Wikipedia:Reference desk/Archives/Science/2018 February 24

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February 24

Weather control to prevent cold snaps and snowstorm s

Has anyone ever proposed weather control methods for preventing cold snaps and snowstorm s? — Preceding unsigned comment added by 73.118.181.38 (talk) 00:00, 24 February 2018 (UTC)

If by "cold snap" you're meaning the sort of unexpected late frost which damages seedlings and grape vines, then yes, they have. "Smudge pots" were a popular attempt, where orchards and vineyards would light smokey stoves between the trees. The smoke reduces radiative cooling, thus the temperature drop overnight. Andy Dingley (talk) 00:11, 24 February 2018 (UTC)

Andy Dingley's example and cloud seeding are about as far as this goes with our technology. Weather is pretty much impossible to control directly, as it is a system with many times the energy output that humanity can produce. Instead of spending energy in a futile effort to change the weather we typically tend to adapt our tools to let it do the work for us: e.g. windmills, wind turbines, hydroelectric power plants... Besides, cold snaps, snowstorms and other apparently extreme events can often be useful to the environment in unexpected ways. For example, sequoia relies on forest fires to procreate, while cold weather in the Arctic winter encases Alaskan and Siberian north coasts in ice, preventing coastal erosion by winter storms. 93.142.92.135 (talk) 01:27, 24 February 2018 (UTC)

I was about to reply with a supporting statement that cold snaps kill off insect pests, but a quick Googling has an amusing fifty-fifty split between articles supporting that it does and articles debunking that as a myth. Whether it kills them or not, my personal observation is that it does induce them to hibernate or otherwise quite being such pains. I quite look forward to it for that reason - and because a sharp frost typically stops my ragweed-induced hay fever. Matt Deres (talk) 15:08, 24 February 2018 (UTC)

Loss of cabin pressure

I was on a plane and they gave me a bread roll in a sealed plastic bag. I didn't eat it, but kept it.

During the trip, the bag got puffed up and under pressure, like a little football. Then it went back to normal.

I understand that the pressure in the cabin must have dropped. But is that normal and okay? Anna Frodesiak (talk) 00:58, 24 February 2018 (UTC)

Was the bag given to you on the ground? After you'd been in the air for a while? Nil Einne (talk) 01:03, 24 February 2018 (UTC)

The bag was given to me an hour into the flight, and it was baggy then. Then an hour or so later it was blown up. Then, hour(s) later, still in the middle of the flight, it went back to being baggy. Anna Frodesiak (talk) 01:08, 24 February 2018 (UTC)

AFAIK, barring stuff like damage, For a given commercial airline plane, cabin pressure will depend mostly on altitude, the higher the plane, the lower the cabin pressure. Nimur or someone would probably explain more, but I believe it's common for cruising altitude to increase over time as the plane gets lighter (burns up fuel), see [1] [2]. So it's not unexpected the cabin pressure will reduce, particularly if it's only been 1 hour. Altitude may also need to be adjusted for other reasons, particularly weather conditions. If you happen to have access (sorry I suspect you don't), you can see how the altitude and cabin pressure varied over one flight from London to Bangkok here [3]. Nil Einne (talk) 02:01, 24 February 2018 (UTC) 03:33, 24 February 2018 (UTC)

I don't think the pressure really depends much on altitude. As I understand it they pressurize the cabin to about 8000 ft for cruising, regardless of cruising altitude (which is obviously always higher than that). Used to be 7000, but 8000 saves on fuel, and makes the passengers slightly groggier and slightly less likely to cause trouble. --Trovatore (talk) 06:17, 24 February 2018 (UTC)

That mostly doesn't seem to be what the Respirology article found or says, unless I misunderstood it. Actually you don't need access to the whole source for this part although the graph may be useful, the abstract itself mentions "There was a linear fall in cabin pressure as the aircraft cruising altitude increased." That said, I did miss until now that this relationship only held during normal cruising, as the cruising altitude increased into the flight. On the 3 occasions the aircraft descended to a lower cruising altitude mid-flight, the cabin pressurisation was higher than predicted. (I'm pretty sure they mean in hPa, i.e. the pressurisation was at a lower altitude equivalent than expected.) It sounds like possibly they didn't have any occasions where the plane went to an unexpectedly high cruising altitude, so we don't know what would have happened then. And finally this was only during cruising, it doesn't apply during the descent and ascent phrase.

