[pct-l] condensation on inside of homemade tarptent

[elee at microsoft.com (Eric Lee (GAMES))]

Scott wrote:
>
>> ...because the fabric is colder than the surrounding air on both
sides...
Eric, you might rethink that statement. :)
>

No, I'm quite sure that statement is correct.  Dew forms on objects that
are colder than the ambient dewpoint while the surrounding air is still
above the dewpoint.  How do objects become colder than the surrounding
air?  Because on clear nights they undergo radiative cooling faster than
the air does.  When the air itself reaches the dewpoint, then you get
fog.

Here's a web site with a lot of good definitions for these types of
phenomena: http://www.research.umbc.edu/~tokay/chapter7.html

In particular, it says this:

Dew: the water droplets formed by condensation of water vapor on a
relatively cold surface of an object. It forms when the temperature of
an object drops below the dew point temperature. 

The practical point I'm trying to make is that people often ask, "When I
wake up in the morning my tarp is soaking wet.  Why is that and how can
I stop that from happening?"  For tarps, the cause is generally
macro-climate dew (i.e. dew is forming everywhere), not micro-climate
condensation (i.e. it's just inside your tarp), and the solution is to
go somewhere where dew isn't forming.  How do you know which is which?
If your tarp is wet only on the underside, then you have condensation
problems.  If your tarp is wet on both sides, then you have dew
problems.  (I'm probably not using those terms according to their strict
definitions, but I hope you get what I'm trying to say.)  In my personal
experience, I almost always have dew problems rather than condensation
problems with my tarp.  In my tent, it's often the opposite.

Like I said in my first post, if the grass and foliage around you is
collecting dew, then your tarp is probably going to get wet and no
amount of creative pitching or VBLs or what have you will stop it.  If
the wind is blowing, sure, it'll help retard dew formation for both your
tarp and everything around it.  But you're at the mercy of the macro
climate around you.  There's nothing you can do inside your tarp to stop
dew from forming, except pitch somewhere else.

Eric