keeping warm when camping or hiking
Introduction
cognition becomes impaired when body temperature falls below 35degC and this is likely to create a vicious cycle of further hypothermia as the person no longer makes good judgements to protect themselves
shivering is an important survival mechanism when the body gets cold and is maximal when core temperature reaches 35degC but shivering ceases when the body temperature falls below 31degC in which case death is likely to ensue as temperature continues to fall without the protective capacity of shivering.
Hypothermia due to immersion in cold water
Exposure time for core temperature to fall to 35.5°C
Water temp | ankle deep | knee deep | waist deep | neck deep |
10-12degC | 7hrs | 5hrs | 1.5hrs | 5 minutes |
13-15degC | 8hrs | 7hrs | 2hrs | 5 minutes |
15.5-18degC | 9hrs | 8hrs | 3.5hrs | 10 minutes |
18-20.5degC | 12hrs | 12hrs | 6hrs | 10 minutes |
>21deg C | no limit | no limit | no limit | 30 minutes |
NB. times are half that (except for neck deep) if it is raining; those with less body fat or higher surface area may have less times;
Keeping warm
be prepared
even on short day hikes, no one expects to get lost, injured or caught in an unexpected storm
be sensible and take precautions pending the risks of your activity
take wet weather gear and some form of shelter even if it is just an ultralight tarp and bivvy bag
take a map and compass and a radio beacon (EPIRB)
protect against frostbite
frostbite (skin death) usually occurs when skin temperature falls below below −2°C, lesser degrees may cause chilblains
frostbite to the exposed face in an otherwise properly dressed person is likely to occur when Wind Chill apparent temperatures fall below minus 25degC which would equate to 50kph winds with ambient temps at minus 14degC
frostbite to fingers and toes will occur at much less extreme cold conditions
when apparent temperatures drop below 4degC, cover your skin especially your feet and hands and keep them dry
take a spare pair of gloves, etc in case they get wet
consider chemical pocket warmers
if frostbite occurs:
do not re-warm the skin if there is risk it will freeze again as repeat warming-freezing cycles cause more tissue loss than delayed re-warming
if further freezing is not expected, rapidly rewarm by immersing in 39-42degC water
start one aspirin per day
do not drink alcohol or smoke
elevate the affected part
manage as per burns
don't get wet
many fabrics such as cotton and down loose much of their insulating properties when they get wet - generally avoid hiking with cotton or denim, and don't get your down gear wet! Consider synthetics if getting them wet is likely as synthetics generally retain their insulating properties better and they tend to dry out faster. A wet canvas swag will act as a cooling fridge and will take a long time to dry out!
don't wear your full arm length puffer jackets while hiking unless it is extremely cold - you will just perspire too much - wear layered breathable clothing even if its raining
dry off as soon as possible
avoid perspiration before going to sleep - avoid exertion, excessive clothes (add layers as temperatures drop)
keep wet gear outside the tent
don't sleep with your head inside the sleeping bag - this will make your sleeping bag wet from your exhaled breaths
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keep thermal wear clean as well as dry
don't get injured in cold weather
have a shelter from the rain and wind
whilst tents are poor thermal insulators, in addition to protection from rain and wind chill, they can create a microclimate by helping to retain some body heat - especially if the tent is small and the canopy is full fabric rather than mesh
a 2-man tent fly with minimal wind will provide 0.4 to 1.4deg warmer microclimate than outside
a mesh canopy combined with a tent fly will probably be marginally warmer than a fly alone as the mesh does reduce wind flow
a 2-man fabric canopy tent with fly and mesh doors with minimal wind will provide approx 4 deg warmer by the end of the night microclimate than outside
