I could be wrong- however I believe the reason your bag frosts up is due to
the condensation
(dew point) not the amount of heat escaping the bag! Pretty sure
anytime I sleep out *and* there is frost on the bag, there is usually frost
on the ground/leaves/pine needles/etc. as well. In fact if you leave a tarp
or something out crunched up so it is not comopletely flat, ie, is in poor
thermal
contact with the ground, there will be a lot of frost on the tarp. Moreso
than your
sleeping bag (as the heat form the bag that does escape ABLATES the frost
off back
into the surrounding air- which must be still as any airflow as you know will
dissapate(sp?) the frost before it forms.
Hopefully I am not screwing things up too much :-)
Rich
At 06:05 PM 11/16/99 EST, ROYROBIN@aol.com wrote:
>In a message dated 11/10/1999 9:34:41 AM PST, calliger@infolane.com writes:
>
><< Specifically, P, the heat which can be dissipated is:
>
> P= Q(a) [(T2/T1)-1] where Q(a) is air flow in ft3/min past your body.
> However radiative cooling is a primary consideration as well, it
>being
> P + e(sigma)(T**4-T(0)**4). Which takes into account the thermal
> emisivity of the radiative body,e, and sigma is the Stefan-Boltzman
> constant in watts/cm**2xK deg. The same equation [my emphasis]
> is used for thermal conductivity to the ground where air-flow is now
> zero. >>
>
>Beg to differ? Radiative heat transfer is, as you say, proportional to the
>fourth(!) power of temperature difference. That's why your sleeping bag
>frosts up when you sleep out on a clear night. (So get under the trees or a
>tarp.) However, conductive heat loss is proportional to the simple
>difference in temperature.
>
>Even so, our main concerns while trying to stay comfortable in our sleeping
>bags are conductive heat loss -- heat transfer from us to the ground --, and
>convection -- heat transfer through the bag to the surrounding air. A good
>insulating layer reduces heat transfer. Between us and the ground, this is
>usually a closed cell foam pad, or the equivalent. Between us and the air,
>it's the dead air space provided by the loft in our down or synthetic filled
>bag. Moisture anywhere in the system increases the rate of heat transfer,
>which is not good if you are trying to retain body heat.
>
=--
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Richard Calliger -- At Home in The Sierra Nevada Mountains; Mi Wuk Village,
California
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Use email: Don't send a tree to do an electron's work!
--RJ Calliger 1994-1999
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