Explanation

Hypothermia is caused by exposure to cold temperatures for a long time.

It can also occur after prolonged immersion in cold water. 

Causes 

Cold temperatures: Exposure to temperatures below 10°C for a long time

Cold water: Prolonged immersion in water below 20°C

Wet clothing: Wearing wet clothing in cold or windy weather

Heavy exertion: Exerting yourself while in cold weather

Dehydration: Not drinking enough fluids in cold weather

Undereating: Not eating enough in cold weather