So my statement was at least a little wrong as the variation only seems to apply during normal cruising and probably won't apply during unexpected changes such as the weather example I mentioned. Also they only looked at 747-400 so it's possible other planes vary.

Note also in this study from 2004 to 2006, they don't seem to have generally reached the maximum allowed cabin pressure of 8,000 feet altitude equivalent. (Maximum seems to be what the sources use since they generally refer to the cabin pressure altitude equivalent.) The time frame suggests that this was probably mostly before high fuel prices really began to hit, and also before budget airlines really began to take off so these may have affected modern practices, even in full service airlines like those studied (Qantas, British Airways and a single Malaysian Airlines). And on that point, maybe only those airlines have/had these practices or even they only do/did it on the London to Bangkok route. Also it's possible that the airlines were aware that the study was being carried out and varied their practices. But I'm not sure as the study seemed to use a wristwatch altimeter and in-flight displays (which I take to mean any data shown on the entertainment systems).

That said, there also seems to be a push to higher Cabin pressurisation (i.e. lower altitude equivalents), e.g. the widely touted 6,000 feet for the 787. Our article also mentions something similar "A design goal for many, but not all, newer aircraft is to provide a lower cabin altitude than older designs. This can be beneficial for passenger comfort."

Our article also says "One study of 8 flights in Airbus A380 aircraft found a median cabin pressure altitude of 6,128 feet (1,868 m), and 65 flights in Boeing 747-400 aircraft found a median cabin pressure altitude of 5,159 feet (1,572 m)." referencing a 2010 study (albeit the link doesn't seem to work for me). So it doesn't seem that the findings of the earlier study on typical cabin pressurisations changed much in those 4-6 years. (I believe I looked into the issue of typical cabin pressurisations a bit more before, it's probably somewhere in the RD archives.) My memory is I found other stuff which supports the view the cabin pressurisation often doesn't reach the maximum allowed.

Anyway ultimately I don't know how well supported outside this study the finding on the variation of cabin pressurisation with cruising altitude during normal flight is either in the ~2005 period or nowadays. (My impression from vague memories of what I read before when looking into average or median cabin pressurisations is this is an area where there tends to be a lot of theory. What airliners actually do isn't well outlined or studied.)

Nil Einne (talk) 09:25, 24 February 2018 (UTC)

Hmm. Well, my phone can do both barometric pressure and GPS altitude, so maybe I'll start taking some data points and see what I can see. I suppose it's slightly less expensive to pressurize to 7000 ft if you're at 25000 than if you're at 35000, so if the airlines actually see any upside to pressurizing more than they have to, then I suppose they could. But I'm a little skeptical that they do — it's not something your average passenger tracks, so it probably doesn't give much competitive advantage. --Trovatore (talk) 09:58, 24 February 2018 (UTC)

Passengers rarely know it explicitly, but they feel more comfortable when cabins are pressurized to lower equivalent-altitudes. This is a major selling point for new airplanes like 787 and 777X: here's a blurb from Boeing.

Most of the time, the pilot configures the cabin pressure for "the lowest cabin altitude the plane can sustain," and that is basically limited by a few factors: how perfectly airtight is the cabin? How much energy can the engine bleed (e.g., the "air pumps") sustain? How much structural strength does the fuselage provide for the gauge-pressure difference between inside and outside?

