Hypothermia is caused by exposure to cold, the wind, and exhaustion and is aggravated by being wet. It leads to mental and physical collapse accompanied by the chilling of the inner core of the body. Normal core body temperature can range between 37.17°C (98.9°F) and 37.72°C (99.9° F). Hypothermia occurs when your internal body temperature drops to 35°C (95°F) or lower. Hypothermia is a major cause of death in outdoor recreational situations.
Most outdoor enthusiasts simply can't believe that hypothermia can occur at temperatures as high as 10°C (50°F). They fatally underestimate the danger of wind-chill or being wet at such temperatures. Hypothermia is often the primary cause of death in cases reported as drownings.
The moment your body begins to lose heat faster than it produces it, you are undergoing exposure and two things will happen. You voluntarily exercise to stay warm or your body makes involuntary adjustments to preserve normal temperature in the vital organs, and you start shivering. Either response drains your energy reserves. The only way to stop the drain is to reduce the degree of exposure. The time to prevent hypothermia is during the period of exposure and gradual exhaustion.
If exposure continues until your energy reserves are exhausted, cold reaches the brain depriving you of good judgment and reasoning power. You will not realize this is happening and you will lose control of your hands. This is hypothermia. Your internal temperature is sliding downward. Without positive change this slide leads to stupor, collapse, and death.
Staying dry is the most important part in preventing hypothermia. When clothes get wet, they lose about ninety percent of their insulating value. Wool loses less as do many of the new synthetics. Cotton and wet down are worthless.
Beware of the wind; a slight breeze carries heat away from bare skin much faster than still air. Wind drives cold air under and through clothing. Wind refrigerates wet clothes by evaporating moisture from the surface.
This calculator is based on the new North American standards that were released in 2001 and will be used North America wide.
If you cannot stay dry and warm under existing weather conditions using the clothes you have with you, do whatever is necessary to be less exposed. The best strategy against the elements is to wear the gear that will protect you in the severest of conditions. It is very important to prepare and even over prepare to prevent hypothermia.