One of the most lightweight forms of camping outdoors is bivvy bag camping, where you do without the need to carry a tent along with you.
A bivvy bag (some spell it bivy, with one V) will keep your sleeping bag dry from any light rain and dew. Since it is also waterproof underneath, the bag presents a barrier to the cold moisture that rises up from underneath your body while you are asleep.
The word bivvy is a soldier’s abbreviation for the French word bivouac, which can be both a noun and a verb. Soldiers are taught to bivouac down for the night. They make a bivouac (shelter) to protect themselves from the elements as best as they can. This is done where carrying and setting up proper sleeping tents would be too time-consuming. However, a bivvy bag is just a bag. It is compact, lightweight and ultra-convenient for tent-free camping.
Most good bivvy bags include a zip-across insect netting to keep the mosquitoes and no-see-ums from eating you alive while you sleep. So you end up with an insect mesh right in front of your face. This isn’t for everybody, especially if you’re inclined to feel a bit claustrophobic.
The best bivvy bags use a breathable but waterproof fabric which allows water vapor to escape yet stops water droplets from coming in. The original type of special fabric was made by Goretex™, but there are other — and cheaper — fabrics available nowadays which stop liquid water, but allow water vapor to escape away from you.
I own two lightweight camping bivvy bags. One cheap and one expensive. The top-quality bivvy bag is made of Goretex™ and has been woven or printed in Australian Army desert camo. It cost me about $400 at a specialist camping outfitter in Kent Street, Sydney, a few years back. The other bivvy bag I own is a cheap blue and black one that’s made in Korea. I think the brand name was Rhinoceros… It cost me about $50, but this bivvi doesn’t have an insect mesh like the Aussie one does. They are both great for lightweight camping or for hiking… and for sleeping outdoors without a camping tent.
You can use a bivvy bag to camp out in most weather, although driving rain will get in and soak you from the head and shoulders area. This would be most uncomfortable, so some kind of mini-tarp would be a good idea to give extra protection from the rain if bad weather is expected.
A bivvy bag will also work to keep you warm and dry even in the snow. However, you will still need to have a thick enough sleeping bag that is rated for those very cold temperatures. And remember that very same bivvy bag of yours will work very well as a survival bag, something that will keep you dry and hopefully save you from hypothermia (exposure) if you are ever caught out in the wilds while hiking in changeable weather.