Wikipedia:Reference desk/Archives/Science/2019 September 4

= September 4 =

obesity with brown fat
Are there people who are at what would be considered an obese weight given their height a(and amount of muscle) so that the extra weight is indeed from fat, but the fat is predominantly brown fat?Rich (talk) 03:07, 4 September 2019 (UTC)
 * perhaps sumo wrestlers?Rich (talk) 03:09, 4 September 2019 (UTC)
 * Brown fat is stimulated by cold exposure and excessive exercise. Therefore, it is expected to be increased in poeople who experience those situations. Inuit are an example of people with documented increase in brown fat stores. Another is football linemen. According to what I've read, both groups of people suffer from severe increases in cardiovascular disease if removed from the environment that promotes brown fat growth. 135.84.167.41 (talk) 11:47, 4 September 2019 (UTC)

Mars zero elevation
Geography_of_Mars says that zero-altitude used to be set at an elevation corresponding to a pressure (theoretical calculated mean pressure?) of 6.105 mbar but that in 2001 it was set to "the equipotential surface (gravitational plus rotational) whose average value at the equator is equal to the mean radius of the planet". By how many meters did this zero elevation change and by how much does that change vary from place to place? If it does vary then I'd love to see a map similar to Mars_topography_(MOLA_dataset)_with_poles_HiRes from Geography_of_Mars but instead showing the difference in elevation between the two definitions.173.72.209.196 (talk) 13:53, 4 September 2019 (UTC)


 * Any variance would likely be dynamic, due to temperature changes between night and day. SinisterLefty (talk) 16:01, 4 September 2019 (UTC)
 * I rather doubt the pre-2001 datum fluctuated with the weather but that it was instead set based on a fixed planet-wide model of mean atmospheric pressure. That is part of what I am trying to figure out.173.72.209.196 (talk) 17:13, 4 September 2019 (UTC)


 * Mars says temps vary from −143°C to 35 °C, and it's hard to imagine that doesn't have a major effect on the pressure of the atmosphere. See ideal gas law. The warm volume could also expand up into space, or winds could blow the warm atmosphere around to the dark side, but all this would take time. Hence, pressure would still increase in the sunlight. SinisterLefty (talk) 17:25, 4 September 2019 (UTC)
 * Are you seriously claiming that a dynamic planetary datum was used such that the elevation of a particular location changed based on the weather? I'm not saying that the atmospheric pressure on Mars is static. It does change and to a much greater degree than you suggest. From Atmosphere_of_Mars: "In summer, the polar dry ice cap can melt and release the CO2 back to the atmosphere. As a result, significant annual variability in atmospheric pressure (~25%) and atmospheric composition can be observed on Mars." What I am saying is that the only reasonable way I can see to interpret the the pre-2001 datum "... arbitrarily defined in terms of a constant atmospheric pressure. From the Mariner 9 mission up until 2001, this was chosen as 610.5 Pa ..." is that it was fixed and based on the presumably average seasonal pressure of some atmospheric model. And what I am asking for is the vertical datum shift from the pre-2001 pressure based datum to the post-2001 MOLA based datum.173.72.209.196 (talk) 21:27, 4 September 2019 (UTC)
 * It's fruitless to discuss such specific details unless we can name the models, cite the published sources, and so on. Somebody, somewhere, may very well have defined a datum using such an esoteric methodology; but that's not the usual way we define the reference datum for Mars (or any other planet or celestial body that educated Earthlings choose to investigate).
 * The most commonly-used reference datum for Mars is the JPL model - the DE431 for the orbital stuff, and the IAU/USGS reference ellipsoid for the planet's shape; e.g. the method and the numbers promulgated in this 2002 paper (or any newer version - it really depends on when you're going to Mars...) And if you actually study data from a specific spacecraft, you almost surely want to use whatever datum the spacecraft science team chose to use.  Consult your individual spacecraft's scientific data product documentation!  (If I had a dollar for every time I had to shout that at an intern...)
 * We discussed this topic on the Science Desk back in October because somebody was looking for a specific crater location, to within ±10 meters.
 * Nimur (talk) 07:00, 5 September 2019 (UTC)
 * Thank you for helping bring this back on track. (I fixed your second link to point where I think you intended.) From that 2004 paper: "... a spherical-harmonic representation of an equipotential surface will be used as the reference for elevations." And presumably there were earlier function describing a nominal 6.105 mbar surface for use with pre-2001 datums, but while I've found the paper you linked and others I've not been successful in finding a detailed description of the earlier elevation schemes or the effect of the change which is why I am asking on the reference desk. Note one significant difference between my question and one regarding location of a feature: While there have been refinements in coordinate systems they have been pretty much the same since 1972 and changes such as going from defining the zero longitude as that which passes through the center of the Airy-0 crater in a crater to that which places the Viking 1 landing site at exactly -47.95137 still had zero longitude passing though the center of Airy-0 just with a precision greater than the center of crater can be specified. But the 2001 zero elevation surface change was a systematic shift from one with a nominal atmospheric pressure equal to that of triple point water to an equipotential one with average value at the equator equal to the mean radius of the planet. That is equivalent to a hypothetical systematic shift of redefining the coordinate system so that zero latitude now passed through the Viking 1 landing site. Had the spacecraft happened to have landed at the meridian of Airy-0 the systematic change would have coincidentally been just a refinement but with such a scheme based on the actual landing location the answer to a query about its effect on coordinates would be that it uniformly shifted longitude values by about 48 degrees. Likewise the systematic change for the vertical datum might coincidentally have been a refinement but it could also have changed altitudes by tens, hundreds, or even thousands of meters for all I know, and that change might have been largely uniform across the globe or might have been highly varied. That is what I'm trying to learn. -- 173.72.213.109 (talk) 09:53, 7 September 2019 (UTC)

