Chilly Out - Question for the Arctic RMC

[Chance]

I've recently been reading another manual with a section on snow shelters, which has the standard (?) 'make a trench for the cold air' advice. This will be pedantry, but I want to make sure that I understand what's meant. Although I've made a variety of snow shelters (kids are a great excuse), I've yet to spend a full night in one; and am looking for a clarification from someone with experience.

Where does this cold air come from ? Wasn't it there to start with ?

I understand the need to have a low entrance, sheltered from drafts; but are you not actually making sure that you're close to any warm air that you create ?
In two otherwise identical shelters, is it better to add a trench to aid some form of stratification, or to minimise the volume to be heated ?

 

[Tadpole]

The cold air comes from all around the empty space, the contact with the ice/snow will cool the air next to it, and it will then sink downwards. The opposite happen when warm bodies come into contact with cold air, the air next to the body will warm and rise.

It’s my understanding that cold air sinks in the presence of warm air, (warm air escaping from your sleeping bag/candle rises). So if there is nowhere for the cold air to sink to, it will mix with the warm air, effectively cooling the warm air. But if there is a place for the cold air to go to, then the warm air can only rise circulate and get warmer.
AS you said stratification, keeping the upper part of the T shape warmer.

 

[Chance]

The cold air comes from all around the empty space, the contact with the ice/snow will cool the air next to it...

That's partly my point: wasn't the cold air there already, until you arrived ?
Once you're inside, what changes is that there is now some warm air which you seek, rather than some cold air which you avoid. I think.

Very pedantic, I know; but it still seems to me that lowering a trench (as opposed to raising a platform) is unnecessary. All else being equal, a quinze/igloo will be warmer with, for the sake of illustration, a shallower trench. Won't it ? There's less of the cold air to mix/warm.

 

[Tadpole]

That's partly my point: wasn't the cold air there already, until you arrived ?
Once you're inside, what changes is that there is now some warm air which you seek, rather than some cold air which you avoid. I think.

Very pedantic, I know; but it still seems to me that lowering a trench (as opposed to raising a platform) is unnecessary. All else being equal, a quinze/igloo will be warmer with, for the sake of illustration, a shallower trench. Won't it ?

In an igloo, I believe, you sleep on slightly raised platform, so there has to be a point, to not sleeping where cold air can collect.

 

[Chance]

In an igloo, I believe, you sleep on slightly raised platform, so there has to be a point, to not sleeping where cold air can collect.

No argument with a raised platform. My only contention is that you're raising yourself to air that you've warmed, not from air that was already cold before you arrived on-site.

To re-phrase: all else being equal, do you (this is a very fine distinction) build a higher ledge or a deeper trench ?
Or: all else being equal, is there any value in making a deeper trench to "collect more cold" ? My contention is that this would be counter-productive.

 

[John Fenna]

Surely - if you are digging a snowhole into a virgin bank the deeper you dig the trench equates to raising the sleeping benches?
To form a distinct sleeping bench/platform you do not pile on snow but you cut the space to sleep in from the mass of snow, giving you the headroom you need, and continuing down, forming the raised platform by removing the floor to give an area of increased headroom - in effect the cold trench.....
Warm air, the cold air that was there when you started and then heated by your body/stove/candle rises to the area you sleep, cold air (the warmed air as it comes into contact with the snow or air that comes in through the doorway to replace warm air that has risen out of your vent hole) sinks - or does not rise in the first place unless drawn up by the chimney effect of lost ait through the vent hole - into the cold trench.
If you have no trench - just a flat floor you will be sleeping in the coldest part of the shelter, if you have a cravass then you will be in the warmest part but potentially will have a lot of space to warm up and have difficulty getting into bed....
Low headroom over the sleeping platforms, a slightly lower section to stand and work at and a deeper area near the entrance seemed to give optimum heat control in the days I went snowholing....

 

[wattsy]

the trench forms a cold sink or a cold well for the cold air to sink into. as the warm air rises it displaces the cold air which goes into the cold well rather than staying around you. if you don't have a trench you'll end up with all the warm air being above you not around you.

 

[tenderfoot]

This is a good one! Its got me thinking....
First i agree with what Tadpole first responded but then Chance made me think that maybe although this is true there is a second stage of thinking to be applied. ( nothing is simple)
and this is thermodynamics so maybe someone with that kind of background might give a definitive answer. Anyone out there?
My two pennorth:- As Chance says the cold air is there to start with.When you inhabit the space you warm the air.This rises. It also expands.This expansion will displace a larger amount of colder denser air.We want it to displace this cold air away from us and to mix as little as possible with our warm air. So we need to make a place for it to go.We want this place to have a minimal area of interface with the warm air to stop mixing and heat loss. so so perhaps a system which "pipes" the cold air away to outside through a relatively narrow diameter pipe (read small tunnel) to outside might be better than a trench. In a snowhole on a hilside this could open out below the level of the snowhole.
That said the native people who invented many of these cold weather shelters did so through many generations of trial and error to find what works so if they used trenches there must be something in it.We may be missing the subtlty of the designs in terms of size ratio of trench to internal volume of shelter etc that they would know from tradition passed down.

