Hurricanes form over warm tropical oceans and are categorized based on their wind speeds using the Saffir-Simpson scale. The most damaging aspects of hurricanes are storm surge, which is a dome of water swept ashore near landfall, wind damage, and inland flooding from heavy rains. Modern warning systems using satellites, aircraft, buoys and radar help detect and track hurricanes, providing critical lead time for coastal evacuations before hurricane watches are issued for threatened areas within 36 hours or hurricane warnings within 24 hours of expected strong winds.