2737Hammock Camping Re: Oh Boy, Cold Wars II, winding up!

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• Sep 2, 2003
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>Wind does not add cold, it removes heat. And it does not have to
>blow into the insulation to do this, it only has to remove the layer
>of air that was warmed up by the insulation that your body warmed up.

We are saying the same thing in different ways. We totally agree.

>You are in effect generating your own bubble of heat. this bubble
>actually extends several inches below your hammock into the night
>air, even with a very good pad.

Yes, we create a bubble of heat in all directions, even beyond the
region insulated with a pad. But the heat dissipates into the air
more quickly than it would into dry ground, and therefore we need
more insulation below us (thicker pads) than we need with the same
weather conditions when sleeping on dry ground. We do not, however,
need thicker sleeping bags.

>If you were to put a windproof silnylon barrier right up against
>that closed cell pad that was losing heat, it would make only a very
>minor difference in your warmth. On the other hand, blocking the
>wind from penetrating a down garment would make quite a difference
>in how warm it will keep you.

According to Newton's Law of Cooling the heat transfer is equal to
the thermal conductivity of the material times the cross-sectional
area times the temperature difference between the two boundaries
divided by the thickness of the material.
The part of this equation that seems relevant is that the rate of
heat loss is proportional to the thickness of the insulation between
the person and the point within the insulation where the temperature
is approximately what it is outside of the insulation.
The actual situation is, of course, more complicated. I have, for
instance, ignored the effect of any clothing the person might be
wearing.

It appears to me that what happens is that without wind you have to
go all the way through the insulation to reach the point where the
temperature is the same as what you find outside the insulation. If
there is wind then you only have to go part way through the
insulation to reach the point where the temperature is similar to
what you find outside the insulation.
This is true no matter what the insulation.
And this is, of course, an approximation.

Therefore, if the insulation is down, and if the wind is strong
enough to have the temperature halfway through the down be similar to
what it is outside of the down, then you have lost half of the
insulating value of the down. If the insulation is closed cell foam,
and the wind is strong enough to have the temperature halfway through
the closed cell foam be similar to what it is outside of the closed
cell foam, then you have lost half of the insulating value of the
closed cell foam.
And if in either case you place a wind-proof layer outside of the
insulation then you will regain the half of the insulating value that
had been lost to the wind.

>The goal of your windproofing efforts should be to keep the wind as
>far from your insulation as possible, not just to keep it out of
>your insulation. The advantage to the skirt and cone ideas is that
>they protect the outer air layer of your heat bubble.

Because of convection if doesn't matter how far away you keep the
cold wind as long as you have the same thickness of the same
insulation in the area protected from the wind.
Again, this is an approximation.

>As for the condensation issues... You need convection, but the
>source of the air does not necessarily have to come through the
>fabric.

From what I have read about the "chimney effect" and tent design it
is necessary to a have a source of cool dry air below and a vent
above where the warm moist air can exit.
I was hoping to find that if was possible to have a windproof hammock
with a large open area above and avoid the condensation problem. It
appears that this only works if the hammock is somewhat windproof.
Therefore it appears that fresh air must come from below and through
the fabric of the hammock.