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Hiking Thermoregulation in Cold Rain

Hiking Thermoregulation in Cold Rain

Thermoregulation is a fancy word for an essential hiking and backpacking skill, namely keeping a healthy body temperature even when the surrounding temperature is too cold or too hot. It goes hand-in-hand with layering your clothes to maintain a comfortable body temperature that avoids hypothermia in cold and/or wet weather or heat exhaustion when it’s hot and/or very humid.

One of the biggest thermoregulation challenges hikers and backpackers face is thermoregulation in cold, sustained rain, below about 50-60 degrees Fahrenheit. This is the temperature range when the occurrence of condensation and perspiration inside a rain jacket can make you uncomfortably chilled and potentially lead to hypothermia.

Put simply, moisture on your skin is 25 times more efficient at cooling you than air, which is why sweating is so effective for cooling you in hot weather. But when the outside temperature is much colder than your core temperature (98.6F), excessive cooling will make you feel chilled. But rain is often unavoidable, so it’s important to understand how to mitigate its chilling effect when temperatures dip.

Why do you get wet inside a rain jacket?

All rain jackets suffer from internal condensation when the interior is warmer than the exterior when it’s raining because they trap your body heat. That temperature differential is what causes condensation, just like in a single-wall tent. When I say “all rain jackets,” this includes waterproof/breathable rain jackets, which can only vent water vapor in a gaseous form before it condenses to a liquid. While waterproof/breathable jackets can help vent moisture from perspiration in its gaseous state, they’re unable to vent it after it’s turned into a liquid.

The Role of a Midlayer Garment

If all of the garments you wear inside a rain jacket get soaking wet and the interior of the rain jacket is wet, the moisture will be very efficient at conducting body heat away from your core. It’d be a lot like being immersed in a vat of cold water.

That’s where a good midlayer garment can save your bacon. What you want is a midlayer pullover, hoodie, or jacket that resists saturation, usually by wicking moisture to its outer layers, while trapping your body heat in air pockets to keep you warm. If you can break that pipeline of moisture conducting heat away from your body, you’ll interrupt the flow of warmth away from your skin.

Hiking on the Maine Appalachian Trail in Rain
Hiking on the Maine Appalachian Trail in Rain

Polyester Fleece Midlayer

The solution is to wear a midlayer between your shirt and rain jacket that blocks the transfer of body heat to your rain jacket. A polyester fleece pullover or polyester fleece jacket is the best type of garment for this because it is naturally wicking, pulling moisture from your base layers away from your skin while maintaining an envelope of warm air around your core. Polyester fleece is also better than wool because it absorbs less water (it’s plastic after all) and dries much, much faster.

Polyester fleece is also fairly inexpensive, very durable, and available in different weights so you can dial in the amount of warmth you need for different times of year or climates. Many manufacturers also make recycled fleece pullovers, hoodies, or jackets to mitigate the environmental effects of creating new fleece material.

Recommended Fleece Types

The best fleece midlayers are fairly dense exteriors with lots of small air pockets and good venting. I’d avoid fleece that is windproof, thick pile, or too porous and thin, like Polartec Alpha. What you want are 100-weight or 200-weight fleece tops equivalent to R1 and R2 from Patagonia or TKA100 and TKA200 from The North Face.

Here are some suggested fleece garments to try:

Down and Synthetic Jackets as Midlayers

What about using a down jacket or a synthetic insulated jacket as a midlayer? While these jackets are more popular than lightweight fleece pullovers or jackets, they’re usually too warm to use as a midlayer garment and are best held in reserve to change into when you stop hiking and want to put on drier and warmer clothes. They also absorb a lot of moisture, even if their insulation has been treated with a waterproofing agent, and take much longer to dry than fleece unless you have a clothes dryer handy.

Wrap Up

For three-season as well as winter hiking, most hikers will find a simple lightweight or midweight fleece to be effective as a midlayer under a rain jacket when hiking in the rain. A fleece vest, quarter-zip or half-zip pullover, or full-zip fleece jacket are all good because they provide venting if you feel too warm. A hood can also be useful.

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More Frequently Asked Questions

19 comments

  1. This is a serious topic. The only case of hypothermia that could have been fatal I’ve ever witnessed was due to a sudden windy thunderstorm in summertime — not winter. Luckily, we had a large group of people and could carry the victim to his car to warm him there, since building shelter and warming him up in place would have been impractical given the high winds.

