What is survival or bivyy bag?

Survival or bivyy bag could be useful in multi-day expedition in case of emergency or hypothermia to keep person warm and stabilise temperature. Because it’s light and doesn’t take a lot of space would be useful to put it in rucksack. In case of hypothermia, lack of survival bag can lead to disability or death. Hypothermia starts when body temperature gets lower than 35 degrees Celsius and slower body functioning. Person gets mentally confused and stressed.