see also:

• bushwalking in Australia
• heat injury, burns, heat rashes and heat illness
• the problem with cold air whilst camping
• tips for better sleeping when camping
• risk management when camping
• comparison of full nylon 2P and 3P tents for colder nights
• shelters and tents for car camping or alpine 4 season use
• tips for camping in the rain
• camping in the snow (in nice weather)
• how to reduce your risks in lightning storms
• hot water and showers for campers
• heaters for camping

• hypothermia is a major risk for hikers and campers in cold conditions especially if they are under-prepared, get wet or they have to stop moving due to having to rest, sleep or from injury
• every 10kph increase in wind speed results in approximately 2degC colder apparent temperature¹⁾

Exposure time for core temperature to fall to 35.5°C

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;

• most will start to feel increasingly cool as temperatures drop below 25degC
  • obviously sunlight will keep you warmer depending upon its strength, and you may tolerate temperatures down to 12degC naked
  • wind chill will significantly impact comfort levels and hypothermia risk
• the lower critical temperature (LCT) is defined as “the ambient temperature below which the rate of metabolic heat production of a resting thermoregulating tachymetabolic animal must be increased by shivering and/or non-shivering thermogenesis in order to maintain thermal balance”. LCT for humans is probably around 25-26degC (23degC if lying on a stretcher bed)
• for a resting naked adut, ambient temperatures below the LCT and above the hypothermia zone are considered to be in the metabolic zone where basal metabolic rate of ~75W increases to its maximum of around 150W to match the extent of dry heat loss before plateauing in the hypothermic zone ²⁾
• shivering is initiated when cutaneous cold sensors drop below 33degC and these fire maximally when they fall to 20degC
• if suddenly exposed to 10degC:
  • 1st shivers appear in 6-7 minutes and they become generalised by 10-11 minutes and can increase heat production of the body 5-fold
• manual dexterity begins to be reduced as ambient temperatures of hands falls below 12degC, while touch sensitivity falls at ambient temperatures below 8degC
• if skin temperatures fall below 10degC, the skin blood vessels start cycles of dilation to provide oxygen and nutrients to skin and constriction to conserve heat but at risk of development of frostbite tissue damage
• a light summer camping quilt will be adequate comfort cover for temperatures down to 13degC but a more insulating quilt is needed for comfort below 12-13degC assuming there is a thermally insulated mattress to lie upon
• one can sleep comfortably in air temperatures of 12-13degC without shivering and without any covering to the trunk if one lies upon a heat source such as a 50W electric blanket and the head, hands and feet are kept covered.
• cold adaptation can modify the tolerance to cold ambient temperatures
  • this may be related to a high basal metabolic rate and a higher amount of brown fat stored which is more readily converted to heat before the need for shivering
  • the threshold set point for shivering is shifted to the left for cold adapted and the elderly (making the elderly more prone to hypothermia)
  • evolutionary changes also have an impact:
    • Inuits have a basal metabolic rate 50% higher than persons living in a temperate climate do, and have relatively more sweat glands in the face and less on the body
    • Australian aborigines are able to sleep naked on the ground even at low temperatures

• CLO is a value that describes the degree of insulation provided by an article of clothing
• A CLO value of 1 is equal to the amount of clothing required by a resting human to maintain thermal comfort at a room temperature of 21degC and is based on a typical business suit, which includes a shirt, undershirt, trousers, and suit jacket.

source: https://roastsurvey.com/blog-post/understanding-clo-values/

• 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
  • frostnip is an early stage of frostbite before the skin cells freeze and often first effects tip of nose, tip of ears and fingertips and can be corrected by covering but may result in sunburn-like effect with skin later peeling off
  • 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
  • minimise and manage condensation in tents
• keep thermal wear clean as well as dry
  • dirt impairs insulating properties
• don't get injured in cold weather
  • this will impair you ability to keep warm by moving and to seek shelter
• 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
    • have additional layers inside your sleeping bag to keep them warm should you need to layer up during the night as the temperature drops
  • 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
  • BUT manage risk of heat injury, burns, heat rashes and heat illness
  • 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
  • consider additional heat sources such as:
    • USB-powered heating pads
      • these are very small and low power output but make great heating pads for small pets
    • 12V car seat heater cushions
      • generally rated at 45W are around 20-24W (2A) averaged as cycle in ~5 min periods of On/OFF and will raise 2P fabric enclosed tent temperature by around 2-4degC as well as providing direct heat
    • 12V electric blanket / throw rugs (usually around 45-60W max)
      • Kickass blanket 3 settings $AU69 1.9A, 2.85A, 3.8A (46W) with 45min auto off
    • 12V heating pads / "electric blankets" to lie on
      • ElectroWarmth T36 mattress pad $AU122 75W 7 power settings, “3.1A” on lowest setting, no auto-off (can get separate controller)
        • Setting 1: seems to run at <3A when ON but has long OFF periods so gives minimal heat
        • Settings 2-7 run at 6.5A when on but the proportion of time it is on increases as setting increases until constantly on at level 7 when it hits surface temp of 48degC but will drain your battery quickly - great to get warm quick in 15 minutes but then turn it back to setting 3 or so!
          • Setting 2 On 1 sec Off 1 sec so average is around 3.1A?
          • Setting 6 On 2 secs Off 0.5sec
          • safety pins to attach to mattress to prevent it scrunching up - consider using bunjee cords instead
    • 12V 130W 10.5A air heater
      • will raise temp from 13 to 19degC within 10 minutes in a full fabric 2 man tent but will drain battery fast if left on!
    • NB. each 10Ah 12V LiFePO4 battery capacity will weigh at least 1.1kg (see battery systems for camping)

