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Thread: Southern Dew

  1. #11
    Cali's Avatar
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    No, but "Southern Dew" will make you care less about any dew on your tarp or UQ or anything else.
    Happy Hangin!!!


    AKA BajaHanger

    You cannot solve a problem with the same mind that created it. -Albert Einstein

  2. #12
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    Here in nor cal, we have fog. Thick, thick fog. Everything gets wet! It worse then rain. Fog comes under the tarp and wets the quilts. Only thing I've found is to pitch tarp as close to the ground as possible and close the doors.

    S

  3. #13
    Senior Member zukiguy's Avatar
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    I just spent a week hiking in the everglades and every morning my tarp was soaked, top and bottom. I was kind of disappointed that it didn't bead up and roll off easily. I carefully took down my quilts and hammock trying not to rub against the underside and get drenched. Then I took a corner loose and shook like mad. It looked like a rainstorm but got most of the moisture off the tarp before packing up.

    I repeated this every morning for 4 days. The temps only got down to 49 or so one night and the rest were warmer. I don't know what the dew point was (not really that much of a weather nerd) but I'm guessing we broke it based on all the moisture.

  4. #14
    Senior Member taylo's Avatar
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    I've been sleeping out in the back yard every night when it's not raining here for awhile now. I've had massive dew on the underside and topside of my tarp just about every night. A few nights, the air was dry enough to stave it off. Dry air just isn't very common in Alabama, and your gear will suffer if you dont have adequate protection. This I've learned.
    And it seems to be worse when it's warm in the day, and drops to cold at night.

    We need a weather man to explain dew point to us.

  5. #15
    Senior Member Frost's Avatar
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    I see a couple misconceptions here about dew that I might be able to clear up, and maybe we can draw up some good ways to counter it with the information. So gather round the weather nerd, folks.

    Fog forms when the air temperature is the same as the dew point. That is called a 'saturated air mass'. It doesn't happen very often, and when it does, it's almost always in the form of precipitation. Fog is a finicky creature, and needs uber-specific conditions to form.

    Dew doesn't 'fall'. That seems to be the most common misconception. It forms where you see it, unless it is dripping off of leaves or the like above you.

    Dew is just condensation. It forms when objects near the ground drop below the dew point, which is usually with in a couple degrees of the ambient air temperature. With out exception, unless there is fog, it can only form on objects that have some kind of help in cooling off. In our case, almost always this is evaporative cooling. If the air temp is much higher than dew point (more than say, 4 or 5 degrees) objects have a hard time cooling down enough to see any condensation at all.

    Leafy objects see the dew first because they are very good at evaporating moisture into the air, which as anybody who's been caught in a summer rain storm with high winds can attest, can cool an object to below the ambient air temperature. This is not normally possible simply by sitting and radiating your heat off into your environment. The moisture that evaporates off of you literally carries the heat away.

    If you put your tarp up over dry, bare ground, and didn't sleep in it, you wouldn't see dew on it, pretty much ever. Dew forms on it, most heavily on the inside, because it acts as its own little weather generator.

    When you pitch your tarp, it does two things. It traps warm air that wants to rise from the warm ground, and from the warm pink bear burrito under the tarp. It also traps moisture. Some of this also comes from the soil and plants beneath the tarp, but a much larger amount comes from us, and not just from breath. Sweat as well. The trouble starts because the tarp is very thin, so it doesn't insulate much. This means that the tarp material itself, while holding a lot of heat and moisture beneath it, will want to sit somewhere near the ambient outside air temperature. This will very often be *well* below the dew point of that warm, moist air trapped under it, so dew forms on the inside of the tarp. Lots of it.

    Now you've got yourself a recipe for a nice indoors monsoon to accompany your breakfast in the hammock.

    In addition, you're sweating into your bag and hammock, as well as breathing *very* humid, nearly saturated, warm air in to it. This lets the exterior of the bag actually get slightly below the ambient air temperature because this moisture will evaporate and cool the surface of your hammock and bag. If the temperature inside your tarp is, let's say 40F, it's very possible that the surface of your bag could be 39F, or even a bit lower. When the difference between the ambient air temperature and dew point is a half a degree or so on those balmy Georgia nights, that's plenty enough to cause dew on your bag and UQ.

    Ok, so Meteorology 101 is over. What can we do about our condensation problems?

    Firstly, and easiest, if you can help it, don't set up on top of lots of leafy vegetation. Grass is not your friend here. It is darn good at pumping moisture into the air. The same goes for damp ground, especially if it's warm, damp ground. The ideal spot to pitch would be straddling a big, dry slab of bedrock. Ice isn't bad either. Snow is okish. Better than warm, wet mud anyway.

    A tarp laid out beneath your hammock will help cut down on this source of radiated heat and moisture. The trouble is, a *lot* more of it comes from you than from the ground.

    So, you're a sweating, breathing, highly efficient humidifier. What do we do about it? Obviously the #1 source is your breath. As folks have mentioned, some kind of absorbent cloth very near your mouth and nose is one good way. It will cool quickly through evaporative processes to the dew point, and collect moisture very well. It won't catch 100%, or even 50% unless it's worn as a mask, but it will help a lot.

