Wikipedia:Reference desk/Archives/Science/2017 August 18

= August 18 =

Low pressure and high temperatures
I've been checking the weather report lately since the weather has been pretty hot this summer over here, and I've noticed that the heat waves were well-corellated with times of low atmospheric pressure. For example see. That seems to be the wrong way around to me, usually we get low pressure with storms, not 24-hour heat. What could cause this? — Preceding unsigned comment added by 78.0.231.47 (talk) 04:25, 18 August 2017 (UTC)
 * Cold front) and warm front have nice tab that will help you understand what's going on. wind direction and strength are also important in that matter.
 * As evidenced in those articles, the pressure in itself is not as important as pressure variation Gem fr (talk) 07:40, 18 August 2017 (UTC)


 * Looking at the temp and pressure diagrams at the first link, I don't see any correlation at all. The 2nd link does seem to show pressure peaking mid-morning each day, and of course temps peak mid-afternoon.  Not sure what would cause the pressure to behave like that.  StuRat (talk) 07:37, 18 August 2017 (UTC)
 * Temperature at high altitude lags behind surface temperature, so you generally get the coldest troposphere in mid-morning. Low temperature means high density, means more weight in that column of air, means more pressure at the surface. So it makes sense. Of course the effect isn't very large. There must be air moving in or out of the column, which happens by high-altitude wind changing with the time of day. PiusImpavidus (talk) 10:39, 19 August 2017 (UTC)
 * When the temperature is high, the density of the air is lower. The column of air above you weighs less, so the air pressure drops. If the pressure gets lower than in surrounding areas, the air will rise, clouds will form and the temperature will drop (during the day). But a lot depends on regional effects. When the pressure distribution is such that the wind comes from the continent, the air is dry, which mean less clouds and higher temperatures in summer. PiusImpavidus (talk) 10:04, 18 August 2017 (UTC)
 * oh dear. your are basically saying that higher temperature lower pressure. How wrong.
 * Ideal gas law P =(n/V)RT do not perfectly apply to air, but it does quite well, meaning, when temperature rises, pressure rises, too (opposite of what you said). The higher temperature air do not rise because of lower pressure, it rises because, for a given pressure, if T rises, then density (n/V) must goes down, hence Archimedes' principle applies and the parcel of air must go up. All that without any pressure effect. Gem fr (talk) 12:34, 18 August 2017 (UTC)
 * I know the ideal gas law. When the air heats up, it expands. The air isn't locked up in a bottle, it's free to expand and flows away horizontally at the tropopause, driven by a slight increase in pressure up there. As the density drops with rising temperature, the pressure difference between the surface and the tropopause (caused by gravity acting on the column of air) drops, so the surface pressure drops too. That's the pressure we measure. The reduced pressure at the surface causes horizontal inflow, continuing as a vertical flow from the surface to the tropopause. That's how Archimedes' principle drives convection. PiusImpavidus (talk) 10:39, 19 August 2017 (UTC)
 * Very complicated way to explain than an isobar process will bring a drop in pressure...which it will not, by definition.
 * I guess you'll explain likewise that vapor bubbles in a column of boiling water will reduce pressure at the bottom of the column? they won't, either. Actually they would slightly increase the pressure, because slightly rising the whole water surface height (hence pressure, since it depends on height)
 * Try again Gem fr (talk) 16:54, 22 August 2017 (UTC)
 * Vapour bubbles will push the surface up, but won't change the pressure at the bottom. You may turn part of the water into vapour, but the weight stays the same. Pressure is the weight of the water+vapour bubbles divided by surface area (+ pressure at the top surface), so boiling the water doesn't change the pressure at the bottom, until the water actually boils away. And all of this is about the average pressure at the bottom, as locally the pressure will change. Where it's heated, the pressure at the bottom drops, where it's not heated, pressure increases, just like in the atmosphere. If heating is uniform over the bottom area, you just get turbulence with random pressure variations.
 * In short, the average pressure doesn't change, but there are local changes. Where heating is going on, the pressure at low altitude decreases and at high altitude increases, where there's cooling, pressure at low altitude increases and at high altitude decreases. Just as the world-wide average air pressure is constant, but pressure can drop locally on a hot day. PiusImpavidus (talk) 13:50, 23 August 2017 (UTC)


