> If I had to sleep outside I would add a tarp […]

Outside you need to add something to insulate you from heat loss to the ground or it is colder than you would want.

Instead of rocks (I’m sure that was a joke), add closed cell insulation, like polystyrene. Five cm of that beneath you, and you will sleep warm and toasty.

Instead of rocks (I’m sure that was a joke)

Not a joke at all. If the tarp is big enough, fold it into a half tent and put rocks at each end, then cover them in snow to create a mini-wall and hold the tarp in place from wind. The tarp itself needs to point up-wind to block it. It will block the wind and new snow. If wearing the right snow clothing the only killer is wind. The snow below the tarp will be fine, again if wearing the right snow clothing. Adding layers above the tarp is great if it's an option. I was just basing that on what can be easily carried in a ruck-sack as I assume the only people sleeping outside can't afford lodging. That insulation roll can be rolled up and attached under the ruck-sack if it is small enough and/or flexible enough.

[Edit] For completeness sake, if they are homeless and do not have good snow clothing then the first thing they should be doing is begging, borrowing or stealing their way onto a train heading towards the equator.