    Next up is the moisture you're pushing into your TQ and UQ. There isn't much you're going to do about this aside from using a water proof shell on them. This will prevent that evaporation that will cool the surface of the bag to below ambient air temperature, and any dew that *does* form, will form on the water proof surface, but then the moisture you are pushing in to the insulation will be trapped there until you turn the thing inside out and let it dry. This is going to be one of those things you should experiment with to see where the cost/benefit balances out. In the spring and fall in the deep south, I think it might be worth it. For most of us in more temperate/dry climates, I doubt it will work out favorably. You'd trap more moisture in the insulation over night than would be collected as dew on the exterior.

    One last trick that helps with the tarp condensation. Don't pitch your tarp perfectly level. If you can, jack the head end up slightly higher than the foot end, and leave the doors open, at least a bit. This will allow a convective flow to establish as your body warms the interior of the tarp. The warm, moist air will flow out the top, and the cool, somewhat dryer air will flow in the bottom, hopefully preventing the build up of really moist air that leads to the morning rain storm in the hammock. You can also pitch your tarp sides as steeply as possible, as this will let the water bead and run a little easier, as well as minimizing the amount of volume the tarp has to collect the warm, moist air at the top where the surface area/volume ratio is stacked against you the most.

    So there you go. A scientific approach to dew. Being a relatively inexperienced hammocker, I'm going more off my physics and meteorology courses than anything else here, so I'm eager to hear from the Hanging Masses if I'm misinterpreting something.
    Last edited by Frost; 02-08-2012 at 21:58.

    If - if he stood! Enough of ifs!
    He knew a path that wanted walking
    He knew a spring that wanted drinking
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    "A Lone Striker" Robert Frost

  6. #16
    Senior Member HappyHiker's Avatar
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    Excellent post Frost!

    Some more info here:
    http://johncwalton.com/WindowOutdoor...0Radiation.htm
    Experience is the worst teacher - it presents the exam first and the lesson later. - Unknown

  7. #17
    Senior Member taylo's Avatar
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    I guess a proof of all this is your window panes in winter when you are running the heat. You have warm ambiet air inside, but the cold air outside causes the window pane to drop below ambiet room temp., and you get lots of moisture on the inside of your window panes.
    I also am a believer in the grass dilemma. My backyard setup is over really thick grass, and I do believe that grass is a major factor in my morning tarp dampness.

    But my main trouble was my topquilt getting wet without a tarp. Im guessing from your teachings that it's my moist body heat evaporating thru my quilt that is causing it too cool down and form moisture on the surface.

    Right?

  8. #18
    Senior Member emcee's Avatar
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    Thanks, "Professor" Frost. Good tips on tarp pitching.

  9. #19
    Senior Member Frost's Avatar
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    Taylo, yes this is one potential reason for the dew on your TQ with no tarp.

    Another way to look at it may be that you're kind of creating your own little microclimate surrounding the hammock. If the air is very still, you would create a warm pocket of air sitting immediately around the hammock. If you moved, or a slight breeze kicked up, some, but not all of that warm air would be replaced with rather cold, dryer air. You'd see a sudden increase in evaporation from your TQ, along with a sudden cooling, and as the warm pocket settles back in around you, moisture condenses.

    Then you've set up a vicious cycle where it has more moisture to evaporate, which cools it further, which creates more condensation, repeat until your butt cheeks freeze together and your down goes back to its roots and heads south looking for warmer weather.

    The ideal solution to this situation would seem to be a genuinely breathable DWR shell on the bag so that your perspiration can evaporate away, and any dew that forms just beads up harmlessly on the top. The only trouble I see there is that, if it beads up that well, it will run off the shell, into the hammock, and you'd awaken in a puddle.

    If - if he stood! Enough of ifs!
    He knew a path that wanted walking
    He knew a spring that wanted drinking
    A thought that wanted further thinking.
    A love that wanted re-renewing

    "A Lone Striker" Robert Frost

  10. #20
    Senior Member HappyHiker's Avatar
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    Quote Originally Posted by taylo View Post
    But my main trouble was my topquilt getting wet without a tarp. Im guessing from your teachings that it's my moist body heat evaporating thru my quilt that is causing it too cool down and form moisture on the surface.

    Right?
    Mostly. It's also the moisture in the surrounding air as well. Even a tarp will not entirely prevent it, unless it's pitched so close as to create a stable micro-clime that will move the dew point out to the tarp surface. Overcovers help in this respect.

    FWIW, I'm notorious for picking the worst spots to sleep (grassy meadows, edge of trees, on a ridge, no cover etc) and without a tarp in order to obtain the best stargazing. What moisture does collect will dry out in short order by simply laying your bag/quilt out for a short while in the sun. It only becomes an issue if you can't dry it out for days on end, or in extremely cold conditions where it can freeze in the insulation and vapor barriers become crucial.
    Experience is the worst teacher - it presents the exam first and the lesson later. - Unknown

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