At what wind speeds will the sea surface disappear?
Wind causes waves and high waves will break. At the regions where waves break, you'll have many air bubbles in the water and many water droplets in the air, so there the water surface will have disappeared. At what wind speed will the entire sea surface disappear? Count Iblis (talk) 14:11, 4 September 2019 (UTC)
 * Jesus, for surface to disappear, you are talking of a wind strong enough to literally blow in the air the whole 1.35 billion cubic kilometers of ocean, which also imply rising it by a few km. Gem fr (talk) 14:58, 4 September 2019 (UTC)


 * Not really. The surface will just cease to be a sharp point and become more of a gradual transition, similar to the change from atmosphere to space. The 400 mile per hour winds on Jupiter, etc., might make that happen, between the gas and liquid components. But it also would be a gradual transition from sharp surface to fuzzy, so hard to define an exact wind speed point of transition. SinisterLefty (talk) 15:51, 4 September 2019 (UTC)
 * You mean, surface tension will just disappear, the air+water fluid will turn supercritical as if we were past critical point? No. That not gonna happen Gem fr (talk) 18:19, 4 September 2019 (UTC)


 * Think of the contents of a powerful blender on high. Can you draw a line above which is air and below which is liquid ? More likely there's an indeterminate layer of foam. SinisterLefty (talk) 18:55, 4 September 2019 (UTC)
 * Well, would be a surface not a line, but YES, I quite easily can draw a line below which is liquid, maybe with some bubbles. Doesn't matter if above is not just air, but foam or whatever. Gem fr (talk) 21:41, 4 September 2019 (UTC)


 * The article Wind wave gives a table of sizes of waves caused by winds of a range of speeds and durations enough to fully develop a wave but not to break the surface. Winds exceeding the strengths, durations and fetch distances given cause the wave tops to form "whitecaps" that break the surface. Waves necessarily break when they encounter shallow water, see Wind wave and Breaking wave. 84.209.119.241 (talk) 15:43, 4 September 2019 (UTC)


 * Note that you might also get a thick layer of sea foam between the water and air, if the water contains the right impurities. SinisterLefty (talk) 17:31, 4 September 2019 (UTC)


