4 ms·

I'm not a physicist but wouldn't this reflective effect be tiny compared to the prevention of convective heat loss?

Cerium 5y ago
I spent a night in the woods (on purpose, not a real emergency) sleeping with an emergency blanket. It was not at all comfortable, but the blanket seriously worked. I would wake up whenever it moved to stop covering me because it would instantly become cold, then feel much warmer as soon as the blanket is back in place. The reflective action is impressive. They also act as a wind break and turn whatever clothes you are wearing into a more effective measure against convective heat loss.
- zwp 5y ago
  A long time ago I similarly tested a (non-heat-reflective) orange plastic survival bag, sleeping overnight behind some rocks on exposed moorland (something like https://www.military1st.eu/cs037-highlander-emergency-survival-bag-orange.html https://www.military1st.eu/cs037-highlander-emergency-surviv...). The poor thermal insulation and condensation were predictable. I hadn't predicted the shifting wind which chose to balloon my orange cocoon with cold air every 20 mins no matter which orientation I chose to lie in. I finished the night with a bungie cord around the sack, under my armpits. That of course left my head, shoulders and arms exposed but kept the wind out. Not recommended. After this experience I upgraded to reflective mylar survival bags. NB "bag". These are a little harder to find than survival blankets but if a bag can be so badly affected by moderate wind, a blanket would be so much worse.
- swader999 5y ago
  I sewed one into my ski jacket. Always good to have in case of injury and help an hour away.
legulere 5y ago
Considering average human body area is around 2 square meters and body temperature is around 37 degrees Celsius I get around 1000 W of radiation from the human body according to this calculator: http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/radfrac.html http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/radfrac.h...
- Dylan16807 5y ago
  Or to put that in terms of net heat loss, in the 0-30C range you're looking at about 10 watts for every degree of difference between your skin and the environment.
- deleted 5y ago
  [deleted]
- dredmorbius 5y ago
  Backing out human power consumption from caloric requirements comes closer to 120W constant output, at 2,500 kcal/day. The range is a low of about 50--75W (1,000--1,500 kcal, effectively starvation levels), and a maximum ceiling of around 485W (10,000 kcal day, perhaps a elite endurance athlete). Well below 1 kW in all cases.
- snewman 5y ago
  Human skin temperature is considerably lower. (Or to think of it from another perspective, the system which is insulating your core temperature includes layers inside your body – skin, etc. – in addition to clothing, blanket, etc.)

I spent a night in the woods (on purpose, not a real emergency) sleeping with an emergency blanket. It was not at all comfortable, but the blanket seriously worked. I would wake up whenever it moved to stop covering me because it would instantly become cold, then feel much warmer as soon as the blanket is back in place. The reflective action is impressive. They also act as a wind break and turn whatever clothes you are wearing into a more effective measure against convective heat loss.

zwp 5y ago
A long time ago I similarly tested a (non-heat-reflective) orange plastic survival bag, sleeping overnight behind some rocks on exposed moorland (something like https://www.military1st.eu/cs037-highlander-emergency-survival-bag-orange.html https://www.military1st.eu/cs037-highlander-emergency-surviv...). The poor thermal insulation and condensation were predictable. I hadn't predicted the shifting wind which chose to balloon my orange cocoon with cold air every 20 mins no matter which orientation I chose to lie in. I finished the night with a bungie cord around the sack, under my armpits. That of course left my head, shoulders and arms exposed but kept the wind out. Not recommended. After this experience I upgraded to reflective mylar survival bags. NB "bag". These are a little harder to find than survival blankets but if a bag can be so badly affected by moderate wind, a blanket would be so much worse.
swader999 5y ago
I sewed one into my ski jacket. Always good to have in case of injury and help an hour away.

legulere 5y ago

Considering average human body area is around 2 square meters and body temperature is around 37 degrees Celsius I get around 1000 W of radiation from the human body according to this calculator: http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/radfrac.html http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/radfrac.h...

Dylan16807 5y ago
Or to put that in terms of net heat loss, in the 0-30C range you're looking at about 10 watts for every degree of difference between your skin and the environment.
deleted 5y ago
[deleted]
dredmorbius 5y ago
Backing out human power consumption from caloric requirements comes closer to 120W constant output, at 2,500 kcal/day. The range is a low of about 50--75W (1,000--1,500 kcal, effectively starvation levels), and a maximum ceiling of around 485W (10,000 kcal day, perhaps a elite endurance athlete). Well below 1 kW in all cases.
snewman 5y ago
Human skin temperature is considerably lower. (Or to think of it from another perspective, the system which is insulating your core temperature includes layers inside your body – skin, etc. – in addition to clothing, blanket, etc.)