So you're miles from your car and a line of thunderstorms is coming at you. You've got minutes to prepare. If your poncho is your only life of defense, you may be in for a miserable time. (Ever tried to keep dry in a poncho in a high wind?) Also, if the rain doesn't stop and you're stuck overnight (as you wait for a flooded stream to go back down, for example), what's your backup plan? Perhaps you should have one, other than standing in the rain all night.
Here's one solution: a 7 by 9 foot (2.1 by 2.7 meter) rectangular nylon tarp, urethane-coated, with grommets and carry bag. With six aluminum stakes, three paracord lines secured with rubber bands, and three small carabiners slipped into the bag, the total weight is 25 ounces (710 grams) and the dimensions are 8 by 6 by 4 inches (20 by 15 by 10 centimeters).
Higher-end tarps have loops of webbing instead of grommets, and tie-ins away from the edges. Also, you can get lighter tarps (or larger tarps for the same weight) by going to other materials. This is just one balance of size, ruggedness, and price. I don't like catenary-cut tarps because I like having the edges of the tarp all the way down to the ground during a storm. Also, if you invest in an ultralight tarp (such as the one I discuss here), be aware that "silnylon" (silicone-coated nylon) is flammable and should be used with caution.
The kit shown above has three paracord guy lines prepared in advance. All have bowlines tied at one end, and that's where the small carabiners go; I attach the guy lines to the tarp as needed by snapping the carabiners through the bowlines and grommet holes. Two of the guy lines are six feet (1.8 meters) long and I create the loops for the stakes by creating a figure eight knot on a bight where I need it, but if you're not handy with knots, pre-tie another bowline at the other end. My third guy line, which I think of as the "master guy line," is twelve feet (3.7 meters) long and the stake end has a big loop with a midshipman's hitch, allowing me to adjust distance and tension as needed. The bowline loop on the master guy line is large enough that a carabiner and the tip of my hiking pole fit, but not so large that the loop will slide past the basket and down the hiking pole.
Next I'll show you a setup you should be able to do in under five minutes, as that storm approaches. (You can see me do the setup with a different tarp, in a video.) Whatever setup you use, be sure to practice it before you go hiking, so you you have all the kinks worked out. A thunderstorm is a lousy time to find out that your tent pegs are too fat to slip through grommet holes, for example—but when you find that out in advance, you can add a small paracord loop at each grommet hole.
First, lay out the tarp, coated side down, so that the long edges are at right angles to the wind. To keep the tarp under control in that wind, put a tent peg in at, say, Corner 1, grab Corner 3 and stretch the fabric tight, and put in a tent peg there.
Now attach the "master" guy line at Midpoint 6. Holding the guy line near that end, pull and lift, creating a taut triangle of fabric whose apexes are Corner 1, Corner 3, and Midpoint 6. You should be pulling in the same direction as a line drawn from Midpoint 2 to Midpoint 6 (directly downwind, if you're doing this right). Take a hiking pole extended to your usual hiking length, turn it upside down, and rest the handle end on the ground. Slip the tip of the hiking pole through the bowline loop, and make sure that the pole is vertical. While maintaining tension on the guy line, extend the line well out from the tarp and peg it in. You want this guy line to be a long one for maximum stability.
If Corners 5 and 7 aren't flapping in the wind, they'll be hanging down loosely, suggesting the next step. Peg down those corners of the tarp to create a triangular opening facing exactly where it should: downwind. Finish by pegging down the tarp at Midpoint 2 (if you have eight stakes instead of six, peg down Midpoints 4 and 8 as well). Adjust the tent peg locations and guy line so that everything is taut. The final product should look like the photo below.
The shelter looks a bit small in the photo, but it's nine feet wide at the back edge. With a 3.5 foot (1.1 meter) height at the peak and a 7 foot (2.1 meter) tarp width, it's 6 feet (1.8 meters) deep. Two or three people will have room to wait out a storm with their gear. One person will have more than enough room for a sleeping bag, a pack, and a pair of boots, so this configuration is also a good one-person shelter for overnight trips. Since the tarp is pegged to the ground on three sides, you'll be protected from most shifts in wind direction.
If the ground is wet, not a problem. Because you now have a shelter, you can pull off (or unpack) your poncho and spread it (coated side up) inside the shelter as a ground cloth.
Usually it's best to keep this shelter no higher than about 3.5 feet (1.1 meters). Increasing the height decreases the floor area and creates more of a sail, encouraging the wind to tear out the tent stakes. You can improve the headroom a bit by jamming a collapsed second hiking pole against the fabric in the back (point down, of course) or by stuffing gear towards the back corners. If you're worried that the wind will pull out the stakes, an easy solution is to rest a large rock on top of each one.
If wind isn't a problem you can open up this configuration by attaching the remaining guy lines to Corners 5 and 7, and pulling those corners up off the ground. You can see this option in the photo below.
There are other ways to set up a tarp, of course. If you do this configuration just upwind from a tree, for example, you can tie the master guy line to the tree, about neck height, rather than use a hiking pole for support. Tarp setup is a form of origami, so don't be afraid to experiment. Just be sure that you have everything (including your knots and your plan) ready in advance, so you don't have to improvise as those first raindrops are splatting around you.
If a grommet or other tie point tears out, or if you need a tie point where one doesn't exist, there's a quick and easy temporary fix. Find a pebble about 1 inch (2.5 cm) in diameter and surround it with tarp cloth. Then take one end of your guy line and tie a tight knot around the "neck" of the little pouch holding the pebble. As long as the knot stays tight, the pebble won't slip loose. This forms as strong a tie point as one attached at the factory. Almost any small object will do, as long as there are no sharp edges or points to cut the tarp.
In the picture I show a pebble tie at a poncho corner where there's a snap but no grommet.
If you're not ready to carry a serious tarp on a day hike, consider packing a plastic drop cloth—cheap, lightweight, compact, and better than nothing. The downside: your "tarp" may not last more than one use (if that). For a more durable lightweight option, go to Amazon.com and search on "Tyvek sheet."
And if you disregard all this advice and get caught out in the wilds overnight, at least know how to create a brush shelter like the one I discuss in a blog.