Bivy tents get their name from the bivouac sack. Which is a bag that a person puts their sleeping bag in to sleep.
The outer shell closes up around the entire sleeping bag and over the persons head leaving just an opening for fresh air
Wikipedia has a great article on the bivouac sack. Today most backpackers use bivy tents instead of the bivouac sack. The tents offer just a little more room.
The main difference being the addition of more outer shell material and poles for support. They are still very lightweight and pack down for easy carrying.
This is a great tent review on a Bivy tent.
If camping in the summer a personal bug hut might be a good lightweight pack addition. Visit our Screen tent page for help keeping the bugs at bay.
Of course, the Sierra Trading post tent buyers guide still applies.
There is a whole set of camping gear options for the backpacker. Weight and size seem to be the key elements as well as quality.
Sleeping bags need to be lightweight and easily compacted and Camp cook stoves also need to be very efficient, work well and be lightweight.
Which is best depends on the need. If a person is kayaking and very limited on cargo space perhaps the bivy bag will be a better choice.
The bags are a little lighter and pack easily. However few are absolutely water proof.
For some sleeping under the stars is a byproduct of what they have set out to do. So, sleep happens when it becomes necessary.
For the person climbing a mountain which requires an over night stay on a cliff the bag might more suitable.
For some, sleeping under the stars is the objective. It is the essence of their outdoor activity. More room for resting a weary body and mind may be a big plus.
These tents have a little more room but are still very light and compact. Most come with a rainfly and some have a vestibule area.
They are more likely to be truly water proof and if there is a vestibule a persons boots can remain outside the sleeping area and still stay dry.
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The AACG Camping Gear Store has a great selection of Bivy tents. Some department stores have a lot to chose from as well.