Of Particular Significance

Happy (Chilly) New Year

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 01/07/2014

Welcome 2014! And quite a start to the year, with a cold snap that rivals anything we’ve seen in two decades. I don’t remember cold like this since the horrid winter of 1994, when the Northeastern U.S. saw snowstorms and extreme cold that alternated back and forth for weeks. Of course, when I was a child in the 1970s, such chills happened a lot more often; I remember a number of New England mornings where I awoke to a thermometer reading of -20ºFahrenheit (-29ºCelsius) [244 Kelvin].

The scariest negative temperature numbers that one hears about from the media are associated with the “wind chill”, which is a number that is supposed to measure how cold the air “feels” to your skin.  But “wind chill” is a rather subjective and controversial measure — there’s no unique way to define it, since you’ll feel differently depending on how much exposed skin you have, on your body weight, on your age and conditioning, etc.  By contrast, the temperature measured by a thermometer is defined independent of how humans feel, and experts agree on what it is and means. Oh sure, people use different scales to measure it: Fahrenheit (F), Centigrade or Celsius (C), and Kelvin (K).  But the differences are no more than the distinction between meters and feet, or between kilograms and pounds; it’s straightforward, if a bit annoying, to convert from one to the other.

So everyone agrees the temperature is and feels extremely cold, But is it, from the point of nature, really that much colder than usual? To say it another way: it was 84ºF (29ºC) in southern Florida yesterday.  How much warmer is that than the -40ºF (-40ºC) that was registered in the cold Minnesota morning?

Well, you might first think: wow, it’s a difference of 124ºF (69ºC), which sounds like a huge difference.  But is it really so huge?

Perception and Reality

We humans experience the world through our perceptions and our consciousness — through what we see, hear, feel, and so forth. And as is natural, we tend to assume that what we see, hear and feel correctly represents the world as it is. But in fact, as scientists have learned over the past centuries, the devices we use for our perceptions — eyes, ears, temperature-sensing nerves, and so on — filter and process the information that they receive from the outside world. Our brain then does further processing.  By the time the information enters our conscious notion of the world, the world has become a caricature, a cartoon, of the real thing.  Much of the world is ignored, and the part that we are conscious of is transformed almost beyond recognition.

Indeed, one of the first things young science students have to do is unlearn almost everything our senses and brains tell us about the world, and relearn it almost from scratch.

So it is with hot and cold. In the real world there is no such thing as a “hot object” or a “cold object.”

However, there really is such a thing as temperature. One can speak of “hotter” and “colder”; if A is hotter than B, then, very simply, A has a higher temperature than B.

But it makes no sense to say “A is hot”. The statement that “A is hot” is a misconception, a trick that our senses play on us.  What this really just means is that, to your skin’s nerves, “A feels hot”. And what that usually means, in the real world, is “A is hotter — has a higher temperature — than your skin”.  As always the full story’s a bit more complicated, but not too much.

It’s always good when you can do a simple experiment, on your own, to reveal some basic fact about the world.  There’s a nice one in this case. Take three bowls; fill one with ice water (literally, water with ice in it), fill a second with water that’s almost too hot for you to put your hand in, and fill the third with ordinary tap water that’s lukewarm. Now put your left hand in the ice water and your right hand in the hot water. After a minute or so, take them out and put them both in the tepid water. How hot is that water?

You will find you are of two minds. Your cold hand will feel the tepid water is hot, and your hot hand will feel it is cold. And this is because the tepid water is hotter than your cold hand, and colder than your hot hand. Thus you may easily learn: what you feel as hot and cold is not a measure of the world, but rather a measure of you relative to the world.

In other words, we humans are not thermometers; our nerves and brains don’t tell us the temperature of a thing, just whether it is hotter or colder than some temperature we find comfortable. [In fact our nerves don’t even tell us how much hotter or colder another object is; they actually tell us something about how quickly heat is flowing out of or into our bodies from the object that we’re sensing, which is why, when you pull clothing from a hot clothes dryer, the metal zipper feels so much hotter than the cloth does. But more on that another time.]

So what is temperature?