 

[Crowe]

Hummm, I note all the above. The shelter entrance is made at the same level as the cold sink; so cold air, and the shelter can vent. The sleeping platform is best above the level of the cold sink and entrance, with a small vent in the roof. That way, you are not sleeping in a cold draft. Afterall its purpose is to protect from the elements. Cold air is already there as noted, but will become stale to breathe.
Hope this helps.
Crowe

 

[Chance]

... and this is thermodynamics so maybe someone with that kind of background might give a definitive answer. Anyone out there?

I'm not sure if I've got it across yet, but I've been (very) pedantically looking at the theory, not the practice; armchair survivalism. To avoid further instruction into the 'how' (I sought the 'why') I will settle for my mistake being that I was reviewing the volume quasi-statically; ignoring expansion and, most importantly, the vent and other leaks.

(it would be foolish to say that I'm still uncomfortable with the implication in some manuals that you're digging a pit in which to trap a stealthy volume of cold air that tries to sneak up on you while your back is turned)

 

[cu14]

my reply may be a bit out of date, i did my arctic survival course in 1985 and my arctic warefare course in 1986, so my theory memeory is a bit lapsed.
from what i remeber:-

the purpose for the cold air trench "air trap" is that it is lower than the entrance, which leads to an elevated living chamber, it works much in the the same was as a "u bend" to insulate from the colder air outside pervading the less colder air inside. (there is not really such a thing as warm air in a snow hole !!!

The physics i could never begin to explain, but when you next try it, build it one night without the trench, then slot it in the following night. i Guarantee you will feel a difference.

 

[Stuart]

you build the sleeping platform up until you are as close to the roof as is convenient (to avoid contact with it during occupation) the floor should remain at door level, there is no benefit to digging lower, in fact if you were to dig down far enough to expose the frozen ground it would cool the inside of the shelter considerably, the layer of snow on the floor insulates you from the frozen ground, which is many times colder than the snow is and would act as a giant heat sink.

(for many years the US military made the amazing error of repeatedly stating in its survival literature that you should dig down to expose the frozen ground as it would warm your shelter!!! Amazing, obviously produced by the same sort of people who wrote in Canadian military literature informing soldiers that the buttons on the issue arctic smock were edible and in fact designed as an emergency survival ration! )

its also worth considering whether a vent in the roof of your shelter is really always necessary? provided the low door of the iglu or Quinze is correctly orientated and left open and you are not operating a stove or burning anything other than a couple of candles or a properly maintained kudlik, you do not always need to put a vent in the roof, a vent in the roof vents all the hot air!

Native iglu's and quinzes did often have a vent in the roof, but it was used to vent excess heat, not for oxygen, it was kept closed with a mitten stuffed in it the rest of the time the temperature inside was not constituting a melt risk, an open door built perpendicular to the prevailing wind should provide adequate ventilation.

Cooking in a snow shelter does of course require a vent, to allow the escape of both fumes and steam; it should be placed directly above the cooking apparatus which is often housed in its own alcove.

Snow shelters should always have a vent if you are sealing up the entrance, but you would be better off with a low entrance, high platform and open door.

 

[johnboy]

Hi,

The trench or 'cold well' in a snow cave serves a lot of purposes other than acting as an area for cold or cooling air to collect. It's a good spot to store your bergen, cook (close to the entrance) and swing your legs into when getting in and out of the cave. It also gets you up and out of the way of the door...

'Warmth' in a snow cave ( or any snow shelter) of course is a relative term it's still going to be well below freezing. In my experience sleeping on a flat floor in a snow shelter is a colder experience than in a well constructed shelter with platforms and a 'cold well'.

[/IMG]

 

[Chance]

Happy now ?

[edit: great, now I have to re-learn how to post images]

 

[bikething]

Happy now ?

[edit: great, now I have to re-learn how to post images]

( looks like you're missing the last bit of the address..)

Cracking pic btw

 

[Chance]

Thanks.
And thanks - I'll pay more attention cutting and pasting next time.

It's getting quite late - I should probably go out and check on her to see if the experiment worked. There wasn't much wax left in that tea light.