Wikipedia:Reference desk/Archives/Science/2022 June 26

= June 26 =

Water lilies
Are water lilies edible? If so, have there ever been attempts to cultivate them for food, or is this only relevant in the context of wilderness survival? 2601:646:8A81:6070:D151:4B2:4CF:6657 (talk) 01:37, 26 June 2022 (UTC)


 * I found that the seeds and flowers and rhizomes of the Fragrant water-lily Nymphaea odorata are eatable (traditionally by N American tribes) and the roots are used medicinally as a tea to treat coughs and diarrhea, but my Peterson Field Guide warns that large doses can be toxic. Modocc (talk) 02:20, 26 June 2022 (UTC)


 * I've eaten Lotus Root Chips (Americans would probably call them fries) "served with lemon & herb seasoning and a side of Wasabi mayo" at the Blue Lotus Water Garden, a tourist attraction not far from where I live in Australia. But I have now discovered that lotuses are no longer considered to be water lilies. So maybe not so helpful. HiLo48 (talk) 02:34, 26 June 2022 (UTC)


 * Our Lotus-eaters article has water lilies as a possible candidate for the eponymous lotus plant that they are supposed to have lived on. Greek mythology is not a good source for survival skills however. Alansplodge (talk) 07:31, 26 June 2022 (UTC)


 * So, some are edible and some are not -- got it! So, one of 2 questions answered -- the other one being, whether water lilies have been cultivated for food?  (And yes, for the purpose of this question, all 3 plant families on the dab page are considered water lilies, so lotuses do count!) 2601:646:8A81:6070:4C2C:9973:74AD:CB94 (talk) 07:15, 27 June 2022 (UTC)
 * The N American tribes knew how to cultivate plants but had their lands taken from them. According to Peterson, the Ojibwa ate the fragrant water-lily's flower bud which has been attributed to preventing scurvy among them. Modocc (talk) 11:04, 27 June 2022 (UTC)
 * Nymphaea nouchali is cultivated for food in Sri Lanka. Modocc (talk) 14:42, 27 June 2022 (UTC)
 * Thanks! So the answer to the second question is also yes -- they have been occasionally cultivated for food (which makes sense, in swampy areas they could be grown instead of potatoes)! 2601:646:8A81:6070:6DDC:7C7B:BDD3:2E97 (talk) 01:19, 28 June 2022 (UTC)

I wonder did you see this video? Regards. Martinevans123 (talk) 14:47, 27 June 2022 (UTC)


 * It's my understanding that unchecked and abandoned cultivars (if such water lilies exist/ed) can revert back to their wild forms so as to become inedible, which might explain any differences between the historical and contemporary accounts assuming they are accurate. Modocc (talk) 17:05, 27 June 2022 (UTC)
 * From what has been said above, some wild species of water lily are indeed edible, and there has been cultivation but not selective breeding of these, so the question of cultivars vs. wild forms is not applicable here. 2601:646:8A81:6070:6DDC:7C7B:BDD3:2E97 (talk) 01:19, 28 June 2022 (UTC)
 * Selective breeding is ancient and the fragrant water lily has not been completely free of toxin. Moreover cultivars can typically  cross-pollinate with wild varieties so the cultivated plants are lost due to natural selection. In others words, little Fido is unlikely to survive in the wild. Like I said, the natives lost their traditional productive lands. Perhaps genetic studies will reveal lost subspecies/cultivars. My field guide is both new and a recent edition but it focuses on the plants' medicinal qualities, so I cannot confirm the sources that our articles are relying on. Modocc (talk) 01:49, 28 June 2022 (UTC)

What's the lowest frequency broadcast by a skyscraper antenna?
The 129 meter tall World Trade Center antenna has VHF and UHF but not AM MF for some reason, maybe it'd be too impractical. Sagittarian Milky Way (talk) 08:13, 26 June 2022 (UTC)


