Basic Shelter

Under adverse conditions shelter can be essential to survival. Even in mild climates, overnight temperatures and rain can easily cause hypothermia, otherwise known as "exposure".  This is the number one cause of death in the wild..  In any case a good shelter will always be a morale boost, think how much better you feel after a good night's sleep. Evaluate what supplies you have and what is in your surroundings that can be used to construct a shelter.  Here are some basic designs which should not be beyond the skills of most people.

Bough Shelter Look for branches that sweep to the ground or fallen boughs that offer protection from the wind-ensure they are secure enough not to fall on you though! You may want to secure them by lashing (see diagram). Weave in other branches to add supplemental protection, conifers are more suited to this technique than broad leaves.

Root Shelter The spreading roots and compacted earth at the base of a fallen tree form a useful storm barrier, if they are facing the right way. Filling in the sides around the roots will increase it's potential and provide a solid base for construction of something more elaborate.

Natural Hollow A shallow depression in the earth will provide some protection from wind immediately, and provides a natural basis for construction of a shelter. However care must be taken in damp areas or on hills or you'll end up under water! Lay a few light logs across the hollow and then a larger bow across them, this will give pitch to short branches laid across the top to keep out rain. Finish with turf or twigs and leaves.

Fallen Trunk A fallen trunk alone provides a good windbreak scoop out a small hollow on the leeward side and construct a lean-to roof of boughs.

Sapling Shelter If you should happen upon a growth of saplings, clear the ground between them and lash their tops together, weave branches between them and consolidate with ferns and turf. A similar effect can be gained by driving pliable branches firmly in the ground.

If you have your "bug-out" or emergency kit you should have access to some form of waterproof sheeting, throw this over the saplings and weight with stones or logs

Bashas With a waterproof poncho, groundsheet, piece of tarpaulin or plastic sheeting you can construct what is often referred to as a "basha". There are a few designs below. Remember, always use natural shelter where possible, always insulate yourself from the ground and always secure the sheeting carefully.

Tepees Best known as the homes of North American Indians, start by tying three or more uprights together to form a cone, you can tie them on the ground before erecting. Cover with hides, sheeting or panels of birch bark. Ensure you leave a hole at the top for ventilation.

http://dnausers.d-n-a.net/prepared/shelter.html

Debris Hut

For warmth and ease of construction, this shelter is one of the best. When shelter is essential to survival, build this shelter.

To make a debris hut

• Build it by making a tripod with two short stakes and a long ridgepole or by placing one end of a long ridgepole on top of a sturdy base.
• Secure the ridgepole (pole running the length of the shelter) using the tripod method or by anchoring it to a tree at about waist height.
• Prop large sticks along both sides of the ridgepole to create a wedge-shaped ribbing effect. Ensure the ribbing is wide enough to accommodate your body and steep enough to shed moisture.
• Place finer sticks and brush crosswise on the ribbing. These form a latticework that will keep the insulating material (grass, pine needles, leaves) from falling through the ribbing into the sleeping area.
• Add light, dry, if possible, soft debris over the ribbing until the insulating material is at least 1 meter thick--the thicker the better.
• Place a 30-centimeter layer of insulating material inside the shelter.
• At the entrance, pile insulating material that you can drag to you once inside the shelter to close the entrance or build a door.
• As a final step in constructing this shelter, add shingling material or branches on top of the debris layer to prevent the insulating material from blowing away in a storm.

http://www.survivaliq.com/survival/shelters_s2.htm#fig5_11

If none of these shelters is possible with the available time and materials, than you have to depend on your emergency survival kit and any other gear you have with you.  A poncho or trash bag can keep you dry.  If you stuff dry leaves and other insulating materials between you and the plastic, this can keep you warm even if the temperature drops.  Remember that more than half your body heat can be lost thru your head if it is exposed.  Always carry a hat or stocking cap with you to conserve your heat.  Above all, stay dry.  If you get wet, much of your body heat will go towards evaporating the water instead of keeping your body warm.