a 2-man fully enclosed fabric canopy tent (apart from opposite ceiling vents) with fly and fabric doors with minimal wind will provide approx 5-6deg warmer by the end of the night microclimate than outside and probably warmer than this if occupied by 2 people. It will probably give at least 3-4deg extra warmth (and much more effective warmth if there is wind chill in the mesh tent) towards the end of the night compared to an all mesh tent with fly - thus you need to compare the weight savings of a mesh tent with the extra weight (perhaps 250g) of a thermal liner for extra sleeping bag warmth, although a liner does not help with the inspired air temperature
reduce your heat losses:
don't get sunburnt during the day - this will increase heat loss from inflamed red skin
don't drink too much alcohol - this will increase heat loss from skin as it opens up the skin blood vessels when they should be closing down
cover your skin including head, feet and hands
generally layered clothing is best so that you can regulate your warmth without getting too hot and sweaty
thin merino long arm tops and long johns are popular undergarment layers for cold weather conditions
use an insulating sleeping pad - much of your heat loss during sleep is via conductive losses to the cold ground, this also applied when sleeping in a hammock
use an appropriate sleeping bag or quilt with appropriate thermal clothing layers as needed without causing perspiration
sleep in the least amount of clothes that will stop you feeling cold
don't sleep in your socks or underlayer (eg. thermal top/longjohns) you have just hiked in - have a clean warm pair of socks and underlayer reserved for sleeping
consider zipping up your jacket and slipping it over the bottom end of your sleeping bag to give your feet an extra layer of warmth
consider putting your DRY, CLEAN clothes in your sleeping bag so they are not so cold when you put them on the next morning and put anything unclean such as boots in a plastic bag inside the tent so they are not freezing in the morning
reduce your surface area by curling up into a fetal position and have hands between thighs
in very cold climates, have a pee bottle - saves you getting out of the tent at night and the bottle will keep you a little warmer, alternatively, ensure you have some sandals to slip on to go out rather than needing to put your boots on
add extra heat
warm food and drinks
radiant heat from a fire outside your tent
hot water bottle such as Nalgene bottle placed in a sock (or perhaps warm stones from the fire)
go for a walk or do gentle exercise prior to sleep to generate some muscle heat but without creating perspiration
sleep with a partner
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USB-powered heating pads
12V car seat heater cushions
12V electric blanket / throw rugs (usually around 45-60W max)
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12V 130W 10.5A air heater
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carbon monoxide poisoning is a major risk in poorly ventilated enclosed spaces such as tents if fuel is burnt inside or nearby
carbon monoxide (CO) is a colorless and almost odorless gas that results from the incomplete combustion of hydrocarbon fuels when oxygen levels are to low (for LPG, the ratio of air to propane needs to be greater than 24:1 to avoid CO production)
2)
levels of CO above 100ppm create risk as poisoning is cumulative with duration of exposure
levels above 800ppm may result in unconsciousness within 2 hours and you may not wake up to extract yourself and ongoing exposure may result in death
levels above 1600ppm (1.6%) may result in death within 2 hours
levels above 1-2% may result in death after only a few breathes
the effects are greater at high altitudes above 3,000m where oxygen levels are low
smoke from a wood fire usually has around 5000ppm CO - does not require much to get into a small tent to be lethal!
for gas stoves with incomplete combustion, the rate of CO production depends upon two factors: the amount of fuel burnt by the stove and the degree of incomplete combustion.
running a small camp gas stove in an A-frame tent with small vents for 15 minutes resulted in CO levels of stabilising around 100-130ppm as production equals ventilation losses of CO, especially if the flames hit the bottom of the cooking pot - elevation of the pot by 25mm significantly reduced incomplete combustion; 2hrs of use at these levels would cause 5% reduction in oxygen saturation of Hb which could be critical at high altitudes
3)
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if you do decide to use a gas heater in a well ventilated tarp or tent space make sure you have a CO detector alarm!
