If the temperature drop occurs in less than four hours it is acute, otherwise it is chronic. Acute hypothermia is also called immersion hypothermia and typically occurs when a person is in cold water. It is important to note the difference between the two since treatment will be different. Hypothermia is considered severe when the body's core temperature drops below 90 degrees F, and mild from normal body temperature to 90 degrees F. Frostbite is another problem in areas of extreme cold. Frostbite is caused by exposure to severe cold.
Frostbite occurs more often when the wind is blowing, quickly taking heat from the body.
The ears, cheeks, nose, toes, and fingers are frostbitten the most frequently. When the part of the body is exposed to cold, the blood vessels constrict. When this occurs the blood supply to the chilled parts decreases and the tissues don't get the warmth they need.