Hurricane Florence has struck the Carolinas, dumping massive amounts of rain that could trigger catastrophic floods inland. Here are five facts you should know about these types of deadly storms:

1. A hurricane is a form of tropical storm that form over warm ocean waters in the eastern Pacific Ocean, Caribbean Sea, southern Atlantic Ocean, and Gulf of Mexico. When the winds of a tropical storm are less than 38 mph, it is called a tropical depression, and when the winds reach between 39-73 mph, it is classified as a tropical storm. When the winds exceed 74 mph, it is classified as hurricane. Scientists aren’t sure what causes hurricanes, but the two necessary ingredients are wind and warm ocean water.

2. The Saffir-Simpson Hurricane Scale defines hurricane strength by categories. A Category 1 storm is the weakest hurricane with wind speeds between 74-95 mph (faster than a cheetah); Category 2 has speed between 96-110 mph (as fast as a baseball pitcher’s fastball); Category 3 has speed between 111-129 mph (the serving speed of many professional tennis players); Category 4 has speed between 130-156 mph (faster than the world’s fastest rollercoaster); and Category 5 has speed greater than or equal to 137 mph (the speed of some high-speed trains).

3. In 1953, the United States began using female names for storms. This was changed in 1978, and thereafter both male and female names were used to identify Northern Pacific storms. The names for Atlantic storms are chosen by the World Meteorological Organization from a list of 21 names that are on a six-year rotation. If a storm is so deadly or costly that the future use of its name on a different storm would be inappropriate, the name is retired (about 86 names have been retired). In the event that more than 21 named tropical cyclones occur in a season, any additional storms will take names from the Greek alphabet.

4. Hurricanes cause damage in four main ways: wind, rainfall-induced flooding, tornados, and storm surge. Wind and rainfall-induced flooding is responsible for much of the structural damage caused by hurricanes. Storm surge is a rapid rise in the level of water that moves onto land as the eye of the storm makes landfall. Storm surge is water from the ocean that is pushed toward the shore by the force of the winds swirling around the hurricane. This advancing bines with the normal tides and can increase the water level by 30 feet or more. Because of the impact of the water—a cubic yard of water weighs about 1,700 pounds—storm surges can cause extensive damage and are the greatest threat to lifefrom a hurricane.

5. Hurricanes have a significant economic impact. Not only do they destroy personal property, they can affect the production of goods and services by damaging machinery, disrupting the local labor supply, and disrupting national supply chains. For example, the two major hurricanes in 2017—Hurricanes Harvey and Irma—are estimated to have each caused between $42.5 billion to $65 billion in property damage, amounting to around 0.2-0.3 percent of GDP. For all United States hurricanes, Katrina (2005) is the costliest storm on record. Hurricane Harvey (2017) ranks second, Hurricane Maria (2017) ranks third, Hurricane Sandy (2012) ranks fourth and Hurricane Irma (2017) ranks fifth. Hurricane Maria is the costliest hurricane on record to strike Puerto Rico and the U.S Virgin Islands.