 * There's an anticorrelation between AM and VHF-UHF, but it's not a hard rule: aviation comms are AM in the VHF band. But the LF-MF-HF broadcasters are going out. Lower frequencies have longer range, which is nice if your target audience is spread over a large area, but it also means there's only room for a limited number of transmitters in the world, or they would interfere. What's more, the available bandwidth is limited. Nowadays, if a radio station wants to reach people far away, they offer a live stream on the web. Ships and aircraft use satellite. The only real use left for lower frequency AM broadcasting is to spread independent news or propaganda to unfriendly countries.
 * There's only a small number of lower frequency transmitters as their range is very large. They need tall antennas and cause significant interference with nearby electrical equipment. Putting them on a skyscraper may not be so practical. PiusImpavidus (talk) 09:31, 26 June 2022 (UTC)
 * Maybe in some countries but in the United States there's so many stations between 1710 AM and 540 AM it's near capacity and every American radio can receive. A big metro area has dozens of them. It's used by people who want to watch sports games or talk shows but can't, old people, and a fairly popular far right (ish) radio network with sometimes offensive content like playing Barack the Magic Negro all the time (there's also National Public Radio for the left to center (public as in non-commercial donation-funded, not public like the BBC, though that's retransmitted in U.S. MF too I think)). Due to greatly increased ionospheric range at night only a few stations per clear channel per continent can use more than a few watts at night and those must use 10-50kW 24/7 so they can be heard far enough to get the special license. But if you want to follow a major sports team in a mundane game within a thousand kilometers of them while driving at night then you probably can. Sagittarian Milky Way (talk) 12:10, 26 June 2022 (UTC)
 * The aerials for lower frequencies are proportional to the wavelength. Droitwitch (which transmits at 198 kHz in the long wave band) has an aerial which is 590 foot long for example.  You'd need a pair of skyscrapers with the aerial stretched between them for LW! Martin of Sheffield (talk) 10:00, 26 June 2022 (UTC)
 * Do you know why parts of the world still have LW AM but not Oceania or the Americas? Those frequencies are used for extra non-directional beacon channels here. We do have a LW station or two in the States for atomic clocks but you probably can't hear LW on most of them so most US radios only go down to about 525 kHz. Sagittarian Milky Way (talk) 12:10, 26 June 2022 (UTC)
 * Well one of the reasons is to carry the shipping forecast. LW reaches far out to sea and is invaluable as a adjunct to NAVTEX, or for small boats as the only source.  198 kHz (used to be 200kHz) has carried shipping forecasts since 1911. Martin of Sheffield (talk) 13:12, 26 June 2022 (UTC)
 * I've heard about it. Never having been lucky enough to sail I don't know what's the American equivalent, I know there's frequencies for maritime use only. Our popular 60 kHz clock station is apparently not intended for humans, so the clocks don't let you hear the station. Sagittarian Milky Way (talk) 14:16, 26 June 2022 (UTC)
 * Because Great Britain and Ireland are small-ish islands, the BBC broadcasts of the Met Office's Shipping Forecast for the sea areas adjacent to land, and the "inshore waters" addendum that circles the British coastline in segments, are of interest to a great many landlubbers in the "NW Europe archipelago". Some of the main forecast's areas extend to the Atlantic, North Sea, Channel and Biscay coastlines of (South East) Iceland, Norway, Denmark, Germany, The Netherlands, Belgium, France and Spain, so there may be some interested non-mariner listeners there, too. {The poster formerly known as 87.81.230.195} 90.201.73.76 (talk) 19:18, 26 June 2022 (UTC)


