The four mechanisms of heat loss are evaporation, convection, conduction, and radiation.
Evaporation refers to heat loss through moisture evaporating off of skin. Sometimes we want evaporative heat loss, like sweating in hot environments. Sometimes we want to avoid it, like while we’re drying off after a swim in an alpine lake.
If you want to protect against evaporative heat loss, be sure to wear wicking, quick-dry fabrics and leave cotton clothing at home. If you want to channel it to regulate your body temperature, cotton T-shirts and wet bandanas on your head or neck can help keep you cool.
Convective heat loss is the flow of warm air from the body to the air around you (typically exacerbated by wind). One way we prevent convective heat loss is by wearing rain jackets or windshirts.
Conduction is the loss of heat from direct contact with a cooler object, like when you sit on a rock. One way we combat conductive heat loss is by sleeping or sitting on pads instead of the bare ground.
Radiation is heat loss to a cooler object without directly touching it. Our bodies are always exuding heat, so radiative heat loss is always happening. Adding layers holds your radiant heat closer to your body, instead of letting it escape to the ambient air.
Principles of Layering
Your sleeping bag is like a thermos: If you put something warm in, it’ll stay warm. If you put something cold in, it’ll stay cold. So make sure you’re already warm when you crawl into your bag by exercising. (It’s also a good idea to empty your bladder before bed, since your body has to use energy to keep that extra fluid warm.)
Combat Radiative and Conductive Heat Loss
- Make sure to keep your sleeping bag on your pad to prevent losing body heat into the ground.
- Zip up the hood on your sleeping bag. You may feel claustrophobic, but it’ll keep you warmer.
What to Wear
- Baselayers (just your long johns), thick socks, and a hat
What not to Wear
- Puffy jackets will make you sweat, and the moisture will leave you colder. Also, large puffies can compress your bag’s insulation if there’s not enough space.
- Rain gear will prevent your body’s radiative heat from warming your sleeping pad.
It’s easier to warm up than cool down, but if you’re in a hot environment, there are still some options to help sleep comfortably:
- First and foremost, open all the vents on your tent. Making sure there’s ample air circulation (and a cross breeze if possible) will help tremendously.
- If that doesn’t work and there’s no rain on the way, then remove your tent’s rain fly for even more circulation.
- If that’s not working, and your tent has this option, try sleeping on your tent’s footprint with the fly over you like a tarp.
- Still hot? Sleep under the stars.
- And of course, open your bag’s zipper to increase ventilation.