Here in nordic countries it is known as "vindsäck" i e "wind bag".

A lightweight shelter for getting out of the wind when taking a break while skiing or as an emergency shelter if caught in a blizzard.

https://www.google.fi/search?q=vinds%C3%A4ck&client=firefox-a&hs=Nmq&rls=org.mozilla:en-US:official&channel=sb&tbm=isch&tbo=u&source=univ&sa=X&ei=VgCgU5bvDvKS7Aaw04CADA&ved=0CCYQsAQ&biw=1280&bih=602

A video demonstrating the use: http://youtu.be/5p1823RjLMo

I am a believer in ponchos, and generally carry 2 for shelter building. Use them in the winter to block wind and rain, in the summer in two layers with air movement in between to give better shade. The military surplus ones are durable but there are lighter ones out there that pack smaller. Make sure they have grommets or loops for building the shelter. I have a Sea to Summit sil-nylon poncho that works well, and a couple of Equinox ultra light ponchos. Space blankets work well but don't pack as small. I have used these types of shelters in many environments, from the sub arctic to the jungle and deserts. I have always found a way to build the shelter even without trees to tie to.

Yeah, exactly. Though can get upto 20 person bothies. Obviously a group shelter is preferable to individual bivi bags in a group, as everyone contributes to heating a single volume.

http://www.terra-nova.co.uk/tarps-bivis-bothies/all-bothies-bothy-bags/

http://rab.uk.com/products/equipment/shelter/

"Bivvy"..."bothy"...."vindsäck"... In my opinion the difference is mainly semantics.

In all cases we are talking about a simple, extremely lightweight, one layer, wind and rain proof shelter. They protect you from the wind, keep you dry, and somewhat hold in body heat. North Americans most often seem to call them "bivvys" while Brits usually use the term "bothy".

There are a variety of designs. They can be made in a pattern somewhat like a sleeping bag (one or two person), or more like a big sack for two or more people (with or without a floor). Climbers often have webbing loops sewn onto the outside of theirs, to attach to anchors so they don't slide off the ledge in the middle of the night. Some have windows or air vents.

Whatever terminology you prefer, they are a handy thing to have when you don't want to carry a tent and sleeping bag, but might have to spend a night out in severe conditions.

Hmm not sure about that.

A bivvy bag is a weatherproof outer for a sleeping bag.

I would expect a properly equipped climber would have both a bivvy bag to sleep in, and bothy to provide larger shelter.

Stopping for a quick brew, a bivvy isn't going to provide much protection.

I have carried a bivvy bag routinely on climbs and other outings for years, but I have never bothered with a bothy.. Are they a fairly recent or have the Brits been shy about revealing them to their Yankee cousins? I have heard of them only recently....

Some folks supplement a bivvy with a tarp for shelter for cooking, etc, but in my experience that is just added weight.

They are definitely fairly old gear, certainly they seemed to be around when I was kid, ~40 years ago.

I guess they are called bothy bags, named after the cabins/huts (usually built from stone) that are called bothies dotted around the UK, particularly in Scotland. Eg http://www.mountainbothies.org.uk/

If I had the need for a lightweight shelter, I think I would get the Hilleberg Bivanorak. http://www.hilleberg.com/home/products/bivanorak/bivanorak.php

Long enough to cover legs and feet. A hood. Fully waterproof. Able to cinch shut the opening for hands and feet and therby have a protectiv sack. Weight only 1 pound 4 oz.

In my daypack I have a 2# thinsulate mummy bag and a 1#bivy as had been suggested to me. Three pounds and fairly compact. That's not much weight considering the alternative of using a space blanket alone. Add a beanie and a heat pack or two and I'm confident I'd survive anything but deep winter here. A cup of tea and a candy bar or meal before turning in would also help make the experience warmer and more comfortable. JMHO.

Light and fast hardman climbers will generally carry whatever is the absolute minimum they think they can get away with. There is an old adage in climbing that goes something like "Speed is safety. If you carry bivy gear, that just guarantees that you will need to bivouac!"

The Spaniard Kilian Jornet Bourgada just set a new record, climbing Denali and back down in less than 12 HOURS! The typical time is 17-21 DAYS.

Of course now and then that approach gets people into trouble.....