 * However, BBC Radio 4's long wave goodbye says that the world's supply of giant glass valves has been bought up to keep the Droitwich transmitter operating, and when the last of those breaks, BBC Radio 4's long wave transmissions in England and Wales will cease (estimated at 10 years in the article published 11 years ago). Scotland and Northern Ireland have smaller LW transmitters that are easier to maintain. Alansplodge (talk) 14:18, 26 June 2022 (UTC)
 * Hmm. Fleming's diode valves, 1904 look familiar ... compare martian from War of the Worlds, 1906. /OR Card Zero  (talk) 15:41, 26 June 2022 (UTC)
 * Have you noticed that the cathode is outside (the loop) and the anode at the centre (the plate or squiggly bit). All diodes I've ever seen have the cathode in the centre (where it can be indirectly heated if required) and the anode surrounding it.  Triodes, tetrodes and pentodes follow the same inside to outside construction (though obviously a little more complex when you get double-diode-triodes or similar). Martin of Sheffield (talk) 17:02, 26 June 2022 (UTC)

Anatomical feature
What is this protrusion on the side of upper leg below the knee (marked with arrow)? From what I've searched, it could be either tendon of biceps femoris or iliotibial tract, but not sure. Thanks in advance. 212.180.235.46 (talk) 17:36, 26 June 2022 (UTC)
 * That's the biceps femoris tendon, partly shaded by the tassel of her handbag, making it look a bit like it protrudes more. nagualdesign 20:02, 26 June 2022 (UTC)

Oh, the humidity!*
I have a building where the humidity needs to be fairly constant; ideally between 45~55% (realistically, 40~60%) Since the humidity is typically 60%+ year round, and 80%+ is not uncommon in summer, I had a dehumidifier installed with the HVAC. In the winter, however, when the (gas) heater is operating, the inside humidity can drop well below 40% despite it being around 65% outside. Why is that? Common sense might suggest that the heater somehow "dries the air", but since humidity is a measure of water vapor (H2O in gas phase), I don't see how heating the air somehow converts water vapor to something else. Surely it doesn't consume or "burn" the hydrogen and oxygen, right? If that were so, then we'd have a nearly inexhaustible supply of cheap energy. --2603:6081:1C00:1187:813A:ACFE:62DF:42DE (talk) 22:51, 26 June 2022 (UTC)
 * The quantity of water vapor air can contain is lower at lower temperatures. We can use a NOAA moisture calculator to find that a cubic meter of air at 5°C has a relative humidity of 50% then it contains about 3 g of water. If that air moves into a home, it will expand (I haven't figured out how much). If the air warms to 20° the amount of water in the air will still be 3 g, but the relative humidity will be about 20%. Jc3s5h (talk) 23:06, 26 June 2022 (UTC)
 * Hmmm... I'll have to think about that a bit. 2603:6081:1C00:1187:813A:ACFE:62DF:42DE (talk) 23:41, 26 June 2022 (UTC)
 * We need to clarify a couple of terms here. Absolute humidity is the amount of water vapour in a given volume of air.  Typically measures in gm-3.  Relative humidity takes saturated air as 100% and relates the current value to it.  For example, air at  can hold 28 gm-3.  A sample of air with 14 gm-3 at 30 °C therefore has a relative humidity of 50%.  Now let's consider what happens in winter.  Outside air might be at  and lets assume 100% RH.  The absolute humidity is 8 gm-3, a cold dank day.  The air is brought in and warmed up to 30 °C (a bit hot, but I have the figures to hand).  It still contains 8 gm-3, but this is now 8÷28 of the maximum or 29% RH.  The reverse effect is common experience in freezers.  Room temperature air enters when you open the door.  As the air cools it passes its dew point (100 %RH) and water vapour comes out of the air and builds up as ice on the inside.  The article humidity explains this in far more detail. Martin of Sheffield (talk) 07:27, 27 June 2022 (UTC)
 * Since the inside and outside pressure are the same, you can use Charles's law to calculate the expansion from 5°C (278.15&thinsp;K) to 20°C (293.15&thinsp;K); it is by a factor of 293.15/278.15 ≈ 1.054, an increase in volume by 5.4%. --Lambiam 08:52, 27 June 2022 (UTC)


