What is the most severe temperature that a proper snow cave can protect you from?

Question

This question was prompted by my answer to Under what conditions does heating a backpacking tent artificially make objective sense?. In my answer, I mention the possibility of hiking across Antarctica during the winter to experience the harshest cold that our planet has to offer.

You are on this Antarctic winter hiking trip, and you are chasing the cold. You know that the record cold is approximately -135F (-95C), and you are expecting you probably will encounter close to that, and you need to prepare for the possibility of a new record being set so you want to be safe down to a few degrees below that in case of emergency.

To assist in keeping safe, you build a snow cave. Though emergency snow caves are often not done correctly, let us assume that you did construct one correctly on this trip (ie: top of door below inner floor to trap heat, properly placed ventilation, doorway plugged up, etc.)

Normally, I hear people say that a good snow cave can keep the inner temperature at a pleasant freezing (32F, 0C) temperature even if the air outdoors is crazy cold. Would this still hold true at -140 degrees? At what point is it just so freaking cold that even a snow cave cannot be kept warm?

The amount of snow that is insulating you does obviously matter. Let's say you dig into a small embankment, so part of it is more than a few feet thick. For the front of it where you dug into the bank and had to then patch up the front of your cave, I would personally assume a few inches at most, as I have read about safety concerns of cave-ins when people craft thick walls. If you are not concerned about such a cave in danger, then feel free to assume thicker if that is truly what you would do.

Assume that there is a modest wind. Enough that heat does not linger well around objects, but not enough to worry about the integrity of the snow cave.

So, what is the minimum temperature at which a properly constructed snow cave can no longer be expected to maintain a constant 32F/0C temperature without artificial heating?

Answer 1

TLDR: -148 °F including windchill has been survived inside of a snowcave.

I went looking for cases of people surviving extremely low temperatures while inside a snow caves, and it looks like the record is held by the climbers who did the first winter ascent on Denali.

On February 28, 1967, Dave Johnston, Art Davidson, and Ray Genet became the first climbers to stand on Mt. McKinley’s 20,322-foot summit in winter. On the descent, the weather turned, and they dug a tiny snow cave above 18,000 feet and hung on for six days and nights, barely able to sit up in what they wondered would be an icy grave. The windchill dropped temperatures as low as -148 Fahrenheit, and their teammates lower on the mountain assumed the worst.

After six days, the men were able to descend – just barely. Davidson’s account of the climb and descent, Minus 148 Degrees, was published in 1970 and instantly became a mountaineering classic. In 2013, the 100th anniversary of the first ascent of Mt. McKinley, Mountaineers Books published an anniversary edition of Minus 148 Degrees with a new prologue and afterword updated with details of more recent winter attempts on McKinley.

Climber Art Davidson on the 1st Winter Ascent of Denali, Adventure, and Surviving ‘Minus 148 Degrees’

The full story is in the book, Minus 148 Degrees

While that at least means that -148 ° F is survivable by some people, other people have frozen to death inside snowcaves at much higher temperatures.