One of the first things that a human needs to do when in a survival situation is find shelter, but why?
The main reason is warmth and so by extension, keeping dry. I’m not aware of any RPGs that make being out in a drizzling rain a dangerous situation. This is probably because most RPGs only use hit points do track detrimental effects to the character and taking off hp because it’s raining is just silly.
What a GM needs to do is not to tell the players their characters need shelter, he needs to start simulating hypothermia. The players will figure out the need shelter on their own. So how can you do that?
Hypothermia can happen at any temperature below 26 degrees Celsius (about 80 degrees F) but the person would have to be sweating or naked and it would have to be windy (70 km/H winds). Even then, at those temperatures it’s not likely to progress any further than the first stages of hypothermia which is shivering and a loss of dexterity.
Once it drops to about 15 degrees C (60 degrees F) is when hypothermia is more likely to progress to severe stages. At these temperatures it will still require a person to be wet or naked and wind will have a strong influence. In most RPGs I don’t expect most characters to be running around naked but wet is another story. Even sweating is a danger in these temperatures if a person is not going to find shelter any time soon.
But how long would it take to become hypothermic? That is to reach the first stages of hypothermia. There is no one answer so I’ll give some conditions. These are for a person in only light clothing. I know that players are likely heavily armored but that would probably have only minor if dubious insulation value.
Disclaimer: There are a lot of factors for hypothermia, don’t try using these numbers in a real survival situation! They’re good enough for an RPG but are on the generous side so that players have less to complain about.
15 degrees C – about 24 hours
0 degrees C – about 2 hours
15 C with sweat or rain or strong wind – about 12 hours
0 degrees with sweat or strong wind – less than 1 hour
15 C soaking wet – about 2 hours
0 C soaking wet – 2-5 minutes
There are three stages of hypothermia before unconsciousness. The first is shivering and a loss of dexterity, this usually lasts for about ten minutes to a half hour depending on how fast hypothermia set in. (-10 Dex -5 Agi)
Next, severe shivering and loss of coordination is likely to set in an twenty minutes to an hour after this. Again depending on how fast hypothermia set in. (-20 Dex -15 Agi)
Shivering stops and confusion and irrational behavior then sets in after another twenty minutes to an hour. (-15 IQ -20 Dex -15 Agi)
Lastly unconsiousness sets in after another twenty minutes to an hour. Death occurs in another after another twenty minutes to an hour.
So that’s hypothermia and why the characters should seek shelter.