• see also the problem with cold air whilst camping
• we have evolved to minimise cold air hitting our lungs - prolonged breathing of cold air is not going to make you healthier and for some will make breathing problematic!
• 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
• first you need a tent that can have ventilation reduced to almost nil - see comparison of full nylon 2P and 3P tents for colder nights
• 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

• the body is constantly losing heat by warming up and humidifying the inspired air
• this can be reduced by warming the air before we inspire it (eg. sleep in an enclosed tent rather than under a tarp)

• 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

• 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:
  • area of skin exposed and the degree of insulation over that skin
    • your head accounts for 10% of the body's surface area but becomes a much higher proportional source of heat loss when the rest of your body is insulated by clothing but your head is not - hence in the cold, wearing a beanie or similar is important in reducing your heat losses
  • wind chill: air temperature and air speed but further compounded if skin is wet
    • see wind chill

• all things radiate heat at a rate to the 4th power of the temperature in degrees Kelvin
  • rate of heat loss = constant x surface area x temp in Kelvin⁴
• 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

• 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:
  • the temperature gradient (this would be equivalent to voltage differential in electric currents)
  • insulation factors (this would be equivalent to resistance in electric currents)
• this is why an insulating sleeping pad is important when camping

• 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

• the first body response to cold is to divert blood from skin to maintain core temperature and this makes skin cold and pale and the increased effective blood volume causes cold diuresis and your bladder will fill up
• shivering is a body response to generate heat and is dependent mainly on the temperature of hands and feet as these are usually the first parts to cool down as the body reduces blood flow to these high surface area regions to retain heat
• shivering will increase respiratory minute volume, oxygen consumption and respiratory quotient and requires energy

• humans have evolved an important mechanism of perspiration to increase evaporative heat loss in hot conditions
• this is less effective when it is hot and humid

• this is why hairy animals which cannot use perspiration as significant form of heat loss (eg. dogs) need to pant in hot weather

• the sympathetic system is activated resulting in peripheral vasoconstriction, piloerection, increased muscle tone, release of thyroid hormones and catecholamines, and activation of thermogenesis in brown fat and of course the renown penile “shrinkage” in the cold (erection is a parasympathetic response not a sympathetic one!)
• reducing blood flow to the skin reduces heat loss from the skin but also risks frostbite
• excessive alcohol can impair this as alcohol is a vasodilator and thus increases risk of hypothermia

• brown fat is rich in mitochondria and readily generates heat when activated
• capacity of brown adipose tissue (BAT) to dissipate energy is mediated via the catecholamine-induced B3 adrenergic receptor upregulation of the expression of brown-fat-specific mitochondrial uncoupling protein 1 (UCP1) which is inhibited by glucocorticoids (accentuated inhibition by falling sex hormone levels)
• BAT is also known to secrete various metabolism-improving factors, collectively called BATokines, that target various cell types.⁵⁾
• It was widely believed that BAT is present in significant amounts only in newborns, infants and patients with phaeochromocytoma, and that it declines with age in adults becoming rare to detect in those over age 60yrs on PET scans. However, exposure to mild cold (ambient room temperature between 16 and 17°C for 6 weeks) ⁶⁾can increase BAT stores in adults especially in the supraclavicular and paraspinal regions, and this may have a role in improving metabolism, insulin sensitivity, and reduce obesity and the “metabolic syndrome”.

• 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.

• the risk of hypothermia significantly increases when you stop walking

• 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

• cheap Mylar ones are crinkly and noisy and not durable, so better options are below
• SOL 1-2 person heatsheet Emergency Blanket
  • 1.52×2.44m reflective 1mill tarp/blanket
  • 95g $AU15
• AMK Nano Heat Travel Blanket
  • 170x147cm; 193g; quiet, soft, reflective fabric $AU66
• SOL Heavy Duty All purpose Emergency Blanket (can be used as tarp)
  • 1.52×2.44m reflective 2.5mill tarp/blanket
  • 223g $AU31 or included in the SOL Emergency Shelter Kit with paracord and pegs for $AU53

• avoid thick air mattresses as they tend to drain warmth from you
• aim for thermally rated sleeping pads

• 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.

• thin silk or cotton liners may add a few degrees warmth
• a thermal liner weighing 250g may add an effective 8 degrees to your sleeping bag rating

• 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
  • if camping in snow, create a snow wall shelter from the wind
• position your tent appropriately for the wind direction (although remember it can change overnight!)
• close the vestibules
  • this will result in the air temperature outside your tent but inside your fly being 0.4-1.4deg warmer than outside and thus creates a better temperature gradient for your tent as well as reducing wind chill and reduce loss of warm tent air through excessive ventilation on windy nights
• 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
  • add a space blanket
    • duct tape or fasten the blanket all over the top of the inside of your tent with the aluminium of the blanket facing the inside.
  • consider extra one or two waterproof tarps over your tent in addition to the fly
• wall insulation
  • consider lining walls with space blankets or 10mm bubble wrap (insulating rating of R1 and can also be used above bed)
• consider a heater
  • a USB heater mat with a power bank will add some valuable warmth to your skin (better still, a 12V car heated cushion at around 2A/hr or a 12V electric blanket - assuming you have an auxiliary 12V battery)