In most ordinary situations, you can think of temperature in a simple way (though I should note that a more sophisticated view is sometimes needed.) Every material object that we find around us — tables, air, bricks, glass, alcohol — is made from molecules characteristic of that material. In any material, the molecules, vastly too small for you and I to see, are moving, fast and randomly, and banging into each other. Some are moving faster than others, and they’re heading in all directions. In a solid, they don’t move far; they just rattle in place. In a liquid or gas, they can move all around, but they don’t go far without banging into each other, and when they collide their speed and direction change. Because they’re constantly running into each other, each molecule is moving sometimes faster, sometimes slower… but the molecules never stop moving.

[This might confuse your intuition. You’re used to large objects that you can see, like soccer balls and cardboard boxes and pens, slowing down and stopping. That’s because, on the Earth’s surface, large objects generally end up sliding over the ground or the floor, and friction slows them down. Molecules aren’t subject to friction, so they keep moving. In fact friction is a process in which large objects gradually lose their motion-energy (technically, “kinetic” energy) and hand it over to molecules!]

The temperature of an ordinary material is simply a measure of the average amount of random, invisible motion that the molecules in that material are undergoing, or, more precisely, a measure of the average speed and motion-energy that those molecules have.  Molecules in colder objects move more slowly than molecules in hotter objects; that’s all there is to temperature!

Here again, our senses fail us. Look at the table in front of you. Your senses do not tell you that the molecules in that table are randomly wiggling around. Of course your senses don’t even tell you that the table is made from molecules, so it’s not surprising they don’t reveal their motion… but there aren’t even apparent fuzzy edges to the table, which you might have naively expected. And our senses don’t tell us that even on a calm day, with no wind, the molecules of the air surrounding us are zipping around at a cool 1600 feet (500 meters) per second — faster than the speed of sound, and around five times the wind speeds of the most powerful hurricanes.

This incredible rain of molecules against your body doesn’t knock you over because, unlike hurricane wind, which pushes you in a particular direction, the random motions of the molecules push you in all directions, and therefore, in no direction at all. The molecules do exert pressure on your skin, but your body pushes back; if it didn’t it would collapse, so of course evolution has assured that it does.  Yet your senses don’t even bother to tell you that all this is happening.  Why?  Because this pressure is just part of the scenery of being alive on the surface of the Earth, and your survival doesn’t depend upon you knowing about it, anymore than it depends upon you knowing that molecules exist at all.

How much hotter is Florida than Minnesota?

Now that it’s clear what temperature is, we can ask how large, really, is the change from the 84ºF (29ºC) [301 K] temperature felt in daytime in southern Florida and the -40ºF (-40ºC) [233 K] temperatures felt in early morning in northern Minnesota? The temperature difference, subtracting the colder number from the hotter one, sounds dramatic, but doesn’t answer to the question. We have to focus something else.

For this purpose, the outdated Fahrenheit scale is hopeless. The number of degrees doesn’t correspond to anything very easy to remember. You just have to memorize how the Fahrenheit scale works: water boils at 212ºF and freezes at 32ºF, and that’s for weird historical reasons.

The Centigrade, or Celsius, system (Celsius invented it in 1743) is much easier to remember, and almost the whole world uses it in daily life. 0ºC is where water freezes; 100ºC is where it boils. But while that’s convenient when you want to know if the temperature is below freezing, it’s not convenient to determine the answer to our question.

No, if you want to understand really how much hotter is air at 84ºF (29ºC) than air at -40ºF (-40ºC), you need the Kelvin scale. The Kelvin scale is very easy to use.  For one thing, it’s really the same as the number of degrees in Centigrade/Celsius scale, plus 273.15 — and that’s it. I’ll just approximate this shift by 273, to keep my numbers simple.

Thus water

  • freezes at 273 K, and
  • boils at 100 + 273 = 373 K.

What’s this magic number “273.15”? Well, 0 K = -273.15 ºC = -459.67 F is the temperature at which the amount of motion-energy per molecule reaches zero, and the molecules stop moving. [Almost true! Due to the jitter inherent in quantum mechanics, the molecules never quite stop moving, and the motion-energy is never quite zero. But the motion and the motion-energy are far too small for us to worry about for today.] In other words, it is impossible to have a lower temperature than zero Kelvin! This lowest possible temperature is called “absolute zero”, which is why the Kelvin scale starts there. 0 K simply corresponds to the lowest possible motion-energy per molecule, which for most practical purposes might as well be literally zero.

So here’s the advantage of the Kelvin scale: the temperature of the object in Kelvin is directly proportional to the energy of its molecules. An object with a temperature of 200 K has twice as much motion-energy per molecule as one with a temperature of 100 K, and half as much motion-energy per molecule as one with a temperature of 400 K.

