write something about two nonpolitical matters that have puzzled me for many years. I invite all the help I can get from those of you who have specialist scientific knowledge that I lack.

The first of these matters is clouds. Back in the day when we all took airplanes, all of us had the experience of flying through discrete layers of clouds – two or even three – at different levels above the ground. I can recall, as every passenger can, looking down 25,000 feet at a layer of clouds and then, as one descends toward landing, going through that layer of clouds back into clear air and then into another layer of clouds and perhaps even into a third layer before landing. My question is this: why do clouds form in discrete layers in this way? I have only the vaguest notion of what a cloud is – a lot of water droplets, I guess – but no idea at all of why these clouds form at one elevation rather than another and at several discrete elevations in the same area. Does anybody know the answer?

The second thing that has always puzzled me is friction and I think I may have actually mentioned this before. What is friction? Why do my clothes slide onto my body easily when I am completely dry and yet catch and stick when I am wet? I’m sure there must be a simple answer to this question but I am utterly mystified by friction. Anybody?

And then of course there is Joe Biden’s poor showing in the polls with Hispanic voters, but I said I wasn’t going to talk about politics so let that go.


Neal said...

I found this clip well done and informative.

Unknown said...


Here is a neat website to answer your question about clouds:,of%20the%20water%20vapor%20occurs.&text=If%20the%20volume%20of%20rising,then%20there%20are%20discrete%20clouds.

Michael Llenos said...

"The second thing that has always puzzled me is friction and I think I may have actually mentioned this before. What is friction? Why do my clothes slide onto my body easily when I am completely dry and yet catch and stick when I am wet? I’m sure there must be a simple answer to this question but I am utterly mystified by friction. Anybody?"

I think the clothing gets heavier and smaller because of the water absorbtion in a weaved shirt or pants material, and there is no more air pockets on the surface of your skin (in the pores) anymore because the pores are filled with H20. Just imagine bubble wrap packaging material. Rub it on your skin. It should make contact smoothly. And imagine exchanging those air pockets for water filled pockets. It probably won't be so smooth anymore because of the heaviness. I'm sure gravity plays a great part in the clinginess and friction of the water soaked clothing. But don't take my word for it, I'm just guessing.

David Palmeter said...

Off Topic:

The Washington Post tomorrow (it's already on line) has a long story about Bob Wodward's new book based on taped conversations with Trump and a number of others:

David Palmeter said...

This is crazy. I pasted a quote in the text box, but then retyped it in and deleted the pasting. I thought it would stick. But no such luck. If you Google Wordward book you'll find. Here's another attempt at posting the quote:

David Palmeter said...

No luck.

Anonymous said...

My hunch is that friction doesn't have much to do with it. Try fluid dynamics.

Anonymous said...

Clouds form at different heights and have different shapes because the atmosphere is not a homogeneous, undifferentiated mass of air, standing still or even moving in a single direction.

Different air masses carry different amounts of water vapour with them and have their own temperatures. And they move both horizontally and vertically all the time.

The best way to see that is observing a sequence clip of weather fronts. Try your local official meteorological body. I am sure they have simple, illustrated tutorials explaining the basics.

- Me, Myself and I

Alex said...

Friction is basically “stickiness” between two layers or two objects in contact. Specifically, friction forces arise which are in the opposite direction of the motion. So if you are pulling your shirt down your back there will be a friction force on each bit of the shirt pointing upwards.

Microscopically, in the case of the wet shirt, I suspect the friction force is the famous adhesion and cohesion property of water: water sticks to itself and to other things. E.g. when you sprinkle water on a surface it forms a drop, rather than spreading out into a layer one molecule thick. Attractive forces between water molecules and between water and the surface hold the drop together despite gravity trying to pull the drop into a thin layer. When you put on a shirt all these tiny forces point in the same direction (opposing the motion) and add up to making it harder to pull it on.

If you want to go even deeper on where these forces come from, it’s because H2O is a “polar” molecule: the positively charged oxygen nucleus pulls the cloud of electrons closer to it than the two hydrogen nuclei can pull the cloud toward themselves. So each water molecule has a part with a slight negative electric charge (near the O) and a part with a slight positive charge (toward the H’s) even though molecule is neutral overall. But the effect is that attractive forces arise between the positively charged parts of one H2O molecule and the negatively charged part of another. So water is kind of “sticky”. By contrast, nonpolar liquids like oils, whose molecules are made of carbon and hydrogen are much less sticky (even nonpolar stuff still has some sickness arising from a different microscopic origin but that’s another story... :)

LFC said...