  2. Do you have any personal experience with the Buffalo Systems jackets, which are a combination outer shell and fleece? Brits seem to swear by them, but they look less flexible than a shell/fleece combo. My experience with UK weather, though, is that days of cool/wet weather on end is quite frequent.

  3. A rainy trip up Mt Moosilauke a few weeks ago revealed flaws in what I thought was an effective wet weather system. My Frogg Troggs Extreme Light jacket’s hood could’ve been a bit bigger to accommodate my now-soaked wool hat, the zipper area of the jacket breached, even with the inside flap, leaving a wet stripe down my front. Finally, I’d left my mitten covers at home so it didn’t take long for my gloves to get soaked. My lightweight fleece mid-layer was a great help! Chilled, I changed out of the wet clothes just after the summit and was glad to have chemical heaters just in case the change of clothes also got wet. I have since bought a better rain jacket and added a disposable poncho to my essentials.

  4. Good reason to consider the wool vs poly debate. Thought wool retains warmth when wet? My old R1 is my usual mid-layer but not great without a jacket in windy conditions below 50 unless I have significant exertion. Bought a Smartwool 250 (150 wasn’t available) and REI fleece pants at a Jackson, WY REI 2 yrs ago when a long cold and rainy stretch started. I was warm in all day light rain, highs in the mid-40’s with only modest elevation gain. The Marmot Precip jacket eventually wetted-out but I stayed warm.

  5. Why do you suggest a dense exterior to the fleece? I used to always use a North Face half-zip fleece I got years ago (much cheaper than $70), but I got a Senchi Lark a few months ago and love it. My base layer seems to wick better (apart from my back), and I can better avoid overheating by venting rather than needing to take off the fleece. It’s not very useful as an outer layer though – if I don’t put on my wind shirt or poncho/cape, the alpha doesn’t insulate at all really.

    • Because polartec alpha is too porous to prevent the massive condensation that forms on the inside of your rain jacket from being absorbed by your base layer. It works great as a warm layer when its worn under a shell who’s interior is dry, but if the inside of the shell gets drenched from internal condensation, which is what happens when copious cold rain hits the outside of a warm shell (heated by your body), Polartec alpha can’t repel the onslaught of moisture as well as a fleece with small densely layered air pockets. There’s a really big difference. I suspect your old TNF fleece was something like a TKA100 and it would do better under those circumstances.

      • That’s a useful comment on alpha. I love my Macpac Nitro but haven’t used it much in cold rain. Are your comments based in practical experience? Maybe an alpha and windproof combination under a waterproof would work well?

        • Of course its based on experience. But if you layer fleeces, you’re going to be way too hot.

        • Not layered fleeces, but an alpha layer and then a breathable shell e.g. Black Diamond Alpine Start or Patagonia Stretch Terre Planing hoody and then a waterproof. Ive been using 100 wt fleece under a rain shell for decades and its worked pretty well in wet conditions, but was hoping alpha would improve on this. I will have to do my own testing and also see if the experience of other differs to yours.

    • I have the Polartech R1 and a Senchi Polartech Alpha. The R1 will keep you long warmer but when it gets saturated it’s even colder to me. It becomes a heavy wet and cold towel around your torso. That was one of my more dangerous hikes in the Whites when it became saturated. The Alpha fabric absorbs little water and dries several times faster than the R1. The Senchi Alpha works best for me in rain when the temps are 40 or higher and of course with a shell when it’s raining. I hiked Flume/Liberty on June 1st this year and it ended up snowing on the summits and raining most of the way down. I had the Senchi on underneath a Lightheart Gear rainjacket and was comfortable the whole hike in part because I was always moving.

      • Alpha direct, Climasheild Apex, & Exceloft all squeeze very usable dry… They are my favorite synthetic insulations currently.

  6. What do you think of the Patagonia R1 Air fabric under a rain jacket in these conditions?

  7. I’ve been hearing about mesh base layers recently, but they aren’t very common in the US yet. Do you have experience with them and would a mesh base layer possibly help in this situation?

    Thank you!
    Scott

  8. A friend of mine and I were just talking about this topic last weekend. Thanks for the info. I will be trying it out.
    I am also curious about mesh for a base layer..Any feedback is appreciated

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