Well, that suggests to us how we should answer the question: How much hotter is object B than object A? Now that we have a scale for which all temperatures are positive, we can take a ratio of their two temperatures, TB/TA. If B is hotter than A, the ratio is greater than 1; if B is colder, the ratio is less than 1.  If that ratio is, say, 1.2 — if the temperature of B is 20% larger than the temperature of A — that tells us that the molecules in B have energy 20% larger than those in A. Remember this is only true if we use the Kelvin scale, and not Fahrenheit or Celsius.

Meanwhile, the reason not to use the temperature difference, TB – TA, is that its meaning is ambiguous. A difference of 5 K would be very large if B is at 6 K and A at 1 K — B is six times hotter than A — but would be a tiny effect if B is at 60,000 K and A is at 59,995 K, in which case the difference in the two temperatures is a very small fraction of a percent.

I hope you now begin to see why scientists use the Kelvin scale. It is a scale that relates a property of visible macroscopic objects, as measured in temperature, to the level of activity of the invisible microscopic molecules out of which the objects are made, as measured in motion-energy (or “kinetic” energy). If the temperature is large, so is the energy per molecule; as one drops to zero, so does the other.

So how cold is this week’s cold snap?

Yesterday, in southern Florida the high temperature was 301 K, and the low temperature was 233 K in northern Minnesota. The ratio of the two temperatures is simply 301/233 = 1.29. We may say that Florida was as much as 29% hotter than Minnesota, meaning, specifically, that the average air molecule in Miami in the afternoon had 29% more energy than the average air molecule in Duluth at dawn.

Is that a lot, or not? Motion-energy increases as speed squared (Emotion = ½mv², for an object of mass m traveling at a speed v that is much slower than the speed of light) so it only takes 14% more speed to give you 29% more motion-energy.  That means a car driving 68 miles per hour (114 km/hour) has 29% more motion-energy than a car driving 60 miles per hour (100 km/hour).  So think about that when you are commuting to work. When you accelerate from 60 to 68 miles per hour, you’ve increased your speed by enough to turn this morning’s Duluth into this afternoon’s Miami.  

That said, the molecules are a bit speedier.  Compared to those slow-pokes in Duluth, which crawl along at a mere 980 miles per hour (1640 km/hour), the average molecule in Miami is traveling about 1100 miles per hour (1850 km/hour), about 14% faster.

So it’s a matter of perspective.  Biologically speaking, the difference between Florida and Minnesota is enormous; you can survive in one for months, and in the other for minutes.  This is in large part because between comfortable Key West and dangerous Duluth lies the temperature where water turns to ice, and so your body, being more than half water, will start to freeze solid in the cold, not to mention being unable to maintain the internal warmth it needs to function.  And yet, for the nitrogen and oxygen molecules in the air, the cool-down is a rather small change… no more than slowing down on the highway from 68 miles an hour to 60 miles an hour (114 km per hour to 100 km per hour.)   They barely notice the difference!

We should be very grateful to live on such a stable planet, whose hottest surface temperature (about 130ºF= 54ºC = 328 K) and coldest surface temperature (about -130ºF = -90ºC = 183 K) are only different by a factor of 328/183 = 1.8 — and of course those temperatures are unusual, and only occur in unique places.  Compare that with the Moon, where high temperatures (214ºF = 101ºC = 374 K) and low temperatures (-300ºF = -184ºC = 89 K) vary by a factor of 374/89 = 4.2.  Out in the wider universe, it’s a little more drastic. The center of the sun reaches a blazing temperature of 27,000,000ºF = 15,000,000ºC = 15,000,000 K (approximately).  Meanwhile the ancient photons (the particles of electromagnetic radiation which make up the “cosmic microwave background”) that fill outer space would warm you to just under 3 K. The ratio of the two temperatures is about 5,000,000.

Home Sweet Home.  Even when it runs a little cold.

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57 Responses

  1. I don’teven know howw I ended up here, but I thought thiis post was good.
    I don’t know who you are but definitely you are going tto a famous blogger if yyou aren’t already 😉

  2. I enjoyed your explanation on temperature and our perception of it. The 3 water bowls experiment is a great way to demonstrate your argument. I have been aware of what you explain however every time one of my children get a fever I still find myself putting my hand to their forehead. And then I think to myself……where is that thermometer?

