If someone isn't wearing a hat, gloves, boots, or a coat in 11 degree weather, it's either because they don't have them, didn't think they needed them, are in some way impaired, are seeking attention, or they didn't want to wear them. If they don't have them, instead of criticizing or pitying them, take them to a store. Empty your wallet and purchase what they need. They'll feel better and, hopefully you will, too.
If they are impaired, if they don't know any better, if they are disabled, and you see what's happening, YOU--not someone else--YOU go to their closet, get what they need, insist that they comply, and help them shield themselves from the elements.
However, if they are in their right minds, possess a variety of garments and accessories suitable for all seasons, in addition to the activity of all of their limbs, but choose not to bundle up, or were too lazy to open their own closet, take them out, and put them on, that's THEIR stupidity. Any sickness that may follow is THEIR fault, and all shock, disbelief, criticism or reprimanding should be directed toward THEM.
Why be upset with, or angrily address anyone else other than the ADULT person who refuses to do what they need to do to protect themselves? Save all of that chatter and ire for the person who refuses to listen to reason. Sometimes we play directly into the hands of those whose need to have everyone fawning over them, causes them to do some pretty boneheaded things. "Where is your coat? It's freezing outside! You're going to catch a cold! Here, wear mine! Let me warm you up!"
Sometimes we KNOW we're not getting through to people, so we need a scapegoat to vent our frustrations. "Why didn't you tell her to put on a coat? Why didn't you let them know how cold it was outside? Why don't you put their hat and gloves by the door so they can see them?" Why don't you shut the heck up and see that you're enabling the coat-less person to think it's the responsibility of someone else to do what they can surely do for themselves? Keep on badgering and blaming the scapegoat. Don't be surprised when that goat vents (or butts) back.
Don't allow anyone to interpret another person's stubbornness, poor judgment, dishonesty, divisiveness, or arrogance as negligence on YOUR part. Set them straight. Time out for being scolded, penalized, or misjudged for what someone else said, or purposely failed to do.
Don't allow anyone to throw any crosses on your back that don't have your name on them. Don't you dare pick them up. Own only those things for which you are responsible.