 * Here's a better picture, the last 7 days. The temperature is red, air pressure is dotted gray. You can see how after 14th August temperature is still rising day-to-day, but air pressure has levelled off. I don't think it has much to do with fronts, the first day of that period there was a cold front, but there were no storms/rain on any other day. Even better, check out the relationship from the middle of this graph to the right . (78.0.231.47) — Preceding unsigned comment added by 93.139.110.217 (talk) 04:21, 19 August 2017 (UTC)
 * You didn't look at cold front, did you? Or did you find it lacking in some way? Gem fr (talk) 16:57, 22 August 2017 (UTC)
 * The cold front article states that after the front passes, the temperature steadily decreases and the pressure steadily increases. There are three such patterns, each lasting 2-3 days, on the 30-day picture here, but what interests me are the longer periods after the last two, where pressure decreases gradually while temperature increases, and some days the pressure is actually lowest right when the temprature peaks. (78.0.231.47) 78.0.246.130 (talk) 18:32, 23 August 2017 (UTC)

Safe not-quite-total eclipse viewing
Is it safe to view the eclipse when it is not quite total for a few seconds? Here is what it will be like in Atlanta - very close to total coverage. Bubba73 You talkin' to me? 06:00, 18 August 2017 (UTC)


 * See here. Count Iblis (talk) 07:30, 18 August 2017 (UTC)
 * And here. tl;dr: If you have to ask, the answer is "no". --47.138.161.183 (talk) 08:09, 18 August 2017 (UTC)


 * [[Image:IMG 1650 zonsverduistering Malta.JPG|thumb|A solar eclipse viewed with a natural camera obscura.]]During a partial eclipse the eye adapts to the lower light level, but the surface brightness of the sun stays the same. That makes looking at the sun even more dangerous than on normal days. So always use a good filter or project the sun on a screen using a small telescope or a camera obscura. I've always preferred projecting with a small telescope. PiusImpavidus (talk) 10:19, 18 August 2017 (UTC)


 * IT IS NOT SAFE. Newscasters have been pointing this out, over and over, and people still don't pay attention. You risk destroying your retina; permanent blindness. To view the partially eclipsed sun safely, go to your eye clinic, or wherever, and get some ISO certified eclipse glasses. Beware of fakes! Or better yet, use one of the methods describe above to project the sun's image. And if you're wearing solar eclipse glasses, do not look through binoculars or telescope - the focusing of the bright light will nullify the effect of the glasses and you'll be blinded. For further details, look for instructions on a TV news website in your area. ←Baseball Bugs What's up, Doc? carrots→ 14:28, 18 August 2017 (UTC)


 * I was at the 1984 annular eclipse and a professional astronomer who was there said that it was safe to look at it for 5 seconds when the Moon was almost covering the Sun. So that isn't true?  Bubba73 You talkin' to me? 18:33, 18 August 2017 (UTC)
 * Five seconds is way too long. Consider how long you can tolerate the full sun before you instinctively avert your eyes and (hopefully) suffer no permanent damage. It's like a fraction of a second. ←Baseball Bugs What's up, Doc? carrots→ 23:24, 18 August 2017 (UTC)
 * I've read that you need to cut out 99.99% of the light. This annular eclipse was close to total (for 11 seconds).  Bubba73 You talkin' to me? 00:01, 19 August 2017 (UTC)


