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Thread: Southern Dew

  1. #21
    Señor Member wisenber's Avatar
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    Quote Originally Posted by HappyHiker View Post
    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.
    Using an overcover tends to replace the dew problem with a trapped condensation problem by removing the ability of moisture to escape.
    Further, hanging a tarp lower and tighter also tends to result in more condensation than less. Closing tarp doors will be worse than that.
    Promoting airflow is the best way to reduce condensation buildup.

    There are really only two ways to dry things: heat or air movement. Conversely, the way to stay dry involves the same two elements.

  2. #22
    Benji's Avatar
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    funny

    Quote Originally Posted by Frost View Post
    the warm pink bear burrito under the tarp.
    I spit coffee in my lap reading that!!!!
    Benji
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  3. #23
    Senior Member TinaLouise's Avatar
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    for me, I've found that I need to do a lot of these things. I pitch my tarp with one end higher. I need air flow under the tarp but still have to be careful of wind and what the temps will be at night. My under quilts have a real DWR coating on the outside of them. And I have to have something inside my hammock around my face to trap the moisture from my breath. I have to be more careful with using this in the winter than in warmer temps. Then comes camp site selection... The worst site I've found is right next to the river. The fog rolls in right off the water and under my tarp. For that, I need some kind of over cover on my hammock. And then I've got to watch that I keep it vented somehow inside the hammock or I'll have a rain event in there with me.
    The more I hammock, the more I learn how to keep dry and warm and comfortable.
    Now during the summer, when temps are way WAY over 100 during the day and the nights are humid and still hot, I throw all this out the window!! That's a whole different ball game and I've found that I need a whole different set of tricks to stay "cool" dry and comfortable.

  4. #24
    Senior Member HappyHiker's Avatar
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    Quote Originally Posted by wisenber View Post
    Using an overcover tends to replace the dew problem with a trapped condensation problem by removing the ability of moisture to escape.
    Further, hanging a tarp lower and tighter also tends to result in more condensation than less. Closing tarp doors will be worse than that.
    Promoting airflow is the best way to reduce condensation buildup.

    There are really only two ways to dry things: heat or air movement. Conversely, the way to stay dry involves the same two elements.
    True. All depends on the OC material - more breathable is better. Hanging the tarp lower will result in more condensation on the tarp, but not the sleeping bag.

    Condensation happens - the question is, where do you want it to occur. Promoting airflow can cause the dewpoint to move to the surface (or even below the surface) of your TQ/SB by removing the microclime of warmer air.

    Conditions vary (temp, humidity, location, air movement) widely, and all effect condensation.
    Last edited by HappyHiker; 02-15-2012 at 00:17. Reason: Clarification
    Experience is the worst teacher - it presents the exam first and the lesson later. - Unknown

  5. #25
    OutandBack's Avatar
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    Post #15-Frost

    That was an excellent read Frost.
    Thank you so much for taking the time to explain how this works. I learned a lot.
    O&B
    Last edited by OutandBack; 03-03-2012 at 08:07.

  6. #26
    Senior Member bwg's Avatar
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    Quote Originally Posted by Frost View Post
    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.

    ....
    I am not sure I understand whether you wrote that a tarp helps prevent dampness when dew forms or whether a tarp assists dampness forming.

    My experience here in Georgia is that my hammock and TQ consistently get wet, without rain, if my tarp is not deployed and this occurs whether I am in or not in my hammock. Two weeks ago I mistakenly left hammock and TQ outside and found them both soaked at 11pm (no rain). I've never had this experience when my tarp was deployed.
    Last edited by bwg; 03-02-2012 at 23:06. Reason: sentence clarification

  7. #27
    DivaB's Avatar
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    Quote Originally Posted by Frost View Post
    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.

    WOW!!!! Thank you for your weather 101 course on dew build up. I learned a lot.

  8. #28
    MAD777's Avatar
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    Great post, Frost! Knowledge is power!
    Mike
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