High winds are a primary cause of hurricane-inflicted loss of life and property damage. Another cause is the flooding resulting from the coastal storm surge of the ocean and the torrential rains, both of which accompany the storm. The Saffir-Simpson scale is the standard scale for rating the severity of a hurricane as measured by the damage it causes. It classifies hurricanes on a hierarchy from category 1 (minimal), through category 2 (moderate), category 3 (extensive), and category 4 (extreme), to category 5 (catastrophic). A supertyphoon is equivalent to a category 4 or 5 hurricane.
Only three category-5 storms have hit the United States since record-keeping began—the 1935 Labor Day hurricane, which devastated the Florida Keys, killing 600; Hurricane Camille in 1969, which ravaged the Mississippi coast, killing 256; and Andrew in 1992, which leveled much of Homestead, Fla. Hurricane Katrina in 2005 was a category-5 storm at peak intensity over the central Caribbean, Mitch in 1998 was a category-5 storm at its peak over the W Caribbean, and Gilbert in 1988 was a category-5 storm at its peak. Gilbert was the strongest Atlantic tropical cyclone of record until Wilma in 2005, which was at its peak while category-5 storm over the W Caribbean. The 1970 Bay of Bengal tropical cyclone killed some 300,000 persons, mainly by drowning, and devastated Chittagong (now in Bangladesh); some 130,000 died when a cyclone struck Myanmar along the Andaman Sea in 2008. The deadliest U.S. hurricane was the 1900 Galveston storm, which killed 8,000–12,000 people and destroyed the city. Hurricane Katrina (2005), one of the worst natural disasters in U.S. history, was economically the most destructive U.S. storm, devastating the SW Mississippi and SE Lousiana coasts, flooding New Orleans, killing some 1,200 people, and leaving hundreds of thousands homeless. Sandy (2012), though technically an extratropical cyclone and no longer a hurricane when it made landfall, was the second most destructive storm economically, affecting New Jersey, New York, and 15 other states. Hugo (1989) in South Carolina and Opal (1995) and Charley, Ivan, and two others (2004) in Florida, and Rita (2005) and Ike (2008) in Louisiana and Texas also caused billions of dollars worth of damage. Weak hurricanes can still cause major flooding and damage, even when downgraded to a tropical storm, as did Agnes (1972) and Allison (2001).
To decrease such damage several unsuccessful programs have studied ways to "defuse" hurricanes in their developing stages; more recent hurricane damage-mitigation steps have included better warning systems involving real-time satellite imagery. A hurricane watch is issued when there is a threat of hurricane conditions within 24–36 hours. A hurricane warning is issued when hurricane conditions (winds greater than 74 mph/119 kph or dangerously high water and rough seas) are expected in 24 hours or less.