  3. This is a very interesting article but is the use of Celsius fair because C is based on the boiling and freezing points of water so we cannot expect accurate results when measuring whether another object is hot or cold, personally, I feel Kelvin is always the best unit when dealing with temperature.

  4. I simply could hardly keep your website prior to implying that i truly treasured the normal facts an individual deliver for your website visitors? Are going to be once more continuously in an effort to scrutinize completely new posts

  5. Two comments:

    Regarding the relationship of climate change and the disruptions of the Polar Vortex, wathc this:


    Regarding the ozone layer depletion problem, which has become less of a problem in recent years mostly due to the fact that proper measures were taken over these years, in no small part the result of the efforts of people like Al Gore and John Kerry, we can say this:

    Ozone is a “weird” molecule of oxygen that contains three oxygen instead of the more common form with two atoms of oxygen. Ozone is very unstable, because it is more reactive than “common” oxygen.

    Ozone requires a source of high energy to form, and that is what happens at the top layer of the atmosphere: there’s lots of high energy photons, mostly interesting, UV photons. So, the process of ozone formation at to the top layers of the atmosphere consumes many of these dangerous photons, so, they are no longer traveling towards the surface of the Earth, where they can cause more damage, like cataracts and skin cancer.

    At the ozone layer, there is an equilibrium of ozone forming and ozone turning back to “common” oxygen or reacting with other stuff, as it is more electronegative than “common” oxygen.

    This story was fine, until the concentration of chlorofluorocarbons started to increase at the ozone layer (these compounds include Freon and other gases used in A/C and refrigerators, as well as in the production of plastic foams like poly urethane and others).

    One of the reasons why chlorofluorocarbons (CFCs) were chosen for HVAC engineering products is because at the sea level they behave quite well, they are not very reactive.

    But at the top layers of the atmosphere, there is going on a very different story with CFCs, and mostly because of the excess of high energy photons.

    CFCs break down into much more reactive molecules, as this break down makes available to the “outside” surface the outer layer electrons of the chlorines and the fluors present in CFCs.

    Both chlorine and fluor are more electronegative than both oxygen and ozone, so, as they are free to react with the ozone, they deplete ozone much faster than the usual rate (the equilibrium that I already mentioned).

    The presence of these CFCs at the upper layers of the atmosphere establish a different equilibrium, as other reaction paths take place, and this is the main reason for the ozone depletion.

    The was predicted by Dr Mario Molina in 1974, and he eventually won the Nobel Prize for his research on the impact of CFCs on the ozone layer.

    Kind regards, GEN

  6. If there is no radiation source, CO2 approaches 0 K because of its emission. It is non-radiative nitrogen and oxygen gases that award the Earth a warm liveable near surface atmosphere. Nitrogen and oxygen gases constitute of around 99% air of the atmosphere. Carbon dioxide constantly gains heat leading to temperature rising by colliding with warmer nitrogen and oxygen molecules.

    There may be a geometrical relation is existing in between electromagnetic field and gravitational field – keep the radiation source. If not, the loss in temperature of photon affects the conservation law of non-radiative gases, chemical reactions and rest mass. ?

  7. Yes, vibrating molecules emit IR, except diatomic molecules like nitrogen, hydrogen, or oxygen. I got tenure measuring this from chemiluminescence, that is, molecules vibrating due to energy released in a chemical reaction.

    1. Your view is ahistorical and anachronistic. (a) Scientists have not “thrown in the towel” on WIMPS, and the search will continue for decades. (b) Scientists were never committed to WIMPs alone anyway. The media mis-portrayed the situation. Axion searches have been going on for decades, and so have searches for primordial black holes; and there are hundreds of papers on other experimental signtures that would come from non-WIMPs. Only the media thought WIMPs were the only game in town.

  8. Is the Energy Frontier Dead? arxiv:1401.0966v1

    Hitoshi Murayama asks the question:What happens if nothing new is found beyond the Standard Model by the LHC?

    1. I asked it first, :-).

      So, where does the 64 questions leads us to? … Are we a product of the butterfly effect, chaos theory?

      What? … if, indeed, zero point energy is infinite, then the one way to explain a very low temperature (minimal vector) is that the field is infinite in all directions (space-time) and the universe is an ensemble of energy stretching and pulling of this very low field. And predict it is, indeed, the gravitational field, the variable curvature of space-time.