 * I think we can address the question theoretically; and we can address it practically; but I doubt either approach will give a very satisfying answer, because the topic is quite complicated.
 * The mathematical modeling for the dynamics of the boundary between a liquid and vapor is very complicated. Wikipedia has articles on cavitation modelling and free surface dynamics; these can point you in the right direction toward the equations of fluid mechanics that one might apply to address this topic; but as you surely know, turbulent and convective fluid dynamics are very hard to model analytically or numerically.  There isn't going to be a simple, well-behaved, universally-applicable model with a nice, sharply-defined threshold or cut-off wind velocity at which the phenomenon is sharply demarcated.  We might use a simplified model, like a perfect laminar flow over an infinite extent of water, and equate the Venturi effect pressure drop (using the Bernoulli's principle) to the vapor pressure of seawater at standard conditions for temperature and pressure... and we could determine an equilibrium airspeed for any specific vaporization value; but, this theoretical model is not the only one we might use; it totally neglects very important effects pertaining to convection and turbulence; and there are many reasons why this, or any other model, would be inapt for the task.  The ocean is large, but it is not really an ideal infinite isotropic laboratory thought-experiment.
 * From a practical perspective, colored by some real-world knowledge from some nerds I know who spend a lot of time at the sea-air boundary: I recall when I learned to sail a tech dinghy that whitecaps form at a sustained wind speed of around 15 knots. That means the steady wind is creating and sustaining steady-state sea-surface waves with foam bubbles on them.  This is also the airspeed at which a standard, properly-calibrated wind sock (like the ones they sell at Sporty's) will become fully extended.  This is also a good, albeit conservative, estimate of when you will have "too much wind" for sporting activities, (±15%, of course, as any rule-of-thumb goes).  It turns out that this is a very old and well-known rule of thumb. Above 64 knots, on the Beaufort scale, we say that the air is "filled with foam," and the surface of the sea is difficult to see because "visibility [is] greatly reduced."  This is not the most modern method, but it is a commonplace and famous one that appears in textbooks; indeed, it is a way of defining the wind-speed based on what symptoms we see, rather than predicting what we might see at a given air-speed if we actually measured the air speed using some type of scientific instrument.  Oh, how science of meteorology has been irreparably altered by the science of metrology!
 * Again, from a practical view, in standard weather reports, we consider "visibility" to be a distinct and orthogonal item in a weather report: we can have observable mist at any air temperature and at any wind speed; physically, the cause of that mist might be wind that kicks up surface water; or heat that evaporates it; or cold that condenses it; or precipitation that permeates the entire area; and practically, we don't distinguish the cause of the mist; instead, we qualify it by how much it impacts visibility.
 * Nimur (talk) 06:32, 5 September 2019 (UTC)


 * This question will not admit a definite answer without a chosen distance above sea level.--Jasper Deng (talk) 06:37, 5 September 2019 (UTC)
 * Not to mention other mandatory specifications for how the water shall be suspended in the air - as vapor, as colloidal suspension/entrainment; as a dynamic or as a steady-state condition; in what ratio, and in what absolute quantities; and so on, ad infinitum... Nimur (talk) 06:42, 5 September 2019 (UTC)

Why do we find the urge to urinate sometimes increases drastically with proximity to a loo
Is there any research that shows that when peeing is convenient, the urge to pee increases? Bogger (talk) 19:06, 4 September 2019 (UTC)


 * Classical conditioning is a learning procedure in which a biologically potent stimulus (e.g. the need to urinate) is paired with a previously neutral stimulus (e.g. the proximity of the loo). As for why we evolved this link, imagine a group of migrating people stopping for a rest. They might designate an area for urination and defecation, and it would be important to go then and there, not once the migration resumed, as that would either slow down the group or they would leave you behind, and that could be deadly. SinisterLefty (talk) 19:09, 4 September 2019 (UTC)

State-dependent memory is also important in this form of conditioning. Yanping Nora Soong (talk) 23:44, 4 September 2019 (UTC)

Ostwald Ripening In Blood
What would be the characteristics of the compound you must add to blood (acidic, polar, surfactant, etc) to observe Ostwald Ripening in the blood/compound mixture?

65.207.73.130 (talk) 20:29, 4 September 2019 (UTC)


 * Not that I have the answer for any possibilities on hand, but it would depend on which solid you're hoping to crystallize. Someguy1221 (talk) 07:01, 5 September 2019 (UTC)