 * From the link I gave above: "You have likely heard that looking at a total eclipse can be dangerous. It indeed can be. And this is because the pupil responds to average light. During a total eclipse, the pupil swells to its full size owing to the twilight's low average intensity. Its diameter swells to about 7mm: so the aperture has fifty times the area it has just before the eclipse begins. If you look at the diamond ring just after totality, you can therefore cop a dose of 20mW20mW or so in the eye. This can be enough to cause thermal damage. Evolution didn't kit us out to look at total eclipses". Count Iblis (talk) 20:05, 18 August 2017 (UTC)
 * It's not the total eclipse that's the problem, it's when it's not total. The advice I've read is that when the first glint of light appears, avert your gazeimmediately. Then if you want to watch, put your solar eclipse glasses back on. The only time it's safe to not wear those glasses is during totality. ←Baseball Bugs What's up, Doc? carrots→ 23:23, 18 August 2017 (UTC)

Bubba73 You talkin' to me? 00:30, 19 August 2017 (UTC)

Cloudy days and humidity.
I heard in the winter time, cloudy days are your friend. That is, clouds trap heat? Keeping the temperature warmer, so on a sunny day with no clouds, temperatures are generally colder?

My 2nd question is, are summer time or winter time more humid? If it matters, for Chicago. Thanks. 12.130.157.65 (talk) 11:36, 18 August 2017 (UTC).
 * Clouds radiate infrared radiation back down to Earth, so the maintain higher night time temperature. Absolute humidity is normally higher in summer. But the hot summer day peaks probably have the lowest relative humidity. I don't know about Chicago. Graeme Bartlett (talk) 12:55, 18 August 2017 (UTC)


 * lapse rate is your friend on the first matter. Basically, humid air has more energy than dry air (the energy required to turn water into vapor), energy released when vapor turn again into liquid, that is, into clouds. This release of energy warms the air, so it is less cold.
 * humidity is the answer (or, at least, an entry point for other articles linked in) to your second. You'll notice that it refers to three kind of humidity: absolute, relative and specific.
 * Gem fr (talk) 13:03, 18 August 2017 (UTC)


 * Clouds make the night warmer and the day cooler, on average. That's for multiple reasons.  1) Clouds are droplets of liquid water, and liquid water has a very high specific heat.  That means it can absorb a lot of energy without changes of temperature, so water has a mediating effect on temperature changes; it both cools off slower and heats up slower.  That makes for lower highs and higher lows.  2) Clouds tend to reflect radiant heat; that is light energy in the infrared range.  That means that clouds during the day tend to reflect sunlight back into space, keeping the ground from getting too hot.  At night, when the sun isn't out, the clouds will reflect heat radiating from the ground back towards the ground.  3) Evaporation is an endothermic process, so it tends to remove heat from the environment, while condensation is exothermic, adding heat to the environment.  As temperatures rise during the day, clouds can absorb the extra heat of the sun by evaporating (that is, going from tiny droplets to water vapor).  At night, condensation takes over, which means the opposite is happening.  Wikipedia has an article called Cloud feedback which, ideally, would explain this better, but it's a bit hard to follow (not well written), but this explains it a little bit.  -- Jayron 32 13:15, 18 August 2017 (UTC)


 * Clouds reduce cooling during the night by radiating heat back the surface and reduce heating during the day by reflecting light back to space. It depends a bit on the type of cloud. Cirrostratus clouds are almost transparent to visible light but not to infrared, so they mostly affect night time temperature.
 * There's also a second way that clouds are correlated with temperature. You get more clouds when the wind comes from the ocean because of higher water content and this obviously correlates with temperature too: in summer, sea air is cooler than land air, in winter it's warmer. So in winter on a cloudy day, you probably have a (relatively) warm wind from the sea. Whether this is enough to make cloudy days warmer than sunny days in winter, despite lower insolation, depends on where you live. PiusImpavidus (talk) 11:24, 19 August 2017 (UTC)
 * Okay looks like the answer is yes and no for winter. Yes in winter because cloudy days means warm air from the sea, no because clouds reflect sunlight and heat back to space. By the way, when you say cirrostratus clouds are most transparent to visible light but not IR, what type of cloud is the most opposite? The most effective to IR? Thanks. 12.130.157.65 (talk) 16:37, 21 August 2017 (UTC).