      We are all from nothing and nothing is everything. No beginning and no end.

      1. Sorry Mr.Oaktree, I repeated your question. I asked the same question above from different direction, on January 7, 2014 at 1:36 PM, kept under “Your comment is awaiting moderation”. – which may be too long.
        I say sorry to Professor also for this.

  9. I agree with the notion expressed in item 3). There is an overwhelming amount of observations that are consistent with the hypothesis that the global climate is undergoing a very rapid and dangerous change that cannot be explained by natural variability alone. We have turned on the heating and we now experience more and more of the effects, many of them predicted and some more unexpected but also dangerous. Concerning the items 1 and 2, some clarifications:

    “1. If global warming is, indeed, real and growing then should we not expect more powerful vortices and hence more polarizing of the energy within the earth’s atmosphere? (conservation of energy).”
    The general effect is that with global warming more energy is available in the atmosphere and in the ocean, giving the potential to stronger flows and currents. However, there are effects working in some regions in the opposite direction. Global warming, as observed so far (and as predicted by many climate models), is particularly strong in the polar regions. This reduces somewhat the temperature difference between the polar regions and it surroundings (sub-polar regions), and may make it easier for the very cold air of the polar regions to escape from there to smaller latitudes. As a result, the temperature difference between poles and mid-latitudes is (to some extent) reduced, which counteracts the “polarizing of the energy”.

    “2. As dangerous or maybe even more dangerous, could this massive accumulation of energy “trapped” in the atmosphere blast huge holes in the ozone layer and further increasing the energy levels?”
    The ozone depletion (or ozone hole) is not caused by energy in the atmosphere that “blasts huge holes”. It is mainly a chemical reaction in the atmosphere that requires very low temperatures over a long time and few mixing of the cold air with surrounding regions. If the temperatures over the poles in the altitude where these chemical reactions occur increase with global warming, and/or if global warming leads to more mixing of the air with surrounding regions (and effectively warming the air, also), then the depletion of the ozone layer might be even less (and not more) then now.
    (However, the depletion of the ozone layer is a rather complex process, and the argumentation here might be to simple to really address the issue.)

  10. I actually used windchill as an example in my P. Chem class. Its a very
    well defined concept. That is, absent wind, heat must get from your face by thermal diffusion. That is, a thermal gradient is established over a distance roughly equivalent to the size of your face. With wind the air very near the
    face is very quickly exchanged for cold air, and so the gradient is much steeper, resulting in faster heat flow out of your face.

    It would make a somewhat complicated homework question by considering two flat plates of different temperatures, one small and one infinite, with and without air externally driven air flow in between (necessarily at the temperature of infinite plate since it would have had an infinite time to equilibrate.) To make it easy assume a rare gas. This is both a thermal conductivity problem and a viscosity problem (since the air slows down between the plates.)

    The same thing applies to the heat flow under ice in a stagnant pond
    versus a frozen river.

    1. I probably wasn’t clear enough. Wind chill is a real effect. However, the precise value of the wind chill is not something on which people can agree, because you must make all sorts of assumptions. Different humans will lose heat faster or slower, so there’s no human-independent definition that actually determines what an individual person will feel or how quickly he or she will lose heat… and that’s why the definition of the wind-chill index has changed over the years and will probably change again in future. Your comment is correct, but not really relevant to the point I was making.

  11. Nice article Matt. I like this sort of thing. In a way all our senses could be motion detectors, maybe even smell. And you can do Compton scattering to convert photon energy into electron motion. Do it again with the residual photon, and in the limit, all the photon energy is converted into electron motion, and there’s no photon left. And yet you could put a photon through pair production to create an electron (and positron). Sure is interesting is motion.

  12. I prefer Fahrenheit to Celsius for everyday use. Because of the offsets from 0 K, neither is really appropriate for scientific purposes. However 0-100 F corresponds roughly to the normal range of temperatures in the places where most people live, and a 10 F difference makes for a qualitative change in how hot or cold it feels.

    1. Thanks for the dissenting view. Personally I think it is quite convenient to have a system (Celsius) where 5 C makes a qualitative change in how hot or cold it feels. And it’s quite easy to remember:

      In Celsius
      -20 is extremely cold
      -10 is bitter cold
      0 is freezing cold
      10 is quite cool
      20 is room temperature
      30 is quite warm
      40 is very hot

      1. Zero in Celsius is at a pretty convenient spot, since it’s relevant for the weather if water is liquid or frozen. The size of the steps in Celsius and Fahrenheit are both pretty good imo. You seldom have use decimals or hundreds.

