Sims won’t change into winter clothes so they freeze to death!

@AmeliaNG  I've had sims die when in an NPC role from freezing too because they were in the wrong clothes.  You didn't say which platform you are playing on or whether you play with mods, but I have found a workaround for this problem that will work on PCs and probably Macs.  I play on a PC and use mods.  The MCCC Dresser module has an option to force changing to a specific outfit for any sim.  That way, when you see a sim who is not in cold weather clothing, you can change them to the appropriate clothing before they freeze.  

If you are playing on a PS4 or XB1, then this won't work but there are a couple of things you can do to head it off.  There is a trait that can be bought for so sims won't freeze.  It's 2000 aspiration points.  That will generally avoid a sim freezing if they have it.

The other thing you can do is when you see a sim that might freeze, you can immediately travel to somewhere else although you have to be pretty quick about it before the sim freezes solid.

Hope this helps.

@AmeliaNG In addition to @CGrant56's excellent tips, the method I normally use is I deactivate weather effects on sims. This can be done in the game save by going to 'Game Options' > 'Seasons' > and un-check the box that says "Enable Temperature Effects On Sims". Personally, I do this because of the issue you're complaining about... the sims do not always automatically change to the weather-appropriate clothes they need to keep themselves from 'overheating to death' or 'freezing to death' (life-threatening conditions). I see this as unfair and inappropriate to proper/logical gameplay, so until this (hopefully someday) is different, I uncheck this box & save settings.

There are some bug reports on freezing and weather effects on the sims at this link: https://answers.ea.com/t5/Bug-Reports/READ-FIRST-Compiled-list-of-reported-Issues/m-p/7962773#Weather14