Concerning Caradhras
Anyone who lives in the Rocky Mountains, Sierra, Alps, Scandinavia, or the Mountains of Australia and New Zealand can attest to just HOW BAD a winter storm can be, in the dead of winter.
Here, in the United States (America) the attempts of frontier people to cross over the Front Range of the Rockies and the High Sierra are legendary (I can only guess how bad the Canadian Rockies must be like.)
The Donner Pass party was stranded in the High Sierra in November, when snow blocked all egress from those high places, and they were required to stay until spring.
Of course, we know that many of them died, and we know how the others survived ... I'm not going into that, here.
It should be remembered that Eriador, like England, has a Marine West Coast climate.
It is not too hot in the summer, and not particularly cold in the winter.
Yet even in Eriador, even in these later, warmer days, the clime worsened and chilled as one went eastward.
Rivendell was warm enough for oak and maple.
In the hills around Rivendell, deciduous trees gave way to evergreens.
And that's fairly close to sea level.
The Redhorn Pass was, at the least, over 5,000 feet up (more like, over 10,000 feet up, but that's just a guess.)
Going over that pass in January meant facing the most awful of winter conditions.
The party should have expected, and prepared for:
Temperatures below - 30 Celsius (- 21 Fahrenheit.)
Windchills below - 50 Celsius (- 60 Fahrenheit), reaching - 70 Celsius (- 94 Fahrenheit) in worse case scenarios.
Winds of gale force, with gusts in excess of hurricane force.
Whiteout conditions.
Deep snow.
Avalanches.
And this, in a normal mountain range.
Caradhros was no normal mountain - it was apparently sentient, and resented intruders on it's flanks, and with some manner of control over the elements did as it could to drive them away (or kill them.)
Had I been Gandalf and Company, I would have brought the biggest, heaviest fur coat I could bear (preferably, one skinned from the bears of the white north of Forodwaith.)
Huge, fur lined boots.
Enormous, fur lined gloves.
Snowshoes.
Face masks.
Eye goggles (if the elves could make such.)
And as much wood as could be carried, by every member in the party.
The party was shivering and shaking while still in the lowlands west of the mountains (Tolkien, to paraphrase: the chill wind cut through every article of clothing they could put in.)
Not so good for them, when one considers what they presumed to take on.
I hope someone who lives in the Rockies, Sierra, Alps, Scandanavia, or the Southern Alps will come on and discuss winter in the mountains.
They would be most enlightening - I live in Michigan, and can only relate secondhand information from what I have heard from others.