  13. 1. Global warming is an issue of climate, a long-term trend. Even a small amount of global warming can have a big effect on weather, short-term ups and downs.

    One way to understand this is that a warmer climate does indeed mean there is more energy available to power storms, both warm and cold ones.

    2. I am not aware of any significant effect on the ozone layer.

    3. The problem is that we won’t hit a wall, most likely; things will just gradually change, which they do anyway, making it very difficult to recognize the change is long-term.

    1. Also, conservation of energy only applies to systems which are closed. The Earth has a continual energy input from the sun, so conservation of energy doesn’t apply. We would expect to have more powerful storms, but not due to conservation of energy.

      1. Thanks — that’s correct of course, but also complicated. There’s both energy coming in from the sun and energy being lost to space — and greenhouse effects shift the latter so that more is stored in the atmosphere. In short, the question is how much is present at any given time that is available to power storms.

        Similarly: Each of us has money coming in and money going out, but we also have a bank account, and the more we have stored in the bank account, the more spending power we have.

        1. This is the main aspect of the greenhouse effect caused by the increase in concentration of certain gases in the atmosphere: the wavelengths of incoming light is short enough (UV, visible light, as well as infrared) for that light to pass through the atmosphere, then it strikes the earth and the water and warms them, and the surface starts to emit infrared ligth because of increase in temperature, but these greenhouse gases are opaque to infrared light, so heat is kept trapped within the atmosphere, and average temperatures over time increase.

    2. Concerning item 2: “any significant effect on the ozone layer”: The ozone depletion depends on very low temperatures and isolation of very cold air masses from its surroundings (few mixing). Both of these conditions are probably modified by global warming, and I think there are some studies to investigate these (for the moment, somewhat hypothetical) effects.

      Concerning item 3: “things will just gradually change, which they do anyway”. The human-induced radiative forcing (mainly by releasing carbon dioxide and other greenhouse gases) happens on a different time scale than the natural variations; they are much faster. In less than 200 years the human-made emissions altered the composition of the atmosphere so drastically as naturally occurs only over the course of many thousands of years. The difference in the time scale allows to relate observed global warming to different contributions, human-made and natural ones, via statistical analysis. The analyses summarized in the IPCC reports show that global warming goes on both in periods where the natural processes alone would cause (a) heating and (b) cooling, so that a significant contribution from human-made processes must be taken into account to explain the observed global warming. Natural processes certainly cause warming in some periods (typically over much longer time periods), but are not able to explain the observed warming in periods where the natural processes would lead to a cooling.

  14. “This incredible rain of molecules against your body doesn’t knock you over because, unlike hurricane wind, which pushes you in a particular direction, the random motions of the molecules push you in all directions, and therefore, in no direction at all.”

    Two thoughts that immediately to mind:

    a) “light (photon) bullets”, is this where there grabbed on to the idea of creating light bullets? If you create a collimated beam, pack it (photon trap) into a short pulse and send it off in “blaze of glory”, 🙂

    b) Nikola Tesla, … gives the following description concerning the particle gun’s operation:
    “[The nozzle would] send concentrated beams of particles through the free air, of such tremendous energy that they will bring down a fleet of 10,000 enemy airplanes at a distance of 200 miles from a defending nation’s border and will cause armies to drop dead in their tracks.”

    However, instead using as a weapon, could we not use it as a external power source for vehicles. Instead of the power plant on the vehicle send the power the vehicle and an AC motor?

      1. Tesla’s “open-ended vacuum tube” wouldn’t work as a weapon (the air would be an issue) but could be used as a way to get an intense electron beam over a short distance without having the beam interact with the glass. I recall reading about a recent experiment using such a device, but I don’t remember where or why it was needed. Probably something to do with semiconductor doping. It’s certainly useless as a weapon (in atmosphere.)

        As an aside, lasers are interesting because they have negative thermodynamic temperature. This makes them “hotter” than any positive temperature, but explaining why without getting technical is a bit difficult.

