those gold/silver emergency blankets: do they really work, and how?
In: 3859
They do. They work by being highly reflective and by just being a blanket. (They are also very light for the amount of insulation they provide, which is important because they’re meant to be emergency gear carried as a precaution rather than a main part of one’s equipment.)
Your body loses heat in three ways.
The first is by *conduction*. Your body comes in contact with your environment, which is cooler than your body. Your body heats up the environment, and loses energy in the process. This is why a coat or a blanket helps: they reduce the rate at which you’re exposed to the outside environment, and thus reduce heat loss. (In many real scenarios, *convection* – the movement of air due to temperature differences – is also important, but the underlying mechanism is still conductive heat transfer.)
[EDIT: some corrections to ^ this paragraph below. TLDR, convection is more separate from conduction than I had realized.]
The second is by *evaporation*. Dry air comes into contact with your skin, and water from your skin evaporates. Evaporating water takes energy, so this process cools your skin. This process is why ~~swearing~~ sweating keeps you cool, and it’s why hot and humid air feels much hotter than hot and dry air.
The third is by *radiation*. Your body, like all objects, glows. For objects at normal human temperatures, this glow is in the infrared, and can’t be seen by human eyes, but it’s the same kind of glow as a red-hot piece of metal (where the glow has begun to shift into the visible part of the spectrum). That glow carries away heat, which is absorbed by nearby objects, allowing heat transfer even when two objects aren’t in direct contact.
For a normal human, conduction and evaporation are most of the loss. But radiation is still significant (it’s around 10% of the heat loss of a typical person in light clothing). If you’re already sheltered from conductive and evaporative heat loss, as you are if you’re wrapped up in something, radiation becomes more important. But if you’re wrapped in a reflective surface, most of your infrared glow gets reflected back to you rather than being absorbed.
An emergency blanket, then:
* Reduces conductive heat loss by blocking airflow. Your body quickly warms the air trapped inside the blanket, and that air can’t exchange with other outside air.
* Reduces evaporative heat loss in the same way.
* Reflects radiative heat loss by being shiny, and
* Is very compact and lightweight. A heavy blanket would do the first two parts better, but would be much harder to carry.
[EDIT: So while this post is not wrong-wrong, in that convection and conduction are closely related, they actually [do have](https://www.reddit.com/r/explainlikeimfive/comments/114swyx/eli5_those_goldsilver_emergency_blankets_do_they/j8zefds/) some different properties and should probably be listed here separately. In short, the first bullet here should be split into two. Thanks to /u/LeGama for the correction.]
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