Role of the client in large infrastructure projects
In a typical large infrastructure project where most of the accountability is on the designers and construction companies, what is the role of the client? Is it mainly to specify the end product and monitor/regulate the project? 82.132.218.121 (talk) 12:06, 18 August 2017 (UTC)
 * The humanities reference desk may be more appropriate for this question. — Paleo  Neonate  – 12:31, 18 August 2017 (UTC)
 * it is mainly to chose people that have the known-how to specify the end product and to run the project (may be different people for those two actions), so that he will be happy and builders will be able to do it, and will indeed do it, preferably on schedule.
 * Design–bid–build seems a good entry, with lots of links to other relevant articles Gem fr (talk) 12:45, 18 August 2017 (UTC)
 * A large project will have a complex client team that includes people that specify the requirement in detail, evaluate the proposals, finance people, testers to check that requirements are met, liaison people to answer questions, internal stakeholder relationship management, and possibly a media team. Graeme Bartlett (talk) 12:52, 18 August 2017 (UTC)

What is the ultimate cause of seasons?
I already know that seasons are caused by the Earth's axial tilt, but how, precisely? Most diagrams depict the Sun's rays shining parallel on Earth to illustrate how the direction it faces matters, but I don't think this is satisfactory. First of all the parallel rays don't explain why some places are hotter than others, since the radiation carries pretty much as much energy reaching the poles (further away from the Sun) than the equator (closer to the Sun), made evident by the fact that the elliptical orbit doesn't affect seasons. Secondly it's an inaccurate illustration. The Sun is larger than the Earth and rays can come in from different directions (which is why solar eclipses occur). So after all this, how does axial tilt lead to seasons? I feel like I've always been told axial tilt = seasons, but something's been skipped in between. Currently I think it is simply down to the centre of the sunlit area being surrounded by other sunlit areas so it doesn't lose heat, whereas towards the edge of the sunlit zone heat is lost to the areas experiencing night time. Is this an adequate explanation? Please correct me if I'm wrong. The Average Wikipedian (talk) 15:24, 18 August 2017 (UTC)
 * Have a look at the article Solar irradiance, in particular the sections on projection effect and absorption effect. --Wrongfilter (talk) 15:45, 18 August 2017 (UTC)


 * It's easiest to think about it in terms of what the Sun looks like as viewed from the Earth. Where I live in California, for example, on Dec 22 the Sun is above the horizon for about 10 hours and reaches a maximum angle of about 30 degrees. On June 22 it is above the horizon for 14 hours and reaches a maximum angle of close to 90 degrees.  The result is far more solar heating during summer than winter. Looie496 (talk) 15:49, 18 August 2017 (UTC)


 * It's true that the equator is closer to the Sun than either of the poles is, but the radius of the Earth 6371 km is so tiny (0.004%) compared with the average distance to the Sun 150 000 000 km (or 1 AU) that this is not enough difference to explain the seasons. The article Season explains how the combination of axis tilt (see diagram) and Earth's elliptical orbit together contribute to the cycle of seasons. Blooteuth (talk) 15:52, 18 August 2017 (UTC)


 * The incoming energy is more spread out when it comes in at an oblique angle. You can convince yourself of this with a flashlight. If you want to brightly illuminate a spot on the wall, you do it straight on. If you hold the flashlight the same distance away from the spot, but closer to the edge of the wall it won't be nearly as effective. ApLundell (talk) 15:56, 18 August 2017 (UTC)


