I've been outside a lot today and it hasn't been nice. Low cloud and rain backed up by 60mph winds have made things pretty hard work ... including smiling. It did however get me thinking about what I'd choose to supply some shelter if I were out all night. Most of the time, if I'm looking to keep a bit of weather off I'd reach for a tarp. The size would depend upon whether I was on my own or not, whether there'd be any additional natural shelter (trees, etc) and what I was expecting weather wise. The smallest, lightest tarp I have is around 2m x 1m, weighs 200g and fits in the palm of your hand. On a summer evening, even with a little light rain expected this can be rigged to keep most of the elements at bay. It's only ever going to supply a simple roof, either as a lean to or set up with a ridge, so on a night like tonight it might be best left at home.
The next step up would obviously involve a bigger tarp. The picture below shows Taylors tarp (with Si under it). It's a 3m x 3.5m and set up as a ridge between the trees, pitched low to the ground and in a position to make the most of any natural shelter, would offer a surprising amount of protection ... it's also pretty spacious in there too, so great if you're not alone (or think you'll get lucky in the woods). On a trip just before Christmas a 3m x 4.5m tarp, rigged as a very simple roof covered 5 1/3 men (sorry Steve). The weight of that tarp was 800g, so while not light in the great scheme of things, it's actually very light for the space/protection it offers.
Returning back to tonight's weather, I might be looking for a little more protection though ... remember the wind driven rain. Well, a tarp can offer as much or as little protection as you want, just as long as you're willing to invest some time playing about. Once you do start thinking about folding rather than just tying, then you can knock up lots of interesting shapes and forms. The top picture is a 3m x 3m tarp, it hasn't been sewn into that shape only folded and pegged. It's enclosed on 3 sides, has a 'sewn in' groundsheet and a beak/porch for cooking and storage. If the weather really took a turn, the beak can be dropped almost to the ground, giving the happy camper inside almost total coverage. Total weight including, pegs, guys and pole is around 800g.
Tarps are always going to be something of an acquired taste. Some people will point out you can get an ultralight tent that weighs the same and that's true, but ... you can't get one that packs as small, is as quick to pitch and strike and then there's the problem of the ultralight tent costing you around 4 times as much ... origami, it's the future!