I live in an extremely rainy environment. It isn't particularly hot or cold but hypothermia kills people in the back country every year. This is typically due to becoming wet and freezing over night, unable to dry off.
So if I were to become stranded overnight during a hike or whatever, my main goal is staying warm and dry. I carry in my day pack:
>Extra sweater
>Extra socks (also used as gloves)
>Small tarp and rope
>Waterproof rain jacket
>Standard fire making and bushcrafty stuff
>Whistle
>Mirror
The rain jacket is the most critically important item. It keeps the wind and rain off you. It can also be stuffed with leafs and other debris to create insulating loft. I honestly think a good jacket and sweater are more important survival items than knives and lighters and that sort of thing. If I slip off the trail and break a leg I doubt I'm going to be running around building fires and shit. But I can throw a jacket on and stuff it with leafs to survive through the night.
Fun fact, I have actually been stuck overnight in bad conditions twice before -
Once due to my truck breaking down on the mountain in the middle of winter. Alternator shit the bed and my battery was drained with no way to recharge. This meant I couldn't run my heater to stay warm, and it was freezing cold outside. Fortunately I keep a sleeping bag and extra clothes in my truck, so this potential "survival scenario" just ended up being an inconvenient camping trip, some missed wages at work, and a big tow truck bill.
The other time was worse. I was fishing at a remote hike-in lake, and a freak springtime snow storm rolled in on us. It was a complete white out situation with extreme wind and snowfall. We were unable to hike out safely, and before long the sun had set. My friend and I had to set up my emergency tarp and do our best to shelter a small fire all night. We threw all our extra layers on and sucked it up. One of the worst, coldest nights of my life, but we made it out okay.