making the air warmer in your tent to minimise asthma attacks
having a lovely warm sleeping bag does not help much if the air temperature drops below 10-12degC and you suffer from cold-induced asthma attacks
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your body is a 100W radiator and can be used to heat the air rather than your sleeping bag, but to make this comfortable, you will need a 12V “48W” thermal mat (these cycle on and off every few minutes so average power use is around 24W) or 12V electric blanket under you and you will need nice warm socks and a quilt to help keep you warm depending upon ambient temperatures
for rapid warming of the air, a brief usage of a 12V 130W car air demister heater +/- 65W laptop will help
a heating mat allows you to sleep naked so your body heat instead of being trapped in a sleeping bag will add around 5-7 degrees extra warmth to the air temperature inside the tent if it is a full fabric tent and can retain the warm air (a further 2 deg warmth if also are running a laptop)
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Mechanisms of heat loss from the body
respiration
evaporation
when moisture evaporates it uses heat to do so and thus cools the skin in the process
this is the main mechanism we manage high temperatures but it also becomes a very important factor in creating hypothermia if we are cold and wet (eg. from perspiration whilst exercising or becoming too warm)
it is also the reason why reducing
condensation in tents is important so you don't get wet while sleeping
convection
if the air layer close to your body is constantly changing, your body loses heat as it is constantly heating up the air layer
this is why wind chill is such an important factor
wind protection such as layered wind-resistant clothing, a tent and sleeping bag all contribute to reducing convective losses
convection loss is dependent upon:
radiation
all things radiate heat at a rate to the 4th power of the temperature in degrees Kelvin
you can retain some of this by use of reflective materials around your body such as “space blankets” and insulating clothing to retain this heat near the skin
if you are trying to warm yourself near a fire, wearing layers of insulation will dramatically reduce the radiant warming effect of the fire so one needs to weigh up the pros and cons of clothing with regard to its protection from wind chill etc vs impairing heat gain from the fire
conduction
heat flows from a high temperature source (your body) to a lower temperature area (eg. the ground you are lying on and the air layer close to the skin, or worse, very conductive materials such as cold metal close to the skin)
the rate of this heat loss is dependent upon:
this is why an insulating sleeping pad is important when camping
urination
this is a less important source of heat loss however if you drink a lot then you will urinate a lot and the urine will be at body temperature so you are losing heat from the body
avoid drinking lots of alcohol!
but drink enough water to stay hydrated
Mechanisms of thermoregulation of the body
perspiration if too hot
hyperventilation if too hot
peripheral vasospasm of skin if too cold
shivering if too cold
shivering generates heat from high levels of muscle work
shivering occurs when the hands or feet get cold and this sends a signal to the brain to commence shivering
maximal shivering generally occurs when core temperature reaches 35degC and ceases when it falls below 31degC in which case death is likely to ensue as temperature continues to fall.
exercise if too cold
digestion generates heat
the process of digesting food generates heat
eating food before going to bed (especially if warm as this adds body heat in itself) not only creates heat from digestion but provides energy for shivering
reflective emergency space blankets
cheap Mylar ones are crinkly and noisy and not durable, so better options are below
SOL 1-2 person heatsheet Emergency Blanket
AMK Nano Heat Travel Blanket
SOL Heavy Duty All purpose Emergency Blanket (can be used as tarp)
thermally insulated sleeping mats
sleeping bag temperature ratings
assume sleeping on a sleeping mat and wearing base thermal layers
women's bag just use comfort level (comfortable posture and not shivering) if only one quoted
men's bags use the colder “lower limit” or “transition” level (curled up but not shivering) if only one quoted
“extreme level” is emergency survival only - you will be curled up and shivering even with layers on
If a bag is too small, you’ll end up compressing insulation and creating cold spots.
if a bag is too big, you spend some of your energy heating empty space. For very cold conditions, many prefer a mummy style bag to reduce empty space at the feet end at the expense of reduced freedom of movement.
it’s important not to get a bag that’s too warm; if you sweat, you’ll wake up cold.
avoid putting heavy layers on top of a down bag as it will compress the down and lose benefits
A +5 to +10 is considered a summer bag. It is roughly as warm as sleeping with a sheet or light blanket over you on your bed at home.
A 0 degree bag is a great all-rounder but won’t keep you warm in the snow or frosty night – it’s about the same as having a regular doona on your bed at home.
A -5 to -10 degree bag is considered a winter bag for typical Australian climates – it should be similar to having flannelette sheets and an extra blanket or 2 on your bed at home.
sleeping bag liners
use the smallest tent suitable for your needs - it is easier to keep warm
use a full fabric tent rather than a mesh tent, especially one with dual fabric/mesh door options
choose a sheltered camp site
position your tent appropriately for the wind direction (although remember it can change overnight!)
close the vestibules
ground insulation
consider an extra waterproof ground sheet (eg. tarp) and thermally insulated padding under the tent (eg. foam rubber thick mats)
consider using corrugated cardboard as this will also absorb moisture
add a space blanket on the tent floor with reflective layer upwards and if possible tape it to the sides of the tent 12-15cm above the floor
top insulation
wall insulation
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