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On backpacking trips, I would like to avoid dew because it means either waiting till the sun has come up and dried my stuff off or packing wet things and drying them later.

My experience is that dew is less likely at higher elevations, is this true or simply a coincidence?

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According to this Physics forum, altitude does not directly correlate with dew point. So it will greatly depend on the other atmospheric conditions which will very from one time to another. So not something you can count on in order to avoid dew formation entirely.

Dew point temperature depends on the partial pressure of water vapor present in the atmosphere. The atmospheric pressure (and hence, altitude) does not affect it.

Reference https://www.physicsforums.com/threads/dew-point-temperature.248410/

They explain it a bit more but that was the most straight forward explanation.

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    I think you might confuse dew point with dew point temperature. The dew point temperature is not affected by altitude and atmospheric pressure. However, the dew point is. Since it gets colder at higher altitudes, the relative humidity increases, until it reaches 100%. The temperature at the altitude, where the relative humidty reaches 100%, is the dew point temperature. So at higher altitude (with temperatures decreasing, getting closer to the dew point temperature), you should be more prone to dew than at lower altitudes, ignoring other factors like for example foehn wind. – Peter1807 Sep 8 '17 at 6:50
  • Hmm... the quote in your answer seems to contradict the answer itself: "partial pressure of water vapor present in the atmosphere". Partial pressure is that portion of the total pressure taken up by something. So (with other variables remaining constant) as elevation increases and total atmospheric pressure decreases, the partial pressure of the water vapor present in the atmosphere would increase, directly correlating to dew point. – Jimmy Fix-it Sep 10 '17 at 14:07
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Cause of dew:

You have a local surface that is below the dewpoint. It has gotten enough colder than the air, that water condenses on it.

Dew is heavy on clear nights (more surface radiation cooling as your sleeping bag tries to warm up interstellar space) Smoke in the air, or overcast will reduce or eliminate the amount of dew.

Dew is heavier when the air is more humid. (Temperature doesn't have to drop as much for water to condense) However water vapour interferes with radiation.

Dew is almost non-existent if there is a breeze. The air next to a surface can't get that bit colder to allow water to condense.

Avoiding Dew:

Move away from water. Running creeks, lakes, swamps will provide a source of water vapour. Large bodies will be warmer than the land at night and will evaporate strongly, putting more water into the air.

Move away from the bottom of the valley. Cold air settles. I've found that being even 15 to 20 feet vertically (5-6 meters) can make a 10 F (5C) difference in temperature.

Put something between you and the sky. This blocks radiation from the surface of your bag, so the surface doesn't cool as much, reducing or eliminating dew. This can be a tarp, a tent, or the canopy of a tree.

Reduce the amount of visible sky This is a variation of the previous point. Radiation is going to be proportional to the amount of sky that your sleeping bag can see. Very little dew at the bottom of a cleft. A ravine can reduce sky by 50%

Dealing with Dew.

If you use a tarp or tent, pack the wet layer in a plastic bag so that it doesn't get your other gear wet, or roll it up in your foam pad to keep it out of your pack entirely.

Site your camp to catch the sun at sunrise. Or as soon as you get up move the wet item to a spot that is sunny. If I've tarped it, I will raise the west corners so that the entire tarp is exposed to the sun.

Get a lighter coloured sleeping bag/tarp/tent. You actually want a fabric that is very reflective in infrared. This radiates poorly. As far as I know no one makes such a fabric for backcountry gear.

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Dew is likely wherever there is atmospheric moisture and an overnight drop in temperature. Dew forms when the ground or any other object radiates it's heat and becomes cooler than the air. The moisture in the air condenses on the cool object just as water condenses on a glass of ice water. This typically happens during the coldest part of the night, which is just before the dawn.

The only way to avoid dew on your sleeping bag is to not expose it to the outside air. This is most effectively achieved by sleeping in a bivy sack, in a tent, or under a tarp.

  • I really rather avoid dew altogether. On one of my last trips I got dew at low elevations and none at high and I am wondering if that usually holds true – Charlie Brumbaugh Sep 7 '17 at 22:21
  • @CharlieBrumbaugh Depends entirely on how much moisture is in the air, you can't guarantee there won't be dew at higher elevations, but other factors such as wind, inversions, etc. may make dew less likely. In general, if you have humidity, you're going to have dew also. – ShemSeger Sep 7 '17 at 22:30

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