      2. A laser is continuous, maybe I should not have used the word collimated, my mistake. Indeed, I am aware of the drawbacks of transmitting through a viscous medium, like air, but there have been experiments to show that sufficient packet of photons can be trapped, both longitudinally and transversally, to send a fair amount of energy to a distant load.

        Any thoughts on this paper? Again, sorry for my off topic questions.


  15. http://www.chem.ufl.edu/~itl/4411/change/C15F14.GIF

    The action is not in the hump, it’s in the tail, The tail’s effects are very sensitive to temperature because its population is small while its reach is large. Cold shuts things down and freezes water into shards of cell-bursting ice with its hump. Heat is much nastier for cooking things.with its long tail.

    Before venturing a cure, take care to identify the cause rather than the result.

    1. This is a good point; the departures from average, especially at the high end, are often very important, and they are relevant in many contexts. For example, they’re relevant in the sun, where fusion, which powers the sun’s furnace, occurs on the high end. However, that’s a more advanced topic, and this was a post aimed more at beginners.

  16. I understand the scientific concepts, but when I go out of my house in Minnesota and it’s 25 below or lower and the wind chill is 50 below, it’s cold.

  17. Hi Matt,
    I had always assumed that wind chill ,which meteorologists announce, was based on some biophysical experiments and theory about effect of wind on skin. I did not think it was that subjective. Human body temperature and skins are approximately same all over. Can you or someone make comments?

    1. There are two aspects to wind chill.
      The first is perceptual, and people have done experiments to create models for the effects of wind on temperature perception. These models apply to bare skin, and the variance between models is generally a few degrees.
      The second aspect of wind chill is that the incoming wind will be of constant temperature, and so change the cooling rate. The rate of cooling of an object is proportional to the difference in temperature between the object and the surrounding material. If you curl up wrapped in a blanket the air inside the blanket will heat up rather rapidly, and you can stay warm even when the outside air is quite cold. Even without the insulating blanket a small amount of the air around you will be warmer than the general outside air, so you will cool less quickly when there is no wind. When there is wind the air around you won’t significantly warm, because as soon as it warms it gets blown away. This principle is used extensively in engineering, forced air cooling systems such as a car radiator blow cool air over a heat exchanger to keep an object colder than it would be if air were not blown over the exchanger.

    1. The earth never left the last ice age, so a new one can’t begin. We look to be ahead of schedule to leave the current ice age, which will happen when the ice caps finish melting. It’s quite unlikely that the current interglacial period will end with a glacial period, it’s much more likely that the ice age will end completely. This will probably be rather bad for most humans, we evolved within an ice age and aren’t well adapted to historical conditions outside of them.

  18. I used to have delivered 25lt flasks of liquid nitrogen to my lab for super chilling PC chips to get some extra work out of them. The liquid felt cold on the skin but not what I would call super cold. I had to use a special technique to stop the liquid boiling on the faces of the chips.

    In some night clubs they use the same liquid for preparation of cocktails !! I could not believe this – but a famous incident in a UK Nightclub a very stupid barman made a raw cocktail out of liquid nitrogen – then gave it to a young girl 22 y.o. to drink. She liked it and ordered another one.

    The end result was an immediate hospital emergency and removal of what was left of her stomach. I think the supplier company whoever it was – probably BOC or Air Liquide should pay the extra millions of $ compensation for supplying a Night Club in the first place !!! As for the barman a term in prison including his manager and night club owners. Amazing how stupid people can be, and something else to worry about if you have teenage or young adult children who like to party.

    1. I disagree with you. Making sellers of raw materials liable for any mistakes their customers make in their use is a horrible and dangerous precedent to set. It would obligate them to deeply investigate anyone who wants to use their products, or even deny them to all but special corporate customers. That would have a paralyzing effect on small business. The responsible party is the bar and/or the bartender, who ought to have known better.

  19. Isn’t there a paradoxical effect, related to uncertainty, where if you try to localize a particle you can only pump more energy into it?

  20. Why cant U.S. switch to celsius scale once and for all and get rid of useless fahrenheit?

    1. In part, I think, because we’re the hyperpower. Power/success breeds intense (even irrational) conservatism. Also, we’re such a large and already-developed country that change is difficult. It’s easier to switch in smaller, more homogeneous countries (where consensus is easier to achieve) or countries with little development yet, where the change doesn’t have much impact.

      That said, I agree with you, and I try to use metric exclusively in daily life. People sometimes roll their eyes when I quote the temperature in Celsius, though!

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