is there any place on earth with humidity lower than inside an airplane?

Factual Questions
1

(yes, I just just got off a 14-hour flight…how can you tell? :slight_smile: This started as a pit thread, but then I decided to calm down)

How do they control the humidity inside passenger jets?

And why is it so damn dry that my nose and throat hurt, even though I drank a lot of liquids during the flight?
Today’s weather in Death Valley, california shows humidity of 9%. I am sure that inside the plane I flew, the humidity was even lower. I asssume that planes must have pretty sophisticated air-conditioning systems to cope with pressure changes, extremely cold temperature of the outside air,etc.The temperature is comfortable;but it seems like the humidity of the air in the system is not regulated at all. Why not?

2

Not sure the answer to your query, but something I do on long flights is to snort some water up my nose every so often. Not a lot, I just put a couple of drops on the end of my fingers then snurf it up. I do this in the bathroom so as not to appear to be some bizarre coke-head, although to be fair, I am a Doper. :wink:

Anyway, this seems to help me a lot. Lozenges might help for your throat. Fortunately it’s been a while since I was on a 12+ hour flight. Most of my recently are 8-12 hours though, which is almost as bad.

FWIW, I doubt that there is much if anything in the way of a humidifier in a plane’s recirculating system.

3

The driest place on Earth might be Chile’s Atacama Desert, which receives less than 1mm of rain annually. I’ve heard that some places in Antarctica have not received measurable precipitation for thousands of years. No idea of the humidity of either place though.

4

I have heard that some places in Australia have very low humidity in certain parts of the year. This is one cause of the damaging bush-fires we often hear about.

5

The reason it’s so dry in the airplane cabin is because it takes in air from the surrounding atmosphere at the high altitude (and compresses it), and the humidity is naturally very low up there. Don’t forget, it’s above all the clouds.

6

Good question. From a pilot’s perspective, humidity is not controlled at all. The air conditioning is on and that’s about it. From an engineering perspective the humidity is controlled to some degree but the method will differ depending on the aircraft. In ours, a Dash 8, there is no humidity control as such. Excess (“bleed”) air is taken from the engines at very high temperature and run through a heat exchanger, it is then compressed and run through the heat exchanger again, twice. The air is then expanded which cools it considerably more. Excess moisture is taken from the air and sprayed on to the heat exchanger to help with the cooling process. The final cold air is mixed with recirculated air from the cabin and hot air direct from the engine to control temperature before being ducted through the cabin.

I know that doesn’t really answer your question :(.

7

Well, when the plane is flying, you’re not actually “on” Earth…

8

The pressurization and air conditioning systems on airliners work fairly simply: air is taken from the engines (bled off, thus called bleed air). This air comes from the compressor section and is very hot. The hot bleed is air is then separated: some of it stays hot, and some of it goes into an air conditioning pack to be cooled.

The cooling of the air is done mechanically - through a heat exchanger and a turbine, which rapidly expands the air. This also produces moisture, which is why all of these systems have a moisture separator in them to rid the air of excess moisture. The air coming out of the expansion turbine is very cool - usually just a degree or two above freezing. Any moisture in it would result in ice buildup inside the ducting, eventually blocking all airflow.

The cold air is mixed with the hot bleed air to come up with comfortable air for the cabin.

So you take dry air from high altitude, run it through an air conditioning pack that removes any residual moisture and you have some very dry air.

The new Boeing 787 addresses this problem, or so I’ve heard. It must re-introduce moisture into the air after the cooling process, but it is supposed to keep cabin humidity similar to normal everyday levels.

9

What has been said above about why the humidity is so low makes sense, but why wouldn’t they just add humidifers after the A.C.? There must be some underlying issue. While it may just be a cost/weight saving, I’ve read on another message board (so take it for what its worth) talking about the new 787 is that a large part of the reason they don’t increase the humidity in the cabin is that it leads to accelerated corrosion of some of the alloys they use. The 787 is largely going to be composed of carbon fiber composite which does not experience similar issue and will allow them to run at a more normal humidity level.

10

The Dry Valleys are the driest, least humid place on Earth as I recall ( but I can’t find an actual number ). They have been used as a stand in for Mars.

About two million years, actually.

11

“I like to put a humidifier and a dehumidifier in the same room and let them fight it out.” - Steven Wright

:smiley:

12

after a long flight, I usually head into the bathroom, turn on the steam with the fan off and let it rehydrate. One of my standard acclimation tips.