 * [Edit Conflict] You are wrong.
 * Firstly, you have ignored the fact that towards higher latitudes the Earth's surface is angled more obliquely to the direction of sunlight, so that (for example) a square metre of sunlight shining vertically on the sub-solar point at noon indeed illuminates one square metre, but the same area/amount of sunlight shining on the tilted surface at high latitudes is spread out over a significantly greater area of surface. This spreading effect varies through the year (because of the constant tilt of the earth's axis): when the tilt effect is at its greatest, the tilt also means that days in that hemisphere are significantly shorter than nights, so the sunlight also shines (and heats) for a shorter time.
 * Secondly, the illustration has to be "inaccurate" to some degree, because it would be impossible to represent the Sun, Earth and the distance between them all to scale on the same page. (For example, the average Sun–Earth distance is about 13,300 times the diameter of the Earth — I have had to wrestle with this problem in editing illustrated science textbooks!) Because of its distance, the Sun, despite being very large, has an angular diameter of only around half a degree, which is close enough to being a point source to make no significant difference in seasonal terms. {The poster formerly known as 87.81.230.195} 94.12.90.255 (talk) 16:09, 18 August 2017 (UTC)


 * In addition to the insolation angle, there's also just more hours of sunlight when your part of the Earth is tilted towards the Sun, and thus more heat accumulates during those times and there are fewer hours at night for the heat to dissipate into space. Note that the difference in number of hours of sunlight is far more dramatic at the poles, hence more difference from summer to winter. StuRat (talk) 16:16, 18 August 2017 (UTC)


 * To somewhat avoid technical terms, just think about where the Sun seems to be in the sky. In the far north, it seems far south, and in the far south, it seems far north - in both cases, near the horizon.  Now it is TRUE that people in those places see the same Sun, the same size (more or less) as they do at the equator.  And it is TRUE that the light and warmth provided if you hold your hand up to the Sun is about the same - just a bit less because the light has to go through extra air when the sun is near the horizon.  So why the difference?


 * Well, consider if you wanted to lay out solar panels on the ground. At the equator, you could lay them out flat and at noon they would all get the absolute maximum energy, intercepting their full area of sunlight.  But in Greenland, you would have to tip each one high up to catch the same amount of light and produce the same amount of electricity or hot shower water.  Now if you have enough space going east to west you can always put them side by side, tilted or not, at least at noon.  But if you don't, and you have to put some north and south of the others -- here's the difference:  you can't put them at the same distance apart as you would at the equator, because the shadow of the south solar panel would block most of the one to the north.  You have to put them much further apart to collect all the energy.  And that reflects the fact that the entire surface of the Earth in the area, like a tilted solar panel, just doesn't block all the light or collect all the energy that one gets when it faces full into the sun. Wnt (talk) 17:46, 18 August 2017 (UTC)

Does relying on refined cooking oil explain why we cannot get EPA and DHA from plant sources?
It is well known that the conversion rate of Omega-3 to EPA and DHA is too low to supply us with the required amounts. However, it could be that this is only what happens in people who eat a normal diet who then get 50% or more of their calories from refined fats and oils that contain very low amounts of Omega-3 fats. A natural diet that doesn't include any refined fats and oils would yield only about 15% of calories from fat, and most of that would be Omega-3 and Omega-6 fats. This doesn't seem to be relevant to the low conversion rate, however, when sticking to such a diet on the long term, most of the body fat would become the Omega-3 and Omega-6 fats that you would then eat. If in the natural context Omega-3 is present in high concentrations in fat tissue, it seems to me that it's there that the enzymes that convert Omega-3 to EPA and DHA should be present. So, the reason that the reaction rate is low would be because we've filled the reaction chamber with junk. Count Iblis (talk) 22:30, 18 August 2017 (UTC)


