Nothing like browsing hurt/comfort on ao3 to really drive home that no one fucking knows how hypothermia works. Jesus guys. Get it together. Don't make me do a psa. It'll be embarrassing for everyone.
If you're getting the bulk of your emergency preparedness from archive of our own or heaven forbid Tumblr dot com on the world wide web, the problem is several steps upstream. I can't help you much with that one. But I can make a helpful critique on writing effective whump because that's what we're here for! Should hypothermia be your plot agent of choice you should recognize that the true horror and tragedy is that you have more time than you think you do and all your mistakes are easily fixable but you can't do a damn thing about them.
Give your character with agency and control issues hypothermia. They will have nightmares about the slow paralysis and lethargy of cold until the day they die. The physical and mental symptoms creep in by degrees that you can easily monitor and recognize but are powerless to stop. The dissolution of initiative and the loss of rationality is terrifying when it happens and it's actually worse after warming up because you can remember being dead certain that striping out of your clothes in the snow was the right thing to do.
The psychology of the situation is as close as reality can get to the frog boiling experiment, where everything slowly gets worse and worse and the only thing that could stop it is getting out of the scenario and since that's an impossibility there's nothing that can solve anything. Bash your whumpee with utter hopelessness while giving them nerve damage, buy one get one style.
If you must get nitpickety about the physics and medical details, hypothermia is when the human body can not maintain internal temperature warmer than the surrounding environment. Homeostasis is maintained by - wait no, too many big words. Drink water, eat food, body needs to keep the guts warm. Food and water are the fuel to keep the guts warm. Guts are 90(ish) degrees, air is usually cooler than that and you can go hypothermic in weather as warm as 60 if you haven't been drinking or eating. The water drinking part is more important than the eating part. Hypothermia at 60 is not gonna be the same as hypothermia in below freezing weather. The nerve damage part is from the equivalent of freezer burn happening in living flesh, so that won't happen if the ambient air temp is above freezing, niether will the frostbite or gangrene. You also dont need to be hypothermic for frostbite to occur on skin and extremities since your core can be perfectly toasty while your toes freeze solid, but it does tend to happen together.
As for your rescuer/caretaker/comfortgiver. This shit is fucking terrifying. The whole cuddle for warmth thing can be actively dangerous since you are also a human being with guts that need to stay warm and a finite amount of body heat to donate. If you're still out in the cold, cuddling with a person sized ice pack is a good way to end up with 2 person sized ice packs. Even if you are in a safe place to get them warm, ouch, frostbite hurts and it only starts hurting when you warm up enough to not be numb, so you can save your person and then forcibly subject them to creeping searing pain that you cannot soothe.
Not to mention the heart attacks. A rush of cold blood from the extremities back to the heart can be enough to cause the cardiac muscle to go wtf and freak out. (Super scientific terminology right there for ya) What would cause a sudden rush of cold blood like that? A bruise. Even a little one. Your patient also happens to be numb, disoriented, and clumsy at the moment. Good luck. Pray for decent baseline cardiac health.
So here's the thing. I dont actually care if the whump is accurate. I want it to be interesting. Go sort through your blorbos for the one who would have the worst time in the above situations. Fly my scarabs. I want those characters tired and chilled, wet and weather beaten. Get after it. On to thermodynamically improbable scenarios we go!
From an allegorical standpoint, being numb means it is really easy to hurt yourself. Go get a hold of your repression princess who shrugs everything off because they're fine guys and have them burn the shit out of their hands trying to warm up. Numb fingers and fire is not a great combo. Numb fingers and boiling water is also not great. All sorts of juicy parallels to be drawn.
Fun tidbit about frostbite specifically, when you freeze a body part the water in that body part expands into nasty little ice crystals that tear up the cell walls and organ tissues. That's where the damage comes from. Once its frozen though, its not as if the ice crystals can do anything else, but if things thaw and refreeze without medical attention or time to heal in between, that's how you lose fingers. The water refreezes into crystals and then there are more more sharp edges on the inside of cells and tissues. Meat turns to goo, meat that is goo doesn't do anything but rot and that's gangrene. Point being, if you're going to warm someone up, make sure you can keep them warm. This is where you put fun metaphors about backsliding and relapses.
Like most situations, everything can be made worse by being drunk. Alcohol acts as a vasodialator and opens up all the capillaries which will bleed heat. Do not get drunk and take a nap in a snowberm, you only feel sweaty and warm, you ain't. Legions of homeless alcoholics have discovered this the hard way.
As for the bit about having more time than you think you do, hypothermia and associated motor skill loss can definitely creep up on you if you aren't familiar with the progression, but it is a progressive problem. Your hands will get stiff and the way you measure how cold you're getting is to touch your thumb to your fingers. Thumb to pinky will be the first to go and as you lose range of motion in your hands it will progress to ring finger. When you can't make your thumb meet your index finger that's the sign that you need to drop whatever you're doing and go get warm because if it gets much worse you won't be able to use your hands. Not having use of your hands make the whole 'getting out of the cold and getting warm' bit that much harder.
Thinking that stripping is a good idea is probably the last idea you will ever have, because that's the point where your body gives up on conserving heat and it all gets dumped back into the extremities. Or at least we think that's what happens, people don't survive that shit often enough for us to actually check.
@goth-automaton seems like a post for you ✨
And to add on and give you another way to torture your characters: Pick the one who is most miserable when they have to sit still and wait. Now give them moderate to severe hypothermia.
OP already explained that a rush of cold blood from the extremities to the heart can cause a heart attack, and you know what else can cause the blood to rush back? Moving. Literally just moving. If your character is found outside acting confused, breathing slowly, shivering or worse, not shivering anymore and all they want is to walk back to where it's warm? Sucks to be them. Every second they do anything but lay down (on a sled, a rescue blanket, a damn tree log, whatever the rescuer has or finds) and don't move a single muscle and just let themselves be transported to safety is a second more for their body to decide that If We Are Moving, We Need Warm Feet And Hands Too, and send that cold blood away from the extremities back to the heart and give itself a heart attack from the cold shock.
So. Sorry whumpee no. 3 but no matter how much you hate getting taken care of or saved or how much trauma you have from someone dragging you around or being unable to move – if you want to survive, you will. not. move. a single muscle.
(Also for the rescuers, if they absolutely have to transport whumpee up or down a hill or stairs or any other incline, they need to keep their body as horizontal as possible, do not make it easier for the body to rush blood up or down the body – even if that means that whumpee who hates touch or physical proximity now needs to be lifted by several people. And if horizontal is really not possible, the rescuers absolutely have to keep whumpee's head above the heart, don't let that cold leg blood fall down towards the heart, the head is the highest point at all times – even if that means that whumpee who has trust issues from the seventh circle of hell and half an aneurysm at the thought of not being able to control everything at all times now needs to be transported head-first and thus can't see ahead where they are going.)