As far as measuring this with your phone - I encourage the experiment, but I doubt it will work... I would not trust either the GPS altimeter and the baro-altimeter to accuracy of even +/- 1000 feet. Those kinds of consumer devices usually use software tricks and sensor-fusion to "improve" the data in a way that makes for very poor scientific instruments - especially when compared to a true altimeter. But if you collect data, please let us know! Nimur (talk) 17:46, 24 February 2018 (UTC)

I use the "GPS Status" app by MobiWIA, which reports the pressure in hPa, not as altitude. So it seems unlikely that it's doing any Kalman filtering or anything like that at the app level. I don't know enough about the internals to know whether the OS could be doing any such thing, but there's no obvious evidence of it as you watch the numbers change. --Trovatore (talk) 20:26, 24 February 2018 (UTC)

Don't pretend to know all the answers to this but here are some more likely possibilities. One doesn't want ones bread rolls to go pop. So when it was a football shape is was possibly out gassing (ie bags are not air tight). Second (you haven't mentioned which type of hairycraft you where in, nor the route) Most jet propelled aluminum cans in service today still use engine bleed air to pressurize the cabin. Bleed air is hot! So to increase the cabin temp, more air is vented out of the cabin (ie pressure is lowered) to allow more warm air to enter cabin from engine. To cool, venting is restricted and cabin pressure increases, slowing down the flow of warm air from engine. The battery in my slide-rule has gone flat so I can't use Log _e but fortunately there is this handy widget air-pressure-at-altitude-calculator. So by regulating the temperature, the cabin altitude pressure can fluctuate between 6000 to 8000 feet. Which equates to 11.78 to 10.92 PSI. Nearly one pound per square inch on the raper of said roll which has degassed whilst at lower pressure of 8000 ft. More than enough to deflate it. --Aspro (talk) 02:45, 24 February 2018 (UTC)

I once bought a soda at 700 feet and broke the seal at only 2 or 3 thousand feet and it burped fog. Air pressure's interesting stuff. (this was natural ground altitude) Sagittarian Milky Way (talk) 03:54, 24 February 2018 (UTC)

Very interesting indeed. Thank you all. I understand now that pressure changes within the cabin are nothing to worry about, and that strange things can happen. Many thanks to all. :) Anna Frodesiak (talk) 05:59, 24 February 2018 (UTC)

The flight attendants have to go through a canned routine set of announcements at the start of every flight, going through the "safety features of our Boeing 737 aircraft". One of the things they always say is ...although we never anticipate a change in cabin pressure.... Liars. Oh well. I know what they mean, and I guess that's what matters. --Trovatore (talk) 06:13, 24 February 2018 (UTC)

You will find they say something like "in the unlikely event of a cabin depressurization ..." The cabin pressure changes almost continuously throughout a flight, but uncommanded depressurization followed by deployment of the emergency oxygen system is indeed a rare event. Dolphin (t) 12:24, 24 February 2018 (UTC)

No, they don't. They say precisely "although we never anticipate a change in cabin pressure". I've heard it enough times that I can recite it in my sleep. This may vary by airline, but I guarantee you that that is word-for-word exactly what they say. --Trovatore (talk) 20:58, 24 February 2018 (UTC)

This seems to depend a lot on the airline and probably even the precise video. This American Airlines video for example says [4] "now, if the airplane loses pressure". This United Airlines video [5] "if necessary, an oxygen mask". (I came across another more standard safety video from United Airlines which said the same thing, but not linking to it because I'm not sure that Global Airline Safety Videos Youtube channel ensured they had the appropriate permissions for redistribution although I doubt UA will actually care. I had higher hopes for another video again with very similar wording albeit also fancy on Canal Plus Producciones's channel but looking a bit more they seem to be a relative small Venezuelan production company so I'm less certain. And I strongly doubt FrienldySKy made sure they had the appropriate permissions although the wording is slightly different on their old video.) This Virgin America one says [6] "if the cabin pressure's changin'". This KLM one says [7] "if there is a sudden decrease in cabin pressure". This Cathay Pacific one says [8] "oyxgen masks will drop automatically if they're needed". (An older one which coming from the production company which made it hopefully they know what they are doing [9] says "should oxygen be required". The smoking version says the same thing [10] although does remind you to extinguish your cigarette later.) Not sure what airline you are thinking of but I wonder if they still say that. With the modern trend of trying to produce fancy inflight safety videos, the would likely have a new one and perhaps the wording was changed. Nil Einne (talk) 02:14, 25 February 2018 (UTC)

