Winding down on our Ten Essentials, today I wanted to talk about Emergency Shelters. Obviously these are something you hope to never need (although many ultralight backpackers might use similar shelters to cut down on weight), but should you ever find yourself in the wrong situation, you'll be glad you have one. Another scenario is that you might find someone else out on the trail, perhaps injured and stranded, and you could use a shelter to help keep them safe and warm until further help can arrive. There are a number emergency shelter options available, each having their own pros and cons.

The primary function of the emergency shelter is to keep you warm and prevent hypothermia. Another consideration is how well it will keep you dry because, lets face it, staying dry helps you stay warm. First, lets consider the five ways the human body loses heat.

1. Convection - when wind blows across the skin, it speeds up evaporation. Normally, evaporation is one of the ways our body reduces excess heat, which is great in the summer. In the cooler temperatures though, this can be costly because we want to maintain the heat that our bodies generate. A key here is to have someway of protecting your body from the wind.

2. Conduction - occurs when your body comes into contact with something that has a lower temperature than your skin. Anyone who has ever attended a high school football game in the northern part of county and sat on metal bleachers for the whole game knows what I'm talking about here. By the end of the game, you're cold! So, you'll need something that acts as a barrier between your body and the cold ground.

3. Radiation - the human body naturally radiates heat. The key here is to not allow it float uselessly off into space. Part of this was covered under the Essential of extra clothing, but your shelter should also find a way to use this form of heat to provide warmth back to your body.

4. Evaporation - one of the primary means of regulating heat is the bodies ability to perspire. When our bodies perspire, the sweat than evaporates off the skin, thereby reducing the temperature of our bodies. When you're trying to stay warm, this becomes an important aspect to limit.

5. Respiration - the simple act of breathing itself is the last manner of heat loss. Next time you're out shoveling snow, watch your breath come out in great waves, like the cooling tower of a nuclear reactor!

So, when considering your options for emergency shelter, consider its ability to prevent heat loss across those five factors. You might also think of this as an emergency shelter system. You may take several pieces that, combined, would prevent against these. For example, a sleeping pad strapped to the outside of your pack could help prevent Conductive heat loss, an extra jacket can help against heat loss due to Radiation, etc.

However, for purposes of a simple emergency shelter that does most of these, you might also want to consider something like the Thermolite II Emergency Bivy Sack or the Heatsheet Bivy (both by American Medical Kits). They are designed to prevent against heat loss due to Convection, Conduction, Radiation, and Evaporation. They are also extremely light weight (around 4 ounces) and pack down to the size of a small orange.

The only manner of heat loss it does not protect against is that due to Respiration. Reviews of this emergency bivy seem positive, with the one consistent drawback being that it does not breathe (a function of its design, it traps the humidity in bivy to prevent against Evaporation), meaning you can wake up with wet clothes. Which takes us back to the importance of having extra clothing as part of your Ten Essentials!