Hurricanes are large tropical storms with heavy winds that form over warm ocean waters. They require warm ocean temperatures of at least 79 degrees Fahrenheit, moisture from the sea evaporating into the warm air, and wind patterns near the ocean's surface that pull winds inward. Hurricanes can last up to 8 days and become very destructive at higher categories, with Category 5 being the strongest designation for a hurricane with sustained winds over 155 mph. Hurricane Katrina, a Category 5 storm, caused massive damage of over $100 million and loss of over 1,800 lives when it struck New Orleans in August 2005.