Most of my flights are on Southwest Airlines. Yes, they definitely still say it. --Trovatore (talk) 03:17, 25 February 2018 (UTC)

Can't find any safety videos. But it seems unlike many other airlines who are competing to have some viral hit safety video, Southwest Airlines instead known for their flight attendants spicing up the safety presentations. So I can find videos of the flight attendants saying this [11] [12] [13] or similar [14]. Although not all seem to say it [15] [16] [17] [18] (while some of these look to be semi spontaneous or invented by the flight attendant, this one at least looks scripted probably by the airline since other than the fact she seems to be reading it, it sounds very similar to this [19] [20]). Nil Einne (talk) 11:51, 25 February 2018 (UTC)

Southwest doesn't use "videos", at least not on their bread-and-butter short-haul flights. The 737s have no video screens. --Trovatore (talk) 20:21, 25 February 2018 (UTC)

Loss of pressure can carry effects, ranging from "nothing" to "passenger discomfort" to "potentially serious physiological response," to "serious medical emergency"... Commercial airplanes never (intentionally) expose passengers to hazardous cabin pressure. In aviation, the two types that are of interest are rapid cabin pressure loss, and slow pressure loss. In the event of a sudden depressurization, most passengers will lose "useful consciousness" within a moment or two. (During my High Altitude Hypoxia training, I lasted almost 300 seconds at equivalent of 29000 feet altitude, with a little fuzziness on the "useful" bit - in this case "useful consciousness" is a technical term meaning that I had enough energy to put on my oxygen mask - barely - I had some blackout periods in there, and could not do basic aviation arithmetic correctly). If you're interested in developing a sense for what altitude hypoxia does, have a look at the famous Four of Spades video. While watching that training video, bear in mind that you are watching a high-IQ, high-adrenaline, college-educated Air Force officer describing his symptoms. Absent oxygen, your brain does things differently. If you want to survive a depressurization event, the only thing your brain must do is to remember is to put the oxygen mask on. If you can not remember to don the mask, it will not matter if you are awake. If the depressurization occurs slowly, you might not notice the onset of hypoxia, and you may lose useful consciousness without noticing it. If the crew informs you to do so, put the mask on immediately. Nimur (talk) 17:59, 24 February 2018 (UTC)

By now we've established that the ambient cabin pressure does change - and this is safe and normal on a commercial airliner. You'll typically lose about 5 or 8 inches of mercury worth of pressure (25 kPa) by the time you hit cruise altitude. At that point, the snack bag (which has sea-level air inside it) will tend to puff up. For me, the most interesting detail is that the snack-bag eventually re-equilibrates to ambient pressure, but only after some delay! Those little bags of cookies or peanuts are frequently bags made from plastic or metallized plastic - sort of like mylar - and sealed using hot-bar pressed seams. Mylar is amazing aerospace material - compared to our best engineered materials, it's darned near airtight - but not actually perfectly airtight! We don't know whether air leaks through the seams or through the material itself. But you can sort of develop an intuition about how fast air flows into and out of the bag based on how effectively it withstands its internal pressure against the ambient cabin pressure! Nimur (talk) 17:46, 24 February 2018 (UTC)

As I understood the OP's comments, the bag was not puffed up when they received it 1 hour into the flight, so the plane was likely already at some level of cruising altitude. This puffing up happened due to changes in cabin pressure (probably due to changes in cruising altitude) after the ascent phase. Nil Einne (talk) 02:14, 25 February 2018 (UTC)