Winter Day Hikes: Tarp or Bivy Sack

My wimpy friends don't seem to want to winter camp so we do a lot of day hikes in winter. It is usually on trails where we don't see anyone else, sometimes on snow. So it is important, me thinks, to carry an emergency shelter or bivy of some sort. My options I own right not are a breathable w/p nylon bivy sack (12.5 ounces) and a coated 8x10 nylon tarp (16 ounces). I would not take both on account of the weight. Of course we always take extra warm clothes and rain gear too. But do any of you have an opinion on bivy sac vs tarp for survival? The tarp can be pitched as a tarp or simply used to wrap up in like a burrito.

It's a day hike. Take them both - the worst case scenario is that it gets late (or you're lost) and it starts raining, snowing, sleeting, or all three. If you have to spend the night out, having both, instead of just one, might just mean the difference between spending a comfortable night and a close call - that could go either way.

The problem with bivy sacks isn't whether they will turn the weather. It's the fact that, at some point, you need to get in or out of the bivy sack - at which point, without a tarp, water gets in.

I'd take both. But that's just me.

The safer option is both. Obviously.

But here's how I would think about the question. I consider the tarp to be more versatile for protection from wet circumstances, such as rain or wet snow, while the bivy probably is best at maintaining a warm air envelope around you and keeping body heat in, at least if the weather is dry. Once wetness invades your bivy its effectiveness is much reduced. So, I'd favor the tarp if it is wet and bivy when its dry.

I agree that taking both is safer not just because I would be doubling up, so to speak, but because they serve two very important different functions (shelter and warmth). Thanks for your thoughts. Maybe just one of our party (of 3) can take a tarp in addition to his bivy and a different person carry the tarp next time.

Yes. As you will see above I agreed both items serve different purposes, just as you said. And yes, I have been in the rain with and without a tarp. without a tarp you have no place to sort through your pack,get undressed/dressed,or cook or anything when it is raining. I did find enough space inside the bivy to read and that surprised me.

Perhaps another subject, but related: My best (Gortex) bivy is a bit heavy at 24 ounces (682g) and my lightest sil-nylon tent is only 8 ounces (227g) more than that so given the convenience of a tent and the requirement of a small tarp over the bivy I never packpack with the Gortex Bivy any longer. Thanks for your thoughts.