 * Think I can see were your coming from. It is the ratio of all these fatty acids which is important. So if one's diet is short of one or more of these essential fatty acids (those which humans can not make) then health will suffer. In this case, Junk is getting too many calories from foods which don't don't supply essential fatty acids, vitamins and minerals. P.S. Do you know that the food refiners that remove those very oils you mention, (for the reason is that they go rancid quickly and shorten the shelf-life of modern cooking oils), go on to sell what they have removed, to companies that reformulate them as health supplements. To be bought at exorbitant prices to make good for what one looses in order to be able to buy long shelf-life cooking oils. Madness! Aspro (talk) 23:08, 18 August 2017 (UTC)
 * Citation? Commercially available "omega-3 supplements" are generally fish oil, krill oil, and flaxseed oil, none of which are widely used as cooking oils because of their tendency to rancidify, among other things. --47.138.161.183 (talk) 06:34, 20 August 2017 (UTC)
 * I see this is mentioned in a section at alpha-Linolenic acid. I haven't looked into it as of yet, but the description is actually rather disturbing  - there is talk of making linolenic acid free soybeans to make the oil more desirable for cooking so it doesn't "have" to be partially hydrogenated.  Note that this runs contrary to the doctrine, frequently espoused in Wikipedia controversies over GMO food, that GMO food is nutritionally equivalent to non-GMO food.  Something similar has been attempted by proxy with omega-3 enriched canola oil for use in fish farms, as I recall.  After the Holocaust-level casualties involved with humanity's first attempt to tinker with fats for convenience (i.e. partial hydrogenation), I would have thought people would be more careful. Wnt (talk) 11:40, 20 August 2017 (UTC)
 * Except that as the very article you linked to says, (I know because I read it before you posted when I linked to it below), this isn't something unique to GMO. It's possible of course to significantly reduce levels levels of omega 3 in soy beans without relying on GMO and this has been done. (Whether this relied in nuking the hell out of the crop and hoping for the best, I don't know.) Not surprising since we humans have been tinkering with our crops and animals for millennia, including with effects on the fats, and often for various reasons of convience, just with a lot less specificity and idea of what we were doing. Nil Einne (talk) 12:55, 20 August 2017 (UTC)
 * No, you can't get EPA and DHA from plants because they're not made by most plants. As the articles state, they're only found in large amounts in fish, seaweed, and algae, as well as, interestingly enough, human breast milk. --47.138.161.183 (talk) 06:34, 20 August 2017 (UTC)
 * I don't think the OP (no idea about Aspro, I didn't bother to read what they said since I've found it best to ignore anything dietary related coming from them) is suggesting you can get EPA and DHA directly from plant oils. Rather they are suggesting that the low levels of conversion of α-Linolenic acid to EPA and onwards to DHA is somehow because of excessive consumption of fats and oils, particularly refined fats and oils. This isn't something I've looked at that well but the earlier linked articles (both mine and yours) along with Omega-3 fatty acid support the view that strictly speaking neither EPA or DHA are essential fatty acids in humans since even though they are important in human physiology, they can be synthesised from ALA which is an essential fatty acids in humans. However the conversion rates are fairly low. There's some limited disputed evidence that the high ratio of Omega-6 fatty acids compared to omega-3 in modern Western diets may be not ideal. See also Essential fatty acid interactions.  But even if this is true, I think it's fairly misleading to suggest it's just because of refined oils. The seem to be a multitude of reasons for this which relate to modern industrial and agricultural practices and the sources we've chosen for eating and feeding (e.g. grain instead of grass for feedint cattle in many places). This includes the fact that the increased rancidification rate means omega-3 is not desirable when it comes to long term storage. But breeding plants for lower levels of omega-3 isn't directly part of refinement, even if perhaps part of the reason it's done is because of the benefit to refined oils. (In other words, you could probably still have poor levels of omega-3 depending on your diet even if you had low levels of refined oil consumption and you could probably still have decent ratios despite high levels of refined oils.)  And notably I don't think it's clear that the levels of oil consumption, as opposed to the ratio, is a problem. And this seemed to be what CI was suggesting. (Note that this could easily be seperate from whether or not high levels of dietary fat are undesirable.) Note also even if the ratio of omega-3 to 6 in modern western diets is a problem, I don't think it's clear that this is because of the effect on levels of EPA and DHA. Nor of course that EPA or DHA levels either from dietary consumption or conversion are a problem. In other words, I agree with you that CI's premise seems to be basically unsupported by anything (as seems to often be the case when CI brings up dietary issues) but not quite on the reasons for that.  BTW AFAIK and supported by both our article and   ALA does significantly come from (certain) plant sources.  Nil Einne (talk) 11:10, 20 August 2017 (UTC)