Thanks to space-age materials, protection against the elements can fit right in your pocket. Since 1964, there have been various forms of lightweight, compact emergency blankets (sometimes called space blankets because they evolved from product development within the U.S. space program) that use a thin sheet of mylar coated with a silver or gold reflective surface capable of reflecting up to 97% of radiated heat.
For outdoor enthusiasts, emergency personnel such as search and rescue teams, and paramedics, these blankets help save lives by reflecting an individual's body heat back toward himself, thus slowing the effects of hypothermia. And in a hot environment, the blanket can be spread as a shade shelter and the reflective surface can protect against the rays of the sun.
The military uses "casualty blankets" that are based on the same principles but built to military spec to endure hard treatment in combat zones.
A company named Adventure Medical Kits (www.adventuremedicalkits.com) makes my favorite emergency blanket. It's called the Heetsheets Emergency blanket, and the reason it's my favorite is because it's built tough enough to take a lot of abuse without falling apart, but is still lightweight and compact enough to carry in a pocket. The blanket is orange on one side and silver on the other, so it serves dual purpose as a brightly colored panel to attract the attention of searchers, and provides the reflective benefits already mentioned. The reflective side can also act as a huge mirror surface (like a signal mirror on steroids) to attract attention to your position.
The blanket is impervious to moisture, so it makes a great rainwater catchment system or a method to pool water grabbed from a stream or lake so you can let the water settle before filtering. The uses of this item of equipment are many and varied, including use as a fire reflector (maintain sufficient distance from the flames), a windscreen, etc.
But Adventure Medical Kits has taken the emergency blanket concept to the next level with their Heatsheets Emergency Bivvy. Basically, they wrapped the blanket into the shape of a sleeping bag and sealed the seams to keep water out. Now, instead of fighting to keep a flat blanket wrapped securely around your body, all you have to do is crawl inside the bivvy and you're totally protected against wind, rain and snow. Body heat is reflected back toward you, so you retain as much of your own warmth as possible.
The bivvy protects against three kinds of heat loss — convective (air movement), reflective, and evaporative. But a thin sheet of plastic cannot provide real insulation, so there can still be conductive heat loss where you are in direct contact with anything colder than your body temperature. For that reason, you need to take steps to provide insulation beneath you (dry grasses, tender tips of evergreen, pine duff, whatever you can find). And it's also a good idea to pile insulative materials over the top of you as well.
The bivvy can be slipped down into a sleeping bag to increase the bag's cold-weather rating. I keep one of these for myself and my wife in our vehicle, and also carry one on my mountain bike and in my gear pack when traveling afoot in the backcountry.