[pct-l] Re: Gear Tradeoff -- Tents versus Sleeping Bags

[Brett Tucker" <blisterfree@earthlink.net (Brett Tucker)]

Actually, the coldest nights - barring a change in weather - tend to occur
on clear nights in the _valleys,_ which pool the heavier, colder air from
the quickly-radiating highlands. This is called katabatic air. The
double-whammy is that the temperature often reaches the dew point under
these conditions, which allows moisture-sapping dew or frost to form on
exposed surfaces, ie the sleeping bag of someone bivy'ing out. Or, the temp
gets at least near the dew point, and moisture permeates the sleeping bag's
insulation either way. Ironically, this phenomenon is less of a problem in
the high country, out of the valleys, at least in summer and fall, because
of temperature inversions and the increased likelihood of there being at
least some breeze to stir the air and reduce radiational heat loss. If it's
windy, the actual temperature is usually warmer than it would otherwise be.
But the wind chill could easily negate this, of course. "Climb high, camp
low. But not too low."

The upshot in these conditions is that we need some kind of barrier between
ourselves and our sleeping gear, to prevent the buildup of heat-robbing
condensation. And if the temperatures are reasonably mild (20's?) and the
wind not too strong, then the best bet sometimes is to use a tarp instead of
a tent. Properly pitched, a tarp is plenty strong - often stronger than
tents with their curved poles in tension. And more importantly in the kind
of weather described here, the tarp breathes phenomenally better than any
tent. I'm talking, no barrier to air flow, either at the openings front and
back, nor along the bottom edges. No-see-um netting, as anyone who has tried
hiking up a steep hill with it on will tell you, breathes surprisingly
poorly, and so traps a certain amount of moisture. Under an open tarp,
condensation will still form on the underside of the fabric, although most
of this will be the hiker's body moisture escaping. The dew will remain off
the hiker's bag, and the humidity under there will be significantly less
than inside a tent. All of this means that the tarp user stands a greater
chance of sleeping warmer than his tent-bound mates. Although they may heat
their enclosed air space more efficiently, they may still find themselves
mired in moisture. It's like breathing into a paper bag: the air in the bag
is suddenly 98.6 degrees F, but by golly if it isn't 100% humidity in there
as well. Again, this is a comparison under the aforementioned weather
conditions. If it's quite windy, then the advantages might be less. Bring on
that no-see-um netting.

- blisterfree, a non-discriminatory tarp and tent user



>>He also pointed out that clear nights
are the coldest nights, because all the solar heat retained by the earth and
rocks during the day just goes out into space at night.  A cloud cover will
reflect some of that heat back to earth, so cloudy nights are warmer. On
cloudy
nights there is typically much less wind as well.   Sleeping in a tent is
like
bringing your own cloud cover with you wherever you go.<<