 * Thanks, y'all. So, when air is heated, the proportion of H2O molecules remains the same; and, since the air expands, the number per unit of volume decreases (right?). 2603:6081:1C00:1187:9D25:B1B4:4C95:1B58 (talk) 07:04, 28 June 2022 (UTC)
 * No. Despite expansion being mentioned here, it's close to irrelevant to the change in relative humidity when air is heated up.  As explained earlier, warm air can carry more water vapour than cold air.  Thus, when cold air at 100% RH is warmed up, it is no longer carrying the maximum water vapour that it could, so its RH is below 100%.  Remeber, RH is the actual amount of water vapour in the air divided by the maximum it could carry at that temperature.--Phil Holmes (talk) 07:40, 28 June 2022 (UTC)
 * The actual amount of water vapor doesn't change? 2603:6081:1C00:1187:60B6:1C14:F06C:8901 (talk) 14:59, 28 June 2022 (UTC)
 * Only if it condenses out as its cooled. That's how air is dried in an air conditioning unit. Martin of Sheffield (talk) 15:18, 28 June 2022 (UTC)


 * I grasp the gist of "what", but the "how" and "why" still eludes me. Empirically, when outside cold and damp air is heated indoors, not only does the metric for humidity drop, the air feels much drier (and stuff that should not be exposed to too-dry air for long becomes vulnerable). 2603:6081:1C00:1187:60B6:1C14:F06C:8901 (talk) 15:35, 28 June 2022 (UTC)

Remember this is the relative humidity that drops, the absolute humidity remains the same, there is the same amount of water in the air. As the air warms the relative humidity drops, and the air is able to take up more water, this is what evaporating is. For instance the moisture in your skin evaporates and the skin feels dry. When breathing very dry air, then the moisture in your lungs and passages evaporates and you experience a dry mouth or nose. Moisture on surfaces is also drawn into the air making them better insulators, with increased ability to store static electricity and deliver shocks. Martin of Sheffield (talk) 16:08, 28 June 2022 (UTC)
 * So, because warmed air has an increased capacity for water vapor (for some mysterious reason), water in liquid phase has a tendency to fill the gap (so to speak)? 2603:6081:1C00:1187:60B6:1C14:F06C:8901 (talk) 19:15, 28 June 2022 (UTC)
 * Approximately. The real answer is tied up with vapour pressure and partial pressure, and that's a whole different ball game. Martin of Sheffield (talk) 20:24, 28 June 2022 (UTC)
 * Thank you for helping to fill the gap between knowing something and (mostly) understanding it.
 * It's actually likely that the absolute humidity indoors is slightly higher than outdoors. Indoors there are humans breathing, sweating, cooking, cleaning the aquarium, watering indoor plants and drying their laundry (not necessarily all of those). At my place, indoor relative humidity reaches a minimum in summer of around 55% (which I find uncomfortably low) and a maximum in autumn of around 97% (which makes wooden furniture sticky).
 * Indeed, warmer air has an increased capacity for water vapour. Actually, the air is irrelevant. At higher temperature, there can be more water vapour in each cubic metre of space. Whether there's also air or helium gas or vacuum in that space is irrelevant. The reason isn't really mysterious, but there's some physics involved. Vapour is a higher entropy state than liquid and higher temperature means higher entropy. When you increase temperature, more of the water will end up in the higher entropy state. Or if you prefer the microscopic version (I don't), higher temperature means faster molecules, which are less likely to stick together when they meet as they have to get rid of more energy. So you have to push them together harder to get liquid water. Pushing molecules harder together means increasing the partial pressure of the vapour. PiusImpavidus (talk) 09:27, 29 June 2022 (UTC)
 * Indeed. But let's keep things simple for the OP, that's why I left it as "Approximately.  The real answer is tied up with vapour pressure and partial pressure ...".  Very few people I know fill their houses with helium or evacuate them! Martin of Sheffield (talk)
 * Thanks for the clarification. [spoken with a squeaky helium voice] 2603:6081:1C00:1187:60B6:1C14:F06C:8901 (talk) 19:56, 29 June 2022 (UTC)