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[Game of Thrones] How deep would the snow have to be...? SPOILERS S05E10

How deep would the snow have to be for Sansa and Theon to survive, relativity unhurt, their jump off the wall of Winterfell?
gpt
*Going to have to go "Doylist" on this one, sorry.*

The Outer wall of Winterfell is about eighty feet high (at least in the books), and this is semi-confirmed with some rough back-of-the-tissue calculations about how long it took "M" to fall to the inner courtyard (~2 seconds, which puts the wall at 80-100 feet).  

Landing in snow is one of the better options, but it's going to depend on a whole lot of factors.

First, the snowdrift needs to be at minimum 4 meters deep to provide adequate cushioning.  Snow is complex - it forms in layers, so top snow (aka 'powder') will have different compression than packed snow further down.  Also, your deceleration needs to be around or about 25g to avoid serious injury and less than 200g to avoid death.

If the snow at the landing point is 3m deep or a light dusting of snow over ice, then both jumpers will suffer horrific injuries, up to and including shattered legs, shattered pelvis and broken back.  This was ably demonstrated by Brandon Stark, falling from a lesser height.   This would be the result of applying over 200g's of deceleration - almost certainly fatal (but not automatically so).
Without immediate medical attention, the jumpers would likely die within hours, faster if we take exposure to elements into account.

If the snow is 4m deep or more, it has enough compressibility to it to provide adequate cushioning force.  To survive, the jumpers will also need to land in the correct manner - relaxed body, feet first, bending knees, and redirecting force instead of absorbing it.  This would apply *very roughly* between 20 and 50g of deceleration on impact - definitely survivable (but with some injuries)

So, if done right, this can mean the jump is survivable.  The jumpers may end up with minor bone breakage, sprains and strains, but are otherwise capable of 'walking away' from the drop.

A depth of 8m or more, plus proper landing technique would also provide adequate protection from the drop, but it would also serve to mire the jumpers in the snowdrift, making them easy targets for archers or soldiers.