Re the Woodward bk revelation(s):

T's defense (so-called) is that he didn't want to "panic the country." Mind-boggling.

Eric said...

I'm certainly no expert, but I learned a little bit about this aspect of meteorology when I got interested in paragliding a number of years ago.

Anonymous @5:44pm is right. The short answer is that there are layers of clouds because the atmosphere is not uniform.

Cloud formation is actually a pretty complex process. As I understand it, the basic principles are as follows:

The sun warms the land; the land in turn warms the air directly above it. The warmed air expands, becoming less dense than the cooler, more-compact air above, and this less-dense warmed air then rises, or floats, above that cooler air. The air continues to expand the higher it rises because atmospheric pressures decrease with altitude. As the mass of air expands, the pressure within it drops, causing the temperature of the invisible water vapor molecules within the rising air to fall. If the temperature falls low enough, the water vapor condenses, forming visible droplets, which become a cloud.

As all of this is occurring, the mass of warm air (called a "thermal") that has been rising has been displacing cooler air above it, which descends around the rising air and goes through reverse processes of contracting and warming. If the descending air warms enough, the visible droplets of water can evaporate; so clouds can form but then also disappear just as a result of vertical movements associated with thermals activity. This vertical movement, the cycles of rising and falling air masses, produces one set of disturbances that lead to differential pressures and temperatures across the lower atmosphere.

Now, this heating of the Earth by the sun is not uniform. The rates at which different areas warm up vary with topography and ground cover. (Just think of a car that's been sitting out in the hot summer sun for hours. Would you rather sit on a seat in that car covered with vinyl or with cloth? Cloth has a higher heat capacity; it takes more energy to heat up a bit of cloth than an equivalent bit of vinyl. Or, put another way, a vinyl seat will heat up faster in the sun than a cloth one will.) Water has a higher heat capacity than land; areas of land covered with pastures and forests have higher heat capacities than areas covered with asphalt roadways or cement buildings. It's for this reason that in many regions adjacent to large bodies of water, there is typically a sea breeze or lake breeze coming inland off the water early in the morning. The air over the land warms up faster than the air over the water. The warming air over the land begins rising as thermals, creating an opening for the cooler air from over the water to rush in to fill. A reverse process occurs at night. Mountain ranges or hills and valleys across the landscape add further complexity.

Eric said...

So patchworks of clouds form from thermals that are rising up as a result of differential heating of the surface due to variable surface features (or to overlying clouds at higher altitudes). Passengers in airplanes can physically experience movement through pockets of rapidly rising and falling air within clouds like this as one form of "turbulence."
As clouds are forming, they may then block the sun, which can result in a slowing of the surface heating, shutting off the thermal fuel and forming a bottom to the cloud. As part of the cloud dissipates (possibly burned off by the sun hitting its upper aspects), or drifts away, the sun's heat is able to build again at the surface and the thermal production can start back up, producing another layer of clouds.

Now add in the effects of winds aloft, masses of air moving in horizontally from other regions with different temperatures and different pressures than in the air rising from the surface as thermals, and the localized clouds start to take on all sorts of different personalities. I think it's those winds that are usually most directly responsible for the layering that RPW asks about. The winds may be pushed by warm or cold fronts. They can carry in clouds at higher altitudes that had formed in other locations; and/or can bring in warmer (or cooler) air with more water vapor from other areas, allowing different clouds to form at higher altitudes than the lower clouds that have formed from air rising up as a result of local surface heating.

Jim said...

Professor Wolff --

I will relate to you how my high school biology teacher described how he came to understand the concept of friction -- a description which has stuck with me all these years. He said that as a kid he had always thought it would be great fun to take a patio umbrella and parachute to the ground from the roof of his family's two-story house. One day when the parents were away, he grabbed the umbrella from the patio table and climbed up onto the roof using a ladder. He then proceeded to march up the sloped roof, umbrella in hand, in order to reach the top and launch his long anticipated leap. Half way up, his feet began to slide a bit down the roof. He froze. "It was at that moment," he said, "when I fully and perfectly grasped the concept of friction."

-- Jim

DDA said...

One feature of friction that is obvious when pointed out is that the coefficient of starting friction is (usually) greater than the coefficient of moving friction.

Danny said...
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