Humidity at 16,000 feet in Tibet (winter or summer) is pretty low.

13

What does it mean to “turn on the steam”?

14

My traveling humidity-retention tips:

  1. A good moisturizing lotion on hands/arms/face/neck
  2. A whisper-thin coating of petroleum jelly (Vaseline or the like) as a moisturizing nostril salve
  3. Isotonic saline solution spritzed periodically in the nostrils (I used a pressurized mist product called ‘Simply Saline’, available OTC at stores everywhere).
15

turn the shower onto the hottest setting with the fan off and sit in the steamy bathroom for a while

16

Going back to why airliners don’t have humidifiers …

The air outside at cruise is around -40C = -40F. That means the outside of the metal skin is roughly the same temp. That means the inner surface of that ~1/8" metal skin is also roughly the same temp.

If there is even a smidgen of moisture in the air, it will condense and freeze on the inner surface of the skin. And the airplane will end up with a load of frost or, on longer flights, ice, attached to its interior.

And when the airplane lands, that ice will melt & the water will soak into the insulation, and also sit there trying to corrode the skin, the wiring, and everything else in there.

The next flight will add another load of ice & water, and another and another. Pretty soon you have both a bunch of weight, and a bunch of damage. Having the skin corrode on your airliner leads to holes & leaks & tears and bad things like that.

This happens now on eveery flight, but because the air is so very dry, the amount of water and the accumulated damage is small enough it can be managed with normal maintenance.
The 787 fuselage is built almost entirely of carbon fiber composites (ie uber-fancy fiberglass), not metal. And so it’s almost completely corrsoion proof. It’s also less heat conductive, which means the interior surface of the skin will be somewhat warmer.

Boeing has invested a bunch of $ in different insulation techniques which they believe will not soak up condensation for the life of the airplane.

All of which is enough enabling technology to make adding cabin humidifiers possible. So they did that too. The fuselage is much stronger than a conventional aluminum one, so they can run the cabin at a lower altitude = higher pressure versus a conventional airplane.

On a current airliner, the cabin pressure at cruise is equivalent to an altitude of 7000-8000 ft. On the 787 it’ll be more like 5000. That difference will reduce the overall fatigue effect of longer flights. It also helps to reduce the total amount of water needed in the air to achieve what feels like normal sea-level dry climate humidity levels.

17

Don’t forget the neutral buffered eye drops … I get these ones in single use droppers. The solution is fractionally thicker than basic saline, so it cushions the eye surface better. They are also great for floating out stray hairs and particulates also. I get wicked bad dry eye on planes if I forget the drops.

And try to wear loose fit clothing, and maybe bring a pair of floppy slippers, the fluids in your body act funny at the lower pressure, I always had issues with edema in my hands and feet until I wore elastic waisted pants and slippers, a big fluffy shirt and a soft nonrestrictive sports bra, and also tried to get up and walk around about one lap around the middle set of seats in cattle class about every 2 hours just to get the body pumping.

18

Point #3 is a lot smarter than my suggestion of just snorting up some ordinary water droplets - much more hygienic. Still, it hasn’t killed me yet so I’ll probably keep doing it. The principle is the same.

19

Looking at my “standard atmosphere” Excel file, I came up with -60F at 35,000 feet, so we’re in the same ball park. Ambient pressure up there is just 3.45 psi.

Then I started looking at the steam tables. At -60F, the saturation vapor pressure of water is just 0.0004977 psi. Assume, for the sake of discussion, that we’re flying through a cloud, and so the ambient relative humidity (outside the plane) is ~100%. Take that air, and compress it to cabin pressure, about 12.26 psi (a cabin pressure-altitude of ~5000 feet). Assuming adiabatic compression, the temperature of that air is now -6F (this will also be true if you take high-pressure/hot engine bleed air and expand it until it’s at cabin pressure). So now you warm it up to 68F.

Due to having been compressed, the water vapor actual pressure been increased to around 0.001768 psi, by a factor of about 3.5. But at room temperature, the saturation vapor pressure for water is 0.3392 psi. So in-cabin, the relative humidity is 0.001768/0.3392 = 0.5 percent.

If my math is right (heck, if it’s even close), that’s pretty dry.

is 0.5 percent relative humidity too much? Do they go to the trouble of drying the air out even further, or does this create an acceptably small amount of frost on the fuselage?

20

Yes. The inside of my nose the first night at the hospital when I